Showing papers in "Cardiovascular Research in 1991"

Transient ischaemia induced by rapid cardiac pacing results in myocardial preconditioning

Abstract: Study objective — The aim was to determine whether rapid ventricular pacing can protect against the ventricular arrhythmias occurring during a subsequent coronary artery occlusion. Design — The effect was examined of two 2 min periods of pacing (300 beats·min−1) in chloralose-urethane anaesthetised dogs on a subsequent 25 min coronary artery occlusion. Ventricular arrhythmias, ST segment elevation, and inhomogeneity of conduction were analysed. Experimental material — 25 anaesthetised mongrel dogs in a restricted body weight range were used. Measurements and main results — Preocclusion pacing reduced the severity of occlusion induced ST segment elevation, degree of inhomogeneity, and arrhythmias: ventricular premature beats were reduced from 528(SEM 40) to 136(45), and there were lower incidences of ventricular fibrillation (0% v 47%) and ventricular tachycardia (30% v 80%). Conclusions — Rapid ventricular pacing “preconditions“ the myocardium in a manner similar to that following short coronary artery occlusions. Short periods of ischaemia no matter how induced protect the heart against the arrhythmogenic effect of a prolonged coronary artery occlusion.

Release of endothelium derived nitric oxide in relation to pressure and flow.

Abstract: Study objective — Endothelium derived nitric oxide (NO) is an important modulator of resting vascular tone. The aim of the study was to investigate the extent by which the rate of NO release is modulated by the two determinants of vascular conductance: pressure and flow. Design and experimental material — Porcine macrovascular endothelial cells cultured on microcarrier beads were used as a model in which the rate of NO release was determined photometrically. Columns packed with endothelial cell covered beads were perfused at different flow rates (2, 10, 20 ml·min−1) and perfusion pressures (ranging from 6 to 200 mm Hg). Measurements and main results — Release of endothelial cell derived NO was continuously quantified under basal and ATP stimulated conditions using a specific difference spectrophotometric assay. Increasing flow from 2 to 20 ml·min−1 enhanced the basal NO release from endothelial cells fivefold. ATP (10−4 M) augmented the NO release from endothelial cells more than 10-fold at each flow level studied. The ATP induced NO release rapidly increased by a factor of 5.8 when flow was enhanced from 2 to 20 ml·min−1 (>180 pmol·min−1·mg endothelial cell protein). Raising perfusion pressure from 6 to 200 mm Hg in endothelial cells did not affect the rate of basal NO release. Conclusions — (1) The rate of NO release from endothelial cells increases when flow is enhanced. (2) Endothelial cells possess a high capacity for NO production, permitting a rapid adjustment of NO release to changes in flow. (3) The rate of NO release is not causally related to changes in perfusion pressure.

Maldistribution of heterogeneous coronary blood flow during canine endotoxin shock

Abstract: STUDY OBJECTIVE - The aim was to investigate whether heterogeneous coronary blood flow is maldistributed during endotoxin shock. DESIGN - Variables were studied before (t = 0) and at t = 90 and t = 120 min after bolus injection of saline (n = 6) or endotoxin (n = 6). SUBJECTS - 12 anaesthetised mongrel dogs, weight 20-27 kg, were used. MEASUREMENTS AND MAIN RESULTS - We studied myocardial blood flows in small tissue sections (of about 1 g in left and 2 g in right ventricle) with radioactive microspheres, together with haemodynamic variables and global myocardial metabolism. At t = 0 min in controls, regional flows per 100 g were heterogeneous and ranged from a factor 0.2 to 2.7 and 0.6 to 1.6 of mean flow per 100 g to the left and right ventricle respectively; heterogeneity was unchanged at t = 90 and t = 120 min. Between t = 0, t = 90, and t = 120 min regional flows correlated: r = 0.78(SD 0.14), n = 18, for left ventricle, and r = 0.70(0.17) for right ventricle. In the endotoxin group, cardiac output and mean arterial pressure decreased by 44(7) and 48(11)% respectively, and lactate increased by 3.2(0.6) mmol.litre-1 at t = 120 min. Global left ventricle blood flow and delivery and metabolism of O2 were unchanged; lactate extraction and external work fell. The ratio between global right ventricular O2 delivery and external work also rose. Regional blood flows ranged from a factor 0.2 to 2.7 and 0.1 to 1.8 of mean flow to left and right ventricles respectively; heterogeneity did not differ from controls and did not change with time. Flow correlations with time were reduced: 0.45(0.24) for left ventricle and 0.45(0.26) for right ventricle (both n = 18, p less than 0.005 v controls). The left ventricular endocardial to epicardial flow ratio fell; flow was redistributed to both layers. CONCLUSIONS - Heterogeneous blood flow is redistributed throughout the heart during canine endotoxin shock so that, at unchanged global blood flow and flow heterogeneity, flow decreases in some but increases in other areas. Flow maldistribution may be associated with focal ischaemia, which may be masked by a rise in O2 uptake for a given workload (contractile inefficiency) in overperfused areas, and may thereby contribute to a fall in global myocardial external work for a given O2 delivery.

Reflection as a cause of mid-systolic deceleration of pulmonary flow wave in dogs with acute pulmonary hypertension: comparison of pulmonary artery constriction with pulmonary embolisation.

Abstract: Study objective – The aim was to examine whether mid-systolic deceleration of the pulmonary flow wave occurred in acute pulmonary hypertension due to pulmonary artery constriction and pulmonary embolisation, and if so whether it was related to reflection. Design – Various degrees of pulmonary hypertension were induced by both pulmonary artery constriction and pulmonary embolisation in dogs. During control periods and during pulmonary artery constriction and pulmonary embolisation, pulmonary flow and pulmonary artery pressure were recorded, and the forward and backward (reflected) flow waves were separated from the measured pulmonary flow wave by the method of Westerhof et al. Materials – 20 adult mongrel dogs were used and 10 dogs qualified for analysis. The other 10 dogs, which died before both interventions were completed, were excluded. Measurements and main results – During pulmonary artery constriction, a distinct mid-systolic deceleration of the pulmonary flow wave was observed in five of the 10 dogs, while during pulmonary embolisation, no mid-systolic deceleration was found in these five dogs. The distinct deceleration of the pulmonary flow wave was related to a steep fall and early negative peak in the backward flow wave. Conclusion – Mid-systolic deceleration of pulmonary flow wave is likely to be related to reflection.

Adenosine infusion during early reperfusion failed to limit myocardial infarct size in a collateral deficient species

Abstract: Study objective — Intracoronary or intravenous adenosine during reperfusion in combination with lignocaine may attenuate “reperfusion injury” and limit myocardial infarct size in the canine heart The aim of this study was to test whether intravenous adenosine also protects myocardium in the rabbit heart, which lacks xanthine oxidase and significant coronary collaterals in contrast to the canine heart Design — Five groups of rabbits underwent a 30 min occlusion of the circumflex coronary artery, followed by reperfusion In adenosine treated groups, either a high dose of adenosine (037 mg·kg−1·min−1) with lignocaine treatment (5 mg intravenously 1 min before coronary occlusion and before reperfusion) or a low dose (015 mg·kg−1·min−1) of adenosine with or without lignocaine was infused for 60 min starting 5 min before the onset of reperfusion Group 1 was untreated, while group 2 received a high dose of adenosine with lignocaine These groups were reperfused for 3 h Group 3 was untreated, group 4 received a low dose of adenosine, and group 5 a low dose of adenosine and lignocaine These groups were reperfused for 72 h Experimental material — 60 anaesthetised open chest rabbits were used Groups 1 and 2 were killed after 3 h coronary reperfusion Groups 3, 4, and 5 recovered from surgery for 72 h and were then killed for further study Measurements and main results — The high dose of adenosine reduced mean blood pressure to 44% of baseline value and diminished reactive hyperaemia in the area at risk by “coronary steal” The low dose of adenosine did not significantly alter systemic blood pressure or heart rate Infarct size did not differ between groups 1 and 2, at 397(SD 201)% of area at risk v 332(159)% (by tetrazolium staining), nor between groups 3, 4, and 5: 503(126)% v 527(156)% v 478(93)% (by histology) Conclusion — Neither a high dose nor a low dose of adenosine limited myocardial infarct size in the rabbit heart even when adenosine was combined with lignocaine treatment

Systemic vasoconstriction induced by inhibition of nitric oxide synthesis is attenuated in conscious dogs with heart failure

Abstract: Study objective – The aim was to test the hypothesis that endothelium dependent vasomotor control is impaired in heart failure. Design and subjects – The haemodynamic effects of NG-nitro-L-arginine (NNA), an inhibitor of nitric oxide synthesis, were studied in five dogs with and without pacing induced heart failure. Main results – In healthy dogs, NNA increased total peripheral resistance and arterial pressure, decreasing cardiac output and heart rate. These effects were partially reversed by L-arginine. In dogs with heart failure, NNA did not change any haemodynamic variable. However, L-arginine decreased total peripheral resistance. Conclusions – These findings support a physiological role for endothelium derived nitric oxide in control of vascular resistance and are in agreement with the hypothesis that endothelium dependent vasomotion may be impaired in heart failure.

Comparative significance in systolic ventricular interaction.

Abstract: Study objective — The aim was to measure the systolic coupling between the ventricles and to determine the relative importance of ventricular interaction in the pressure development of each ventricle. Design — Acute studies were done in dogs to measure the changes in right and left ventricular pressures (dPr, dPl) caused by sudden changes in left ventricular pressure (dPl') with release of an aortic constriction, and sudden changes in right ventricular pressure (dPr') with release of a pulmonary artery constriction, respectively. The instantaneous cross talk gain [dPr/dPl' (Klr) or dPl/dPr' (Krl)] was calculated during the ejection phase. The potential systolic pressure generated by the contralateral ventricle was evaluated as the cross talk gain multiplied by the contralateral systolic developed pressure. Experimental material — Studies were done in eight random source dogs (12-18 kg), anaesthetised with sodium pentobarbitone. Measurements and main results — The maximal Klr was lower than the maximal Krl, at 0.09 (SD 0.05) v 0.25 (0.06), and the mean Klr also was lower than the mean Krl, at 0.04 (0.02) v 0.10 (0.03), p<0.05. The potential right ventricular pressures developed by the left ventricle [maximum 10.3(5.6), mean 4.8(2.7) mm Hg] were not significantly different from the potential left ventricular pressures developed by the right ventricle [maximum 8.8(2.7), mean 3.4(0.7) mm Hg]. However, the ratio between the potential transmitted pressure and the measured developed pressure was greater in the right ventricle [maximum 39.0(21.1), mean 17.8(8.9)%] than in the left ventricle [maximum 11.1(7.1)%, p<0.05; mean 3.9(1.5)%, p<0.01]. This suggests that about 20-40% of the right ventricular systolic pressure may result from the left ventricle and about 4-10% of the left ventricular systolic pressure may result from right ventricle. Conclusions — Although the pressure coupling was greater in right to left ventricular interaction, right ventricular pressure generation may be more dependent on the left ventricle. Systolic ventricular interaction may be more important for right ventricular systolic function. Further, the parameters of right ventricular systolic function currently used may be considerably affected by the left ventricle.

Sequential echocardiographic-Doppler assessment of left ventricular remodelling and mitral regurgitation during evolving experimental heart failure.

Abstract: Study objective — The aim was to study the nature, magnitude, and time course of left ventricular structural adaptations to evolving heart failure. Design — 17 male mongrel dogs, weight 24.9(SD 3.7) kg, underwent rapid ventricular pacing (250 beats·min−1) until severe heart failure developed. Two dimensional echocardiographic and Doppler studies were performed at control, then weekly to severe heart failure. Haemodynamic measurements were made at control and severe heart failure. All studies were performed with the animals conscious during temporary sinus rhythm. Measurements and main results — Left ventricular diastolic volume gradually increased and the left ventricle assumed a more globular shape associated with significant wall thinning. Both the change in diastolic volume after one week of pacing and at the time of severe heart failure correlated with the time to peak heart failure. Mitral regurgitation was mild after one week of pacing, became moderate in most animals at severe heart failure, and lagged temporarily behind the increase in cardiac dimensions. The percentage increase in mitral annular size was significantly less than the increase in left ventricular cross sectional area. Conclusions — In pacing induced heart failure (1) marked left ventricular remodelling occurs, (2) the extent of left ventricular dilatation, both early and late, correlates directly with the time required for the development of severe heart failure, (3) mitral regurgitation is an epiphenomenon and is most likely to be caused by the increase in left ventricular cross sectional area.

Importance of basal nitric oxide synthesis in regulation of myocardial blood flow

Abstract: Study objective — The aim was to investigate whether basal coronary vascular tone and myocardial perfusion depend upon endothelial nitric oxide (NO) synthesis. Design — Myocardial blood flow and vascular resistance of the left and right ventricles were studied before and after intravenous infusions of either NG-nitro-L-arginine (L-NA), a specific inhibitor of NO synthase, or L-arginine, the precursor of NO synthesis. Radiolabelled microspheres were used to study myocardial blood flow in small tissue sections. Experimental material — 14 anaesthetised male cats, weight 2.1-3.5 kg, were used. Measurements and main results — Measurements were made before and 15 and 40 min after L-NA treatment (30 mg·kg−1 bolus followed by 1 mg·kg−1·min−1 infusion; n=8), and before and 15 min after L-arginine treatment (30 mg·kg−1 bolus followed by 10 mg·kg−1·min−1 infusion; n=6). L-NA significantly reduced coronary blood flow to the left and right ventricle, by 30(SEM 9) and 48(6)% respectively, after 15 min, but only to the right ventricle, by 45(8)%, after 40 min. Mean arterial pressure and myocardial vascular resistance were raised during the L-NA infusion. In contrast, L-arginine did not elicit any change in the variables studied. Conclusions — The conductance of the coronary vascular bed and the resting myocardial blood flow is regulated by L-arginine derived nitric oxide, and exogenous L-arginine availability is not a limiting factor in this NO generation.

Postischaemic reperfusion injury in the isolated rat heart: effect of ruthenium red.

Abstract: Study objective The aim was to investigate the effect of attenuating mitochondrial calcium uptake with ruthenium red on myocardial function and the resultant necrosis following prolonged ischaemia and reperfusion in isolated rat hearts. Mitochondrial dysfunction, secondary to increased calcium uptake, has been implicated as an important mediator of reperfusion injury in the heart. Design To examine the role of mitochondrial calcium uptake in mediating ischaemic and reperfusion injury, isolated rat hearts were perfused with ruthenium red (n = 6), a polysaccharide dye which inhibits calcium uptake by mitochondria, and were compared to control perfused hearts (n = 7). After stabilisation, hearts were subjected to 60 min no flow ischaemia, immediately followed by 40 min reperfusion. Experimental material Hearts were used from male Wistar rats weighing 300-350 g. Measurements and main results Cardiac high energy phosphates (ATP, phosphocreatine, inorganic phosphate) and pH were continuously monitored during ischaemia and reperfusion using phosphorus magnetic resonance spectroscopy. Contractility (dP/dT), coronary flow, creatine kinase release, and the time to the onset of ischaemic contracture were also measured. No differences in metabolic abnormalities or time to peak contraction during ischaemia were found between groups, suggesting that ruthenium red does not alter the metabolic consequences of ischaemia. However, upon reperfusion, the following differences in the ruthenium red perfused hearts were observed when compared to control hearts (p less than 0.05): ATP and phosphocreatine recovery were more complete, myocardial contractility was greater, coronary flow was greater, and myocyte necrosis was attenuated. Conclusions Combined with the known inhibitory effect of ruthenium red on mitochondrial calcium uptake, these data suggest that an important component of myocardial injury following ischaemia and reperfusion in the isolated rat heart is the result of mitochondrial calcium accumulation.

Nicorandil suppresses early afterdepolarisation and ventricular arrhythmias induced by caesium chloride in rabbits in vivo.

Abstract: Study objective — Outward K current of cardiac membrane has been shown to be suppressed by caesium chloride (Cs) and enhanced by nicorandil, a coronary vasodilator. The aim of this study was to assess die effects of nicorandil on the Cs induced early afterdepolarisations and associated ventricular arrhythmias in the rabbit heart in vivo. Design — Intravenous bolus injections of Cs (1 mmol·kg−1) were given three times at 20 min intervals. Monophasic action potentials of the left ventricular endocardium and the ECG (lead II) were recorded simultaneously over 60 min, under the intrinsic (sinus node) cardiac rhythm. Experimental material — Eight rabbits were treated with Cs alone (control group); seven other rabbits were first treated with an intravenous infusion of nicorandil (0.2 mg·kg−1) (nicorandil treated group) and the effects of Cs were then examined. Measurements and main results — In the control group, Cs produced early afterdepolarisations, premature ventricular beats and ventricular tachycardias. The ventricular tachycardias included two different types: (1) non-sustained polymorpohic ventricular tachycardia mimicking the torsade de pointes in patients with long QT syndrome; (2) sustained monomorphic ventricular tachycardia. In the nicorandil treated group, the amplitude of the early afterdepolarisations and the incidence of ventricular tachycardias were significantly less than in the control group. Conclusions — Nicorandil suppresses the early afterdepolarisations and ventricular tachyarrhythmias induced by Cs, possibly by increasing the membrane K conductance.

Endogenous plasma Na,K-ATPase inhibitory activity and digoxin like immunoreactivity after acute myocardial infarction

Abstract: Purpose of investigation – The aim was to look for the presence of circulating factors) with Na,K-ATPase inhibitory properties and digoxin like immunoreactivity in patients after acute myocardial infarction. Design – Venous blood samples were obtained when the patients were admitted and different methods were used to monitor the plasma concentrations of factors) with properties of digitalis. Subjects – These were 26 patients of both sexes (mean age 57.7 years, range 40 – 72) during the first 24 h of a first transmural acute myocardial infarct, 11 male patients with unstable angina pectoris (52.5 years, 45 – 67), and 18 healthy male controls (25 to 50 years). Measurements and main results – There was significant inhibition of ouabain sensitive Na,K-ATPase in intact erythrocytes in patients with myocardial infarction [1.4(SEM 0.15) μmol Pi·mg−1·h−1] compared with patients with unstable angina pectoris [3.1(0.4), p<0.01] and healthy controls [3.4(0.25), p<0.01]. In myocardial infarction complicated by ventricular fibrillation (n=5) Na,K-ATPase activity was significantly lower than in the other 21 patients [0.95(0.2) and 1.55(0.11) μmol Pi·mg−1·h−1 respectively, p<0.05]. There was no change in erythrocyte Na,K-ATPase activity in myocardial infarction complicated by acute pulmonary oedema, nor was there any difference in activity in erythrocyte ghosts obtained from the patients with myocardial infarction v healthy controls, at 0.47(0.13) v 0.50(0.02) (μmol Pi·mg−1·h−1. Boiled plasma supernatants obtained from the patients with myocardial infarction inhibited Na,K-ATPase in erythrocytes from healthy subjects. This inhibitory effect was antagonised by antidigoxin antibody. Plasma inhibitory potency was correlated with erythrocyte Na,K-ATPase activity in the patients with myocardial infarction (r= –0.65, p<0.001, n=23). There was a 2.5-fold increase in plasma digoxin like immonoreactivity in the patients with myocardial infarction [1.65(0.5) ng·mT−1] using delfia fluoroimmunoassay as compared with five healthy controls [0.04(0.12), p<0.05] and nine patients with unstable angina [0.48(0.11), p<0.05]. There was no difference in plasma digoxin like immunoreactivity in myocardial infarction complicated or not by ventricular fibrillation, but there was very low digoxin like immunoreactivity in patients with myocardial infarction complicated by acute pulmonary oedema [0.26(0.08) ng·ml−1, n=7]. There was no correlation between plasma digoxin like immunoreactivity and either plasma Na,K-ATPase inhibitory potency or erythrocyte Na,K-ATPase activity. Conclusions – The results show that plasma factor(s) with some of the properties of digitalis are increased in acute myocardial infarction.

Relationship between placental blood flow and combined ventricular output with gestational age in normal human fetus

Abstract: Study objective — The aim was to quantify the changes in placental blood flow and combined ventricular output with gestational age in normal human fetuses to determine the percentage of total cardiac output that placental blood flow represented, and whether this changed with age. Design — Two dimensional echocardiographic images of the umbilical vein, the proximal aorta and proximal main pulmonary artery were obtained, and cross sectional areas calculated from vessel diameters . Doppler velocity signals were recorded from each vessel and digitised to obtain velocity-time integrals. Placental blood volume flow and combined ventricular output were calculated as the products of flow velocity time integrals and cross sectional areas of the umbilical vein, and of the great arteries respective. Subjects — Subjects were 64 normal human fetuses aged between 20 and 42 weeks gestation. Measurements and main results - Placental flow and combined ventricular output both increased exponentially with gestational age (r = 0.79, and r = 0.84; both p<0.001). Placental flow correlated Linearly with combined ventricular output (r = 0.69; p<0.01) and comprised almost one third of total cardiac output throughout the second and third trimesters. Conclusions - These data describe the relationship between placental blood flow and combined ventricular output with age in the normal human fetus and provide a substrate by which placental insufficiency and resulting intrauterine growth retardation may be recognised early.

Pharmacological evaluation of early afterdepolarisations induced by sea anemone toxin (ATXII) in dog heart.

Abstract: Study objective — ATXII is a polypeptide toxin isolated from the sea anemone, Anemonia sulcata, known to delay sodium inactivation markedly and to induce early afterdepolarisations The aim was to investigate the mechanism of its action Design and materials — The mechanism of ATXII induced early afterdepolarisations was investigated in vitro in canine endocardial preparations using standard microelectrode techniques Measurements and main results — ATXII (2 × 10−7 M) induced cycle length dependent prolongation of plateau, more marked in Purkinje than in muscle fibres, and early afterdepolarisations in Purkinje fibres only The calcium channel antagonists verapamil (10−6 M, 10−5 M) and cobalt (2-4 mM), and drugs that block calcium release from the sarcoplasmic reticulum, ryanodine (10−6 M, 10−5 M) and caffeine (10 mM), did not antagonise the ATXII effects However, tetrodotoxin (5 × 10−6 M) and lignocaine (4 × 10−5 M) shortened the action potential and suppressed early afterdepolarisations The effects of lignocaine were seen at concentrations that did not significantly affect Vmax Conclusions — ATXII induced early after depolarisations are due to the effects of ATXII on Na+ entry, probably via a slowly inactivated Na+ channel population Calcium entry through the sarcolemmal Ca2+ channels and cyclic Ca2+ release from the sarcoplasmic reticulum are not required for the genesis of early afterdepolarisations in this model

Effects of digoxin, propranolol, and verapamil on exercise in patients with chronic isolated atrial fibrillation

Abstract: Study objective — The aim was to evaluate the effects of digoxin, propranolol, and verapamil on exercise in patients with chronic isolated atrial fibrillation. Design — Patients with chronic isolated atrial fibrillation underwent maximal exercise testing before and after the administration of digoxin, propranolol, or verapamil. Heart rate, oxygen uptake and oxygen pulse were observed at rest, at gas exchange anaerobic threshold, and at peak exercise. Subjects — The subjects were 10 patients (aged 48-78 years, mean age 60, SD 9, years) with chronic isolated atrial fibrillation. Measurements and main results — During exercise without medication, the heart rate was 85 (SD 8) beats·min−1 at rest, 127(19) at the level of anaerobic threshold, and 175(17) at peak exercise. With digoxin, heart rate was reduced to 75(9) beats·min−1 at rest (control v digoxin, p<0.01). However, reduction of heart rate was not seen at anaerobic threshold or at peak exercise. With propranolol, heart rate was 63(7) beats·min−1 at rest, 99(16) at anaerobic threshold, and 138(28) at peak exercise (control v propranolol, all p<0.01). Heart rate with verapamil was 70(13) beats·min−1 at rest, 107(30) at anaerobic threshold, and 138(28) at peak exercise (control v verapamil, p<0.05 at rest and at anaerobic threshold, p<0.01 at peak exercise. Neither digoxin, nor propranolol, nor verapamil changed the oxygen uptake during exercise. Without medication, oxygen pulse was 6.5(2.0) ml·beat−1 at anaerobic threshold and 7.7(2.1) ml·beat−1 at peak exercise. With digoxin, the change of oxygen pulse, versus without medication, was not significant at rest or at anaerobic threshold but was increased at peak exercise, at 8.3(2.1) v 7.7(2.1) ml·beat−1, p<0.05. With propranolol, oxygen pulse was increased to 8.2(1.9) ml·bear−1 at anaerobic threshold and 9.2(2.3) ml·beat−1 at peak exercise (control v propranolol, both p<0.01). With verapamil, oxygen pulse was increased to 8.7(1.8) ml·beat−1 at anaerobic threshold and 10.0(2.1) ml·beat−1 at peak exercise (control v verapamil, both p<0.01). Conclusions — Digoxin was effective in reducing heart rate at rest, but failed to reduce it during exercise. Propranolol and verapamil reduced heart rate at all levels of exercise as well as at rest. Oxygen uptake during exercise (total exercise capacity) was not reduced with propranolol or verapamil; this was thought to have been accomplished by an increased oxygen pulse.

Protective effect of preconditioning on reperfusion induced ventricular arrhythmias of isolated rat hearts.

Abstract: Study objective — The aim was to investigate the effect of preconditioning on reperfusion induced arrhythmias in the isolated working rat heart. Design — Rat hearts were perfused with modified Krebs-Henseleit bicarbonate buffer. A 15 min period of global ischaemia was performed followed by a reperfusion period of 5 min. This procedure was repeated three times. Hearts were divided into the control group (n = 10) and the free radical scavenger group (n = 11). Hearts in the latter were reperfused in the presence of scavengers (SOD 3.5 × 104U·litre−1 and catalase 5.0 × 105 U·litre−1). Experimental material — Hearts were obtained from male Sprague-Dawley rats (270-350 g). Measurements and main results — The incidence of reperfusion induced ventricular fibrillation in the control group decreased from 100% to 70% (NS) and to 40% (p<0.01) during the second and the third reperfusion periods, respectively. In the scavenger group, the incidence of ventricular fibrillation was the same in each reperfusion period at 45%. The time to onset of ventricular fibrillation increased at the second reperfusion in both groups. Conclusions — (1) The protective effect of preconditioning on reperfusion induced arrhythmia is evident in the isolated rat heart. (2) Addition of free radical scavengers reduced the incidence of reperfusion arrhythmia. However, the protective effect of preconditioning was lost.

5-Hydroxytryptamine mediates endothelium dependent coronary vasodilatation in the isolated rat heart by the release of nitric oxide.

Abstract: Study objective – The aim was to investigate further the endothelium dependent vasodilator action of 5-hydroxytryptamine (5-HT) on the rat coronary circulation. Design – Using saponin to damage the endothelium and L-NMMA (NG -monomethyl-L-arginine), the selective inhibitor of nitric oxide formation, we examined the role of endothelium and nitric oxide in causing 5-HT induced vasodilatation in the isolated rat heart. Experimental material – 56 rat hearts were excised and perfused on a Langendorff preparation. Measurements and main results – 5-HT at (10−9 to 10−5 M) and glyceryl trinitrate at (10−5 to 10−3 M) (n = 24) induced a dose dependent increase in coronary flow. Chemical removal of the endothelium by exposure to saponin (30 μg-ml−1) (n = 8) abolished the 5-HT induced vasodilatation but had no effect on the response to glyceryl trinitrate. Pretreatment of rats (n = 8) with L-NMMA (100 mg·kg"1) unmasked a strong vasoconstrictor effect of 5-HT but did not affect the glyceryl trinitrate induced vasodilatation. Perfusion of L-NMMA pretreated hearts with L-arginine at 10−4 M (n = 8) restored the vasodilatation induced by 5-HT but L-arginine perfusion had no effect on the extent of 5-HT or glyceryl trinitrate induced vasodilatation in normal hearts (n = 8). Conclusions – In the coronary circulation of the isolated rat heart, 5-HT mediates its vasodilator effect via endothelium dependent release of nitric oxide.

Reduction of experimental myocardial infarct size by oral administration of α tocopherol

Abstract: Study objective – The aim was to determine whether high dose dietary vitamin E could improve myocardial resistance to ischaemia and reperfusion. Vitamin E is an important physiological antioxidant which can be accumulated to high levels in the myocardium, without toxicity, by chronic dietary supplementation. Design – Subjects were fed a standard laboratory feed and water ad libitum for 10 d, plus either d-α-tocopoheryl acetate 200 IU·kg−1·d−1 orally (vitamin E group), or no supplement (control group). The animals then underwent either 60 or 180 min of left anterior descending coronary artery ligation, followed by 6 h reperfusion. The area at risk was identified by colloidal carbon, and necrosis by triphenyl tetrazolium chloride and light microscopy. Subjects – Studies were performed on New Zealand white rabbits weighing approximately 3.5 kg. Measurements and main results – In the 60 min ligation study, the control group had 30.5 (SD 4.0)% necrosis of the area at risk but the vitamin E group had no necrosis (n=5 per group, p≤0.0001). In the 180 min ligation study, the control group had 74.1 (11.5)% necrosis of the area at risk whereas the vitamin E group had 23.1 (7.2)% (n=5 per group, p≤0.0001). Conclusions – High dose dietary supplementation with vitamin E can improve myocardial tolerance to ischaemia and reperfusion, significantly reducing myocardial infarct size.

Effect of glibenclamide on extracellular potassium accumulation and the electrophysiological changes during myocardial ischaemia in the arterially perfused interventricular septum of rabbit

Abstract: Study objective – The aim was to study the effects of glibenclamide on the rate of rise of extracellular potassium concentration ([K+]) and the electrophysiological changes that occur during myocardial ischaemia. Design – The study was performed in isolated, arterially perfused interventricular septa from the rabbit. Six septa were treated with glibenclamide 10−6 mol·litre−1 and there were six untreated controls (vehicle only). [K+]o and electrophysiological variables were compared before and during a 30 min period of global zero flow ischaemia. Measurements and main results – Prior to ischaemia, the extracellular potassium concentrations measured using potassium sensitive valino-mycin electrodes were similar in the control and glibenclamide groups being 4.0 (SEM 0.1) and 4.0 (0.1) mmol·litre−1 respectively. [K+] rose during ischaemia in both groups, and at 30 min was 13.3 (0.7) mmol·litre−1 in the control group. The increase in the glibenclamide group was less marked, reaching 9.2 (0.5) mmol·litre−1 (p<0.0005; unpaired t test). Glibenclamide had no electrophysiological effects prior to ischaemia. However, during ischaemia the decrease in action potential amplitude, action potential duration (APD), maximum upstroke velocity of the action potentials (dV/dtmax), and the extent of resting membrane potential (EM) depolarisation were less in the glibenclamide group than in the controls. The effective refractory period (ERP) progressively shortened over the 30 min of ischaemia in both groups, to a similar extent. When taken in conjunction with the relative changes in action potential duration the degree of post-repolarisation refractoriness (ERP - APD) that developed was less in the glibenclamide group than in the controls. Conclusions – Glibenclamide attenuated the ischaemic rise in [K+], with preservation of both membrane potential and action potential amplitude, duration, and upstroke velocity together with less post-repolarisation refractoriness. These effects could be potentially antiarrhythmic in acute myocardial ischaemia.

Functional and electrophysiological effects of oxidant stress on isolated ventricular muscle: a role for oscillatory calcium release from sarcoplasmic reticulum in arrhythmogenesis?

Abstract: Study objective — The aim was to investigate the cellular basis of oxidant stress induced arrhythmias by studying the influence of oxidant stress on the contractile and electrophysiological function of isolated cardiac muscle. Design — Oxidant stress was induced by the photoactivation of rose bengal added to the solution superfusing isolated ventricular muscles from a number of species. Measurements of contractile and electrophysiological function were made under control conditions, during exposure to oxidant stress, and under a number of experimental conditions. Experimental material — Isolated supervised papillary muscles or trabeculae from rat, rabbit, or frog hearts were used in all studies. Measurements and main results — The contractile response to oxidant stress was assessed by measuring isometric developed tension and resting tension throughout the experiment, and the electrophysiological response was assessed by recording action potentials using conventional 3 M KC1 filled intracellular electrodes. Oxidant stress induced a transient positive inotropy, after contractions, and eventually contracture. Associated with these contractile changes were prolongation of the action potential, early afterdepolarisations, oscillations in resting membrane potential, and automaticity. These effects were concentration and species dependent and the oscillations in both tension and membrane potential were abolished by inhibition of calcium release from the sarcoplasmic reticulum with caffeine. Conclusions — The contractile and electrophysiological effects of rose bengal induced oxidant stress are consistent with a cellular calcium overload. The observation that the oscillations in tension and membrane potential were abolished by caffeine and that these effects were species dependent (rat>rabbit>frog) suggests a role for oscillatory sarcoplasmic reticulum calcium release in these effects. The oscillations in membrane potential and the automaticity induced by rose bengal are likely to underlie the arrhythmias observed in isolated hearts exposed to oxidant stress that have previously been described.

No upregulation of digitalis glycoside receptor (Na,K-ATPase) concentration in human heart left ventricle samples obtained at necropsy after long term digitalisation.

Abstract: Study objective — The aim was to evaluate the hypothesis that digitalis glycosides increase the concentration of their specific receptor (Na,K-ATPase) in human myocardial tissue, thereby possibly reducing the inotropic effect of long term digitalis treatment. Design — Intact samples of left ventricle were obtained at necropsy from patients who had been on long term treatment with digoxin and from patients not previously given digoxin. Digitalis glycoside receptors were quantified using vanadate facilitated 3H-ouabain binding before and after washing samples in buffer containing excess digoxin antibody fragments for 16 h at 30°C. This washing procedure has previously been shown to reduce prior specific digoxin binding in human left ventricle by 95% and to allow subsequent vanadate facilitated complete quantification of 3H-ouabain binding sites. In this context it was performed to reduce occupancy of digitalis glycoside receptors by digoxin, caused by digitalisation before 3H-ouabain binding. Subjects — 11 patients who had been on long term treatment with digoxin and eight who had not previously been given digoxin were studied. Left ventricle samples were obtained at necropsy at around 15 h after death. Measurements and main results — Standard 3H-ouabain binding was 39% less in samples from digitalised than from undigitalised subjects (p 0.10) in patients exposed to digoxin compared to left ventricle samples from individuals unexposed to digitalis glycoside treatment. Calculating 3H-ouabain binding relative to dry ventricular muscle weight confirmed the results obtained using wet weight as reference. Conclusions — The results suggest that digoxin treatment in life is associated with a 34% occupancy of digitalis glycoside receptors with digoxin. In the human heart there was no evidence for upregulation of digitalis glycoside receptor concentration due to long term digitalisation. Thus at receptor level there was no evidence for development of tolerance to digoxin therapy. The lower digitalis glycoside receptor concentration in the left ventricle observed in the heart failure patients may support the report of a relationship between Na,K-ATPase concentration as evaluated by 3H-ouabain binding and left ventricular function.

The rabbit dual coronary perfusion model: a new method for assessing the pathological relevance of individual products of the ischaemic milieu: role of potassium in arrhythmogenesis

Abstract: Study objective — The aim was to develop a new model for determining which factors associated with ischaemia and reperfusion are sufficient for arrhythmogenesis, and to use the model for examining regional hyperkalemia and K+ washout. Design and experimental material — Rabbit hearts (n = 150) were perfused with a buffered solution containing K+ in the normal range (2, 3, 4, or 5 mM). The circumflex coronary artery was perfused independently with a similar solution at a similar rate. A regional increase in K+ concentration was produced, followed by restoration of control K+ to mimic regional changes in K+ during ischaemia and reperfusion. Measurements and main results — Regional hyperkalemia (K+ = 9, 12, 15, or 18 mM) mimicked (concentration dependently) the known effects of regional ischaemia on the ECG in three important respects, producing ventricular arrhythmias, regional changes in ECG configuration, and regional alternans. The relationship between arrhythmias and K+ was bell shaped with a peak in susceptibility at 15 mM K+. Arrhythmia susceptibility was reduced and onset delayed by raising the K+ concentration delivered to the adjacent coronary bed. Arrhythmogenesis could be replicated in five or more successive runs in a single heart, indicating a lack of preconditioning. Readmission of control K+ (washout of high K+) mimicked the effects of reperfusion by rapidly causing new episodes of ventricular arrhythmias. The concentration depend ence of this effect was exponential, not bell shaped, with washout of 18 mM K+ most arrhythmogenic. There was no preconditioning phenomenon. Conclusions — Regional hyperkalemia and K+washout are factors sufficient to account for arrhythmogenesis during ischaemia and reperfusion, respectively. The new model is suitable for assessment of whether these factors are also necessary for arrhythmogenesis (by equivalent evaluation of other putative arrhythmogens).

Effects of oxygen tension on endothelium dependent responses in canine coronary microvessels.

Abstract: Study objective — The aim was to determine the direct effects of oxygen tension on endothelium dependent vasodilator responses in canine coronary microvessels. Design — Coronary microvessels were isolated and studied in vitro in a no flow constant pressure state using a video dimension analysing system. Microvessels were exposed to different partial pressures of oxygen. Endothelium dependent responses to acetylcholine and A23187 calcium ionophore were obtained with and without indomethacin during hyperoxia and normoxia, and compared to responses during hypoxia. Dose-response curves were also obtained to the direct smooth muscle dilator nitroprusside during normoxia and hypoxia. The reversibility of the effects of hypoxia on the acetylcholine response was studied after return to hyperoxic conditions following hypoxia. Experimental material — Coronary microvessels (58-150 micron diameter) were obtained from adult mongrel dogs of either sex. Measurements and main results — Exposure of preconstricted microvessels to hypoxia alone [Po25.8(0.4) kPa] resulted in a 25.9(SEM 6.8)% relaxation that was abolished by indomethacin [0.35(2.9)% relaxation]. Acetylcholine elicited dose dependent vasodilatation, with no significant differences in sensitivity between normoxia [Po2 14.6(0.04) kPa] and hypoxia: EC50 = 0.023 v 0.027 μmol·litre−1, respectively. During hyperoxia [Po2 80.2(6.0) kPa] there was a significant increase in the EC50 value to 0.09 μmol·litre−1 (hypoxia and normoxia v hyperoxia). After inhibition of prostaglandin synthesis with indomethacin, the sensitivity to acetylcholine was significantly decreased during hypoxia (EC50 = 0.16 μmol·litre−1) when compared to normoxia and hyperoxia. Indomethacin alone did not alter the acetylcholine response during normoxia and hyperoxia. As with acetylcholine, the sensitivity of indomethacin treated microvessels to A23187 was also decreased during hypoxia when compared to hyperoxia. There was no difference in the nitroprusside response during hypoxia and hyperoxia. The decreased vasodilator response to acetylcholine after hypoxia was persistent up to 2 h after return to hyperoxic conditions. Conclusions — Hypoxia decreases vasodilatation due to endothelium dependent relaxing factor, and oxygen tension has an important influence on both receptor dependent and receptor independent endothelium dependent vasodilator responses in coronary microvessels. Hypoxia also induces a prostaglandin mediated dilatation of preconstricted coronary microvessels. The effects of hypoxia on endothelium dependent responses are persistent up to 2 h.

Role of the endothelium in modulation of the acetylcholine vasoconstrictor response in porcine coronary microvessels.

Abstract: Study objective – The aim was to investigate the role of the endothelium in modulating the acetylcholine response in porcine coronary microvessels and compare the results with simultaneously studied large coronary arteries. Design – Coronary microvessels [104 (SEM 3.3) microns; range 38-150] were removed from fresh porcine hearts and studied in vitro during no flow constant pressure conditions. Endothelium derived relaxing factor (EDRF) activity and the role of the endothelium in modulating the acetylcholine response in microvessels was assessed by measuring changes in intraluminal diameter using a video tracking device. Large coronary arteries were simultaneously studied using conventional isometric ring techniques. Experimental material – Fresh porcine hearts were obtained from a local slaughterhouse. Measurements and main results – Acetylcholine was a potent vasoconstrictor (EC50=0.17 μM.) of passively distended microvessels. The effects of EDRF were studied by either inactivation with haemoglobin or inhibition of EDRF synthesis with N-Ω-nitro-L-arginine. Preconstricted microvessels exposed to either N-Ω-nitro-L-arginine or haemoglobin constricted further, consistent with basal release of EDRF. Neither drug affected passively distended microvessels. The acetylcholine vasoconstrictor response was potentiated after exposure of microvessels to either drug. Atropine, but not indomethacin, blocked the acetylcholine response in microvessels. As with microvessels, acetylcholine was a vasoconstrictor (EC50=0.3 μM) of large coronary arteries. In contrast to microvessels, indomethacin antagonised acetylcholine vasoconstriction in vessels with intact endothelium. Bioassay experiments using indo-methacin-treated large epicardial donor artery segments showed basal release of EDRF but no EDRF release in response to acetylcholine. Conclusions – The results show the microvessels and large coronary arteries are similar in their vasoconstrictor response to acetylcholine, that both release EDRF basally, and that vasoconstriction to acetylcholine is importantly modulated by the endothelium. In large arteries, acetylcholine does not stimulate EDRF release and, in contrast to microvessels, a cyclo-oxygenase product influences the vasoconstrictor action of acetylcholine.

Diltiazem at reperfusion reduces neutrophil accumulation and infarct size in dogs with ischaemic myocardium

Abstract: Study objective – The aim was to demonstrate the ability of diltiazem to protect the ischaemic myocardium in the course of coronary reperfusion, and to establish if an interaction with neutrophils is implied. Design – Ischaemia was induced by occluding the left anterior descending coronary artery for 90 min followed by 6 h of reperfusion with a residual critical stenosis left in place. Three groups were studied: group 1 (control) received a saline perfusion; group 2 was given a bolus injection of 400 μg·kg−1 of diltiazem 10 min before reperfusion, followed by 4 μg·kg_1-min_1 perfusion until termination of experiment; group 3 was made neutropenic by injecting a neutrophil antiserum produced in rabbits and was then treated with diltiazem, as in the second group. Subjects – 60 mongrel dogs of either sex were allocated at random into one of the three groups the day before the experiment. Measurements and main results – Diltiazem plasma concentrations ranged from 68.6(SEM 10.0) to 102.5(15.2) μg·litre−1 during the study. Transmural collateral blood flow, measured with 153Gd microspheres 15 min after occlusion, and area at risk, evaluated by Evans blue perfusion, did not differ among the three groups. Infarct size, estimated by triphenyltetrazoiium staining of heart slices and expressed as a percentage of area at risk, was less (p<0.05) in the diltiazem [20.5(5.2)%] and diltiazem plus neutropenia [17.6(5.4)%] groups compared to controls [39.8(6.9)%] but neutropenia added no significant benefit to diltiazem alone. The animals treated with diltiazem alone had lower serum creatine kinase levels than controls, at 5719(891) v 14333(2885) IU·litre−1, p<0.05. The neutrophilia seen in controls was virtually absent in diltiazem dogs. Myocardial neutrophil accumulation estimated by scintigraphy of In labelled autologous neutrophils was much less in diltiazem than in control dogs, at 3948(1228) v 11021(2081) 111In-neutrophil-g"1 of infarct, p<0.02. Conclusions – Diltiazem given during reperfusion reduces infarct size by a mechanism that includes an inhibition of neutrophil accumulation in the post-ischaemic myocardium.

Development of mock circulation models for the assessment of counterpulsation systems

Abstract: Study objective — This study entailed the development of mock circulation models to assess and compare the haemodynamic efficacy of extra-aortic counterpulsation (using trained skeletal muscle wrapped around the proximal descending aorta) and conventional intra-aortic balloon counterpulsation. Design and experimental materials — Hydraulic Windkessel type lumped parameter models were used either in conjunction with native skeletal muscle or as a dynamic simulation of counterpulsation. The haemodynamic performance of the wrapped latissimus dorsi muscle of the normal sheep was assessed using an artificial load to simulate the pressurised proximal descending aorta. Mock circulation models of counterpulsation comprised Windkessel compliance chambers, laminar flow resistors, a blood analogue, a prosthetic blood pump, and a purpose made hydraulic counterpulsator. Measurements and main results — An electrically stimulated muscle wrap, 5 cm in length, previously trained for 2 weeks at 3 V and 35 Hz, was assessed for haemodynamic performance in a mock circulation: volume of fluid displaced = 14.1(SD 1.8) ml; pressure increase from 100 mm Hg = 14.9(2.1) mm Hg; external work per contraction cycle = 180(70) mJ; external mean power = 800(100) mW. In a simulation of intra-aortic balloon counterpulsation, haemodynamic benefit (ie, an increase in proximal flow rate and endocardial viability ratio and a reduction in left ventricular stroke power) was assessed with respect to defined parameters. Conclusions — This paper demonstrates the potential of the mock circulation models both for the investigation of muscle wrap performance and for the comparison of extra-aortic muscle with intra-aortic balloon counterpulsation.

Phosphodiesterase inhibitors and the cardiac sarcoplasmic reticulum calcium release channel: differential effects of milrinone and enoximone

Abstract: Study objective — The aim was to investigate possible interactions between milrinone or enoximone and the calcium release channel from cardiac sarcoplasmic reticulum. Design — A membrane preparation enriched with “heavy” sarcoplasmic reticulum vesicles containing the calcium release channel was prepared from sheep myocardium. The incorporation of these vesicles into artificial lipid bilayers permitted investigation of the effects of the drugs on single calcium release channels under voltage clamp conditions. The effects of the drugs on radiolabelled ryanodine binding were also investigated as a functional probe for the activity of large populations of channels. Measurements and main results — Milrinone (100 μM-2 mM) caused a reversible activation of channel opening when added at the cytoplasmic face of the channel. Lifetime analysis suggests this activation is synergistic with the effects of calcium on the channel. Milrinone also stimulated [3H]ryanodine binding, consistent with the proposition that it is an activating ligand of the calcium release channel. Enoximone (100 μM-1 mM) was without effect on both single channel activity and [3H]ryanodine binding. Conclusions — Activation of the calcium release channel probably contributes to the positive inotropic action in vivo of milrinone but not enoximone. Other drugs which activate the calcium release channel have been shown to be cardiotoxic, but it is not known whether this is a specific effect of channel activation or a more general result of raising cytoplasmic calcium concentration within the myocyte. Further research is required to determine accurately the mechanism of action of drugs with phosphodiesterase inhibitory activity.

Effect of BN 50739, a new platelet activating factor antagonist, on ischaemia induced ventricular arrhythmias in isolated working rat hearts.

Abstract: Study objective – The aim was to investigate the role of platelet activating factor (PAF) in myocardial ischaemia by using BN 50739, a new specific PAF receptor antagonist with a hetrazepine framework. Design – Isolated working rat hearts were subjected to regional ischaemia, induced by ligation of the left main coronary artery for 30 min, followed by reperfusion. BN 50739 was applied at concentrations of 10−7, 10−6, 10−5 and 5 × 10−5 M, and its effects on the incidence of ischaemia induced and reperfusion induced ventricular tachycardia and ventricular fibrillation and heart functions, such as heart rate, coronary flow, aortic flow, left ventricular developed pressure (LVDP), its first derivative (LvdP/dtmax), and left ventricular end diastolic pressure (LVEDP), were determined. Experimental material – Studies were performed on isolated working hearts of male Sprague-Dawley rats weighing 300-360 g. Hearts were perfused with BN 50739 dissolved in dimethylsulphoxide. Control hearts were perfused with the vehicle. Measurements and main results – Regional ischaemia triggered ventricular arrhythmias showing high incidence between 12 and 20 min with peak appearance at 16 min. BN 50739 induced dose dependent protection against ventricular tachycardia and fibrillation: incidences declined from their respective control values of 91% and 75% to 33% (p<0.05) and 8% (p<0.05) after exposure to 10−5 M, and to 25% (p<0.05) and 8% (p<0.05) after exposure to 5 × 10−5 M concentrations. Reperfusion of the ischaemic myocardium resulted in an immediate appearance of ventricular tachycardia and fibrillation, but these were not suppressed by the PAF antagonist. Regional ischaemia slightly reduced heart rate, markedly decreased coronary flow, aortic flow, LVDP and LvdP/dtmax, and increased LVEDP. With the exception of LVEDP, these variables were not influenced by the drug. BN 50739, applied at a concentration of 5 × 10−5 M, reduced LVEDP significantly during the whole ischaemic period. Conclusions – Under in vitro conditions PAF is likely to be involved in the genesis of ischaemia induced ventricular arrhythmias since BN 50739, a specific PAF receptor antagonist, exerts a protective effect against these rhythm disturbances. This suggests that PAF antagonists may have benefit in the clinical management of acute myocardial ischaemia.

Myocardial protection with verapamil during ischaemia and reperfusion: dissociation between myocardial salvage and the degree of ATP depletion during ischaemia.

Abstract: Study objective – The aim was to evaluate the protective effect of verapamil during myocardial ischaemia and reperfusion. Design – In vivo phosphorus-31 (31P) magnetic resonance spectroscopy was performed on rats pretreated with verapamil (mg·kg−1 intraperitoneal) and controls during a 45 min left coronary artery occlusion and 60 min reperfusion. In separate groups of animals, haemodynamic measurements were taken at baseline, during ischaemia, and during reperfusion. Infarct size was determined by staining with triphenyltetrazolium chloride. Experimental material – Female Sprague-Dawley rats were used (control group n = 25, experimental group n = 24). Measurements and main results – Infarct size was significantly reduced in the verapamil group compared to controls: 9.9(SEM 2.3)%, n = 19 v 28.5(2.7)%, n = 19, p<0.001 (infarct % of left ventricular mass). Myocardial phosphocreatine and ATP levels were reduced to similar levels in both verapamil and control animals after 45 min ischaemia: 56.8(3.4)%, n = 10, v 61.4(1.8)%, n = 11, NS; 67.7(2.7)%, n = 10 v 69.7(2.9)%, n = 11, NS (% of baseline value). After 60 min reflow, there was significant recovery of phosphocreatine [91.1(4.2)% of baseline, p<0.05] and ATP [86.8(2.7)% of baseline, p<0.05] in the verapamil group, but no recovery of high energy phosphates in controls [66.3(2.8), NS; 69.6(2.7), NS]. The left ventricular systolic pressure, heart rate, rate-pressure product, and maximum rate of left ventricular pressure development were similar prior to ischaemia, and during ischaemia in both groups. There was an inverse correlation between infarct size and the degree of phosphocreatine recovery after 60 min of reperfusion (PCr recovery (%) = -0.99 × infarct size (%) + 101; r = 0.91; p<0.01; n = 14). Furthermore, in a separate group of animals (n = 9), there was a significant correlation between the size of the ischaemic area at risk and the degree of phosphocreatine decline after 15 min of coronary occlusion (PCr reduction (%) = 0.91 × risk area (%) + 5.6; r = 0.97; p<0.01). Conclusions – Pretreatment with verapamil extends the ischaemic time after which reperfusion results in myocardial salvage in this model of ischaemia and reperfusion. This protective effect is independent of the haemodynamic determinants of myocardial oxygen demand and the degree of ATP and phosphocreatine depletion during the ischaemic period. In this model of reversible ischaemia, 31P magnetic resonance spectroscopy is useful for quantitating both the size of the ischaemic region during coronary artery occlusion and infarct size after reperfusion.