Showing papers in "Circulation Research in 1980"

Some statistical methods useful in circulation research.

Abstract: Some statistical techniques for analyzing the kinds of studies typically reported in Circulation Research are described. Particular emphasis is given to the comparison of means from more than two populations, the joint effect of several experimentally controlled variables, and the analysis of studies with repeated measurements on the same experimental units.

The role of adenosine in the regulation of coronary blood flow.

Abstract: THE coronary vasculature is influenced by physical (arterial pressure, extravascular compression, hematocrit), neural, and chemical factors, and although there is considerable interplay among these various factors, the predominant one is chemical. Furthermore, the chemical factors of greatest importance are those that take origin from the cardiac parenchyma! tissue and serve as a link between myocardial oxygen needs and supply. One of the principal contenders that can serve as a mediator of coronary blood flow (CBF) regulation in response to cardiac metabolic activity is the nucleoside, adenosine (Berne, 1963).

Flow of "injury" current and patterns of excitation during early ventricular arrhythmias in acute regional myocardial ischemia in isolated porcine and canine hearts. Evidence for two different arrhythmogenic mechanisms.

Abstract: SUMMARY We recorded 60 DC-extracellular electrograms simultaneously from epicardial and intramural sites of the left ventricle of isolated perfused porcine and canine hearts during the first 15 minutes after occlusion of the left anterior descending coronary artery. During coronary occlusion, maximal current flow across the ischemic border occurred when normal cells had repolarized and ischemic cells had not. At that moment, maximal current sources at the normal side of the ischemic border were in the order of 2 /iA/mm 3 and maximal current sinks were — 5 /tiA/mm 3 . During propagation of a broad wavefront in nonischemic myocardium, current sources in the wake of the wavefront were about twice as large. Ventricular premature beats usually followed deep negative T waves in ischemic myocardium, when "injury" currents were maximal. Earliest activity always occurred at the normal side of the ischemic border, and whenever Purkinje activity was recorded it preceded myocardial activity in both single premature beats and the initial beats of ventricular tachycardia (VT) or ventricular fibrillation (VF). For later beats of VT, circus movements with a diameter of 1-2 cm were responsible for continuation of the arrhythmia. Dimension and position of the reentrant circuit changed from beat to beat. In VF, fragmentation of wavefronts occurred, and multiple wavelets followed tortuous paths. Circus movements were seldom completed; when they were, their diameter was 0.5 cm. It is concluded that two mechanisms are responsible for the very early ischemic arrhythmias: one, a "focal" mechanism located at the normal side of the ischemic border, possibly induced by injury currents in normal Purkinje fibers and, two, macro- and micro-reentry in ischemic myocardium. Circ Res 47:151-165, 1980

Inhibition of rat arterial smooth muscle cell proliferation by heparin. In vivo studies with anticoagulant and nonanticoagulant heparin.

Abstract: Heparin inhibits the proliferation of intimal smooth muscle cells which occurs after denudation of endothelium by air-drying injury in the rat carotid artery. We determined (1) whether the antiproliferative effect of heparin is secondary to effects on platelet adherence to subendothelium or endothelial regeneration and (2) whether the antiproliferative and anticoagulant activities of heparin are related. Morphometric observations by scanning electron microscopy showed that heparin did not alter platelet adherence 5 days after arterial injury and had little or no effect on endothelial regener- ation at 5 and 10 days. To study the relationship between the antiproliferative and anticoagulant effects, we fractionated heparin by affinity chromatography on antithrombin-Sepharose into purified anticoagulant and nonanticoagulant fractions. These heparin fractions were administered to rats in doses which were equivalent either in terms of anticoagulant activity or in terms of mass to the dosage of unfractionated heparin known to inhibit myointimal growth. Additionally, some rats received nonanticoagulant heparin at a dose which was greater in terms of mass than the highest dose of unfractionated heparin which could be administered without inducing fatal hemorrhage. Inhibition of myointimal growth, determined by morphometric analysis of total plaque volume 2 weeks after arterial injury, correlated with total mass of heparin administered but not with anticoagulant activity. Non- anticoagulant heparin given at high dose caused 77% inhibition of myointimal growth (P = 0.02 vs. controls). Heparin inhibition of arterial smooth muscle cell proliferation does not appear to be mediated either by effects on other cells at the level of the arterial wall or by antithrombin. This study should direct attention toward a potential growth regulatory role for arterial glycosaminoglycans. Circ Res 46: 625-634, 1980

Correlation between acute reductions in myocardial blood flow and function in conscious dogs.

Abstract: Decreases in regional endocardial function (ultrasonic dimension technique) and blood flow (radioactive microsphere technique) were correlated in 14 conscious dogs with acute graded levels of coronary stenosis. Coronary stenosis affected overall ventricular function only slightly, but induced gradual reductions in regional blood flow (BF) and segment length (SL) shortening in the ischemic zone. The relationship was best fit by an exponential function relating % change in SL to % change in BF; i.e., SL(% delta) = -161.6 e -0.047BF(% delta) (r = 0.92). In 14 segments, where no change in function was observed, blood flow fell by 6 +/- 1%. However, 10--20% reductions of blood flow impaired function significantly. Severe reduction of blood flow was required to reduce function completely. In 12 segments exhibiting paradoxical motion, blood flow fell by 95 +/- 2%, a value greater (P < 0.01) than in four akinetic segments, i.e., those with no shortening or lengthening (-82 +/- 4%). These data, which show on one hand that only 10-20% reductions in blood flow impair function significantly and that, on the other hand, severe reductions in blood flow are required to abolish active shortening completely, indicate sensitive coupling between blood flow and function in the conscious dog with acute myocardial ischemia.

Altered myocardial mechanics in diabetic rats.

Abstract: Diabetes mellitus is associated frequently with congestive heart failure in humans, even in the absence of associated coronary disease or hypertension. Nevertheless, the effects of the diabetic state on myocardial mechanics have not been studied. Accordingly, diabetes was induced in female Wistar rats by injection of streptozotocin (60 mg/kg). Left ventricular papillary muscles were studied 5, 10, and 30 weeks later and compared with controls. Relaxation was delayed significantly and velocity of shortening was depressed at all loads. However, the passive and active force-length curves, as well as the series elastic properties, were not altered. The changes in cardiac performance were found over a range of muscle lengths, stimulus frequencies, and bath concentrations of calcium, glucose, and norepinephrine. The duration of diabetes had no major effect on the mechanical changes observed. The possible influences of drug-induced cardiac toxicity, malnutrition, and altered thyroid hormone levels have been considered; the latter two factors could not be excluded completely from having some influence on the mechanical properties of diabetic cardiac muscle. Evidence is cited showing abnormalities in calcium uptake by sarcoplasmic reticulum and depressed actomyosin ATPase activity. Thus a cardiomyopathic state has been produced in the rat consequent to the induction of experimental diabetes mellitus. Various mechanisms for this entity have been suggested.

Functional and morphological organization of the rabbit sinus node.

Abstract: In isolated right atria of the rabbit heart, we studied the activation pattern within the sinus node, using the microelectrode technique. After the electrophysiological experiments, the preparations were subjected to a correlative morphological investigation, using light or electron microscopy. Different criteria for defining the dominant pacemaker were compared. A group of at least 5000 cells, located within the central part of the node where the most characteristic tissue architecture was found, was considered to be responsible for generation of the impulse. At the ultrastructural level, this leading cell group appeared to be part of a larger uniform cell group. The number of gap junctions observed suggests that all nodal cells are coupled by these structures. Toward the periphery, the excitation wave was propagated preferentially in an oblique cranial direction toward the crista terminalis. Neither morphologically nor electrophysiologically specific pathways were found for the conduction, but the preferential direction could be explained by the tissue architecture.

Tension development and sarcomere length in rat cardiac trabeculae. Evidence of length-dependent activation.

Abstract: The slope of the sarcomere length-tension relation in cardiac papillary muscle suggests length-dependent activation of the contractile system (1). However, there are no data on the effect of inotropic interventions on this curve. Only muscle length-tension curves are available, which give conflicting results (1,2,3). The purpose of the present study was to examine the effect of varied calcium concentrations on the sarcomere length-tension relationship, using laser diffraction techniques.

Inhibition of rat arterial smooth muscle cell proliferation by heparin. II. In vitro studies.

Abstract: We studied in vitro the effects heparin on the growth of rat aortic smooth muscle cells. Measurements of growth were monitored by [3H]thymidine uptake and changes in cell number over a period of 3 days. Our results show that heparin-highly anticoagulant or nonanticoagulant-significantly inhibits growth of smooth muscle cells. We also show that this is a highly specfic interaction with regard to molecule and cell type: i.e., other polyanions, except for a low molecular weight dextran sulfate, do not have the same effect on growth, and not all cells are inhibited by heparin; e.g., endothelial cell growth actually is enhanced. After removing antithrombin from our media, we carried out experiments which show that heparin is effective even though thrombin, a potent mitogenic agent, is still present and active. We also found that passing the platelet extract over a heparin column did not remove all of the motogenic activity of the platelet preparation. Both experiments indicate an inhibitory role for the heparin molecule, per se. Our results support the findings of a recent paper (Guyton et al., 1980) showing that heparin can limit the size of myointimal plaques in rats after carotid injuries by inhibiting smooth muscle cell proliferation.

Functional, morphological, and metabolic abnormalities of the cerebral microcirculation after concussive brain injury in cats.

Abstract: We induced experimental concussive brain injury by a fluid percussion device in anesthetized cats equipped with a cranial window for the observation of the pial microcirculation of the parietal cortex. Brain injury resulted in transient but pronounced increases in arterial blood pressure and in sustained arteriolar vasodilation associated with reduced or absent responsiveness to the vasoconstrictor effect of arterial hypocapnia and with reduced or absent ability of the vessels to undergo autoregulatory vasodilation in response to reductions in arterial blood pressure. Such vessels had reduced resting oxygen consumption in vitro. Electron microscopic examination of the same vessels that were studied physiologically disclosed the presence of discrete endothelial lesions consisting of either vacuolization or crater formation. Occasionally there was extensive destruction and necrosis of the endothelial cells. There was little or no morphological evidence of vascular smooth muscle damage. There was a close association between the presence of endothelial lesions and vessel dilation and unresponsiveness, suggesting a causal relationship. In cats in which the transient posttraumatic hypertensive episode was prevented, the vessels retained their normal caliber, remained normally responsive, and had no endothelial lesions. The results show that the vascular lesions in the pial microcirculation following this type of brain injury are due to the rise in arterial pressure. Circ Res 46: 37-47, 1980

Dual control of relaxation. Its role in the ventricular function in the mammalian heart.

Abstract: II y a toujours plus de chaleur dans le coeur qu' en aucun autre endroit du corps. Cette chaleur est capable de faire que, s'il entre quelque goutte de sang en ses concavite's, elle s'enfle promptement et se dilate, ainsi que font ge'ne'ralement toutes les liqueurs, lorsqu'on les laisse tomber goutte a goutte en quelque vaisseau qui est fort chaud. Ces gouttes se rare'fient et se dilatent, a cause de la chaleur qu'ellesy trouvent, au moyen de quoi, faisant enfler tout le coeur, elles poussent et ferment les cinq petites portes*; et continuant a se rare'fier de plus en plus, elles poussent et ouvrent les six autres petites portes'f qui sont aux entrees des deux autres vaisseaux par ou elles sortent; le coeur, incontinent apres, se de'senfle.

The effect of diabetes on performance and metabolism of rat hearts.

Abstract: To explore the effects of diabetes on myocardial function and metabolism we injected male rats with streptozotocin and studied their hearts 8 weeks later. Blood sugar levels in the treated rats were about 600 mg/100 ml. Body and heart growth rates were diminished. When studied in an isolated working rat heart apparatus using 5.5 mM glucose, hearts of diabetic animals showed diminished cardiac output and stroke work at high filling pressures. There also were significant depressions in peak left ventricular systolic pressure, peak aortic flow rate, maximum negative dP/dt, myocardial oxygen extraction, myocardial lactate production, and effluent lactate:pyruvate ratios. Myocardial glycogen stores, calculated glycogen utilization, and pyruvate production were increased in hearts of diabetics, and myocardial oxygen consumption was the same as in control hearts. The end-diastolic pressure-volume relationship was shifted to the right in hearts of diabetics. Most of the abnormalities observed in hearts of diabetic rats persisted when insulin and 15 mM glucose were included in the perfusion medium. Hearts from young rats or from age-matched food-restricted rats with heart weights similar to those of diabetics did not show depressed function or a pressure-volume shift. Our findings indicate that streptozotocin diabetes in rats results in abnormal myocardial performance. This is not due to restrictions in coronary flow or myocardial oxygenation and is not correctable by the provision of high glucose plus insulin in the perfusion medium.

Hemodynamic effects of exogenous and endogenous vasopressin at low plasma concentrations in conscious dogs.

Abstract: The possibility that vasopressin plays a role in cardiovascular control arouses increasing interest. We studied in unanesthetized dogs the hemodynamic consequences of 1-hour vasopressin infusions that modified plasma concentrations over a range similar to that found in physiological situations. We also examined the cardiovascular events following the stimulation of endogenous vasopressin release by an increase in plasma osmolality. In dogs with baroreceptor reflexes intact, vasopressin infusions which increased plasma vasopressin concentration by 2-20 fmol/ml did not affect mean arterial pressure. However, they significantly decreased cardiac ouput (measured by an electromagnetic flowmeter) and increased total peripheral resistance. After baroreceptor denervation, vasopressin infusion rates as low as 40 fmol/kg per min (0.017 microU/kg per min) led to an increase in mean arterial pressure. Cardiac output was unaffected until much higher infusion rates were used. Changes in total peripheral resistance were very similar to those calculated in dogs with intact baroreceptors. The release of vasopressin following infusions of hypertonic solutions either intravenously or into a carotid artery induced detectable hemodynamic changes which appeared in many respects similar to those following low infusion rates of vasopressin. We conclude that physiological plasma concentrations of vasopressin have hemodynamic effects even though they do not normally modify arterial pressure, presumably because of some particular interaction of vasopressin with the baroreceptor reflex.

Receptors under pressure. An update on baroreceptors.

Abstract: BARORECEPTORS are nerve endings that respond to deformation or strain of the vessel walls in which they are located. They do not respond to pressure per se because they are not activated by pressure changes in the absence of deformation (Hauss et al., 1949; Angell-James, 1971). Pressure is sensed by the baroreceptors in a multi-step process that includes pressure-mechanical deformation in the vessel wall followed by mechano-electrical transduction in the receptors themselves. The relationship between wall deformation and intravas-cular pressure is not direct. Wall deformation may be quantified as strain that is calculated from the ratio of the change in wall radius produced by wall stress to the initial unstressed wall radius. Wall stress is calculated as the product of the distending pressure and the radial distance over which it acts divided by the wall thickness. Wall deformation is also viscoelastic in nature, due principally to the elastin, collagen, and smooth muscle present in the vessel wall. When the wall is strained by pressure, the receptor is deformed by an unknown coupling mechanism probably located in the processes described by Krauhs (1979) and presumably viscoe-lastic, which influences strongly the dynamic response of the receptors. The preceding steps are the mechanical parts of a sequence linking intravascu-lar pressure to receptor discharge. The major components are shown in Figure 1. The remainder of the sequence is electrical in nature and is determined by the electrophysiological properties of the baroreceptor membrane and axon. By analogy with other mechanoreceptors (Terzuolo and Knox, 1971), receptor deformation generates a receptor potential and thus a receptor current which flows outward across the spike-initiating zone thereby producing baroreceptor discharge (Kuffler and Eyzaguirre, 1955) (Fig. 1). Since baroreceptor discharge is slowly adapting (Bronk and Stella, 1935), the most suitable receptor analogies are provided by slowly adapting distortion receptors, e.g., crayfish stretch receptor, rather than by phasic mechan-oreceptors, e.g., Pacinian corpuscle. In the latter case the phasic characteristics are determined by a specialized structure, the lamellated capsule (Loew-enstein, 1971), whereas specialized structures of this sort are not present in crayfish stretch receptors or aortic baroreceptors although a few lamellated bar-oreceptors have been identified in the carotid sinus of the rabbit (Dropmann, 1967). The muscle spindle is a much more complicated end-organ, but its coupling to surrounding muscle has been particularly well studied (Fukami and Hunt, 1977) so that it also might provide useful comparisons. In the crayfish stretch receptor, a …

Morphometric study of early postnatal development in the left and right ventricular myocardium of the rat. II. Tissue composition, capillary growth, and sarcoplasmic alterations.

Abstract: The absolute and differential growths of the capillary network and of myocyte cytoplasmic components in the left (L) and right (R) ventricular free walls were measured morphometrically from 1 to 5 days and from 5 to 11 days after birth. From 1 to 11 days, capillary length, luminal volume, luminal surface area, and endothelial cell volume each increased 2-3 times more rapidly than myocardial mass or myocyte mass in each ventricle. Mean intercapillary distance and the transverse crosssectional area of the average capillary decreased markedly. The mean number of capillaries across the ventricular walls increased from 16 to 79 (L) and 14 to 22 (R). Maturation of the cytoplasm of left and right ventricular myocytes from 1 to 11 days included increases in the volume percentage of myofibrils(1.2-fold), mitochondria (1.8-fold), and smooth endoplasmic reticulum (2.1-fold) and increases in mean mitochondrial size [1.9-fold (L), 1.2-fold (R)] and number per cell [2.7-fold (L), 3.6-fold (R)]. Despite a 2-fold greater overall left ventricular growth, the myocardial compositions of both ventricles were nearly indistinguishable at 1 and 11 days. Both subcellular and microvascular changes, however, were generally achieved more rapidly in the left ventricle from 1 to 5 days of age, demonstrating many structural differences and a lagging development in the right ventricle at 5 days. Thus, postnatal myocardial adaptation to the altered work demands on the left and right ventricles shortly after birth resulted in morphological changes that could be the basis for a transient disparity in ventricular functions at about 5 days of age. Circ Res 46: 503-512, 19S0

An analysis of the mechanical disadvantage of myocardial infarction in the canine left ventricle.

Abstract: An isotropic, initially spherical, membrane model of the infarcted ventricle satisfactorily predicts ventricular function in the infarcted heart when compared to clinical information and available ventricular models of higher complexity. Computations based on finite element solutions of this membrane model yield end-diastolic and end-systolic pressure-volume curves, from which ventricular function curves are calculated, for infarcts of varying size and material properties. These computations indicate a progressive degradation of cardiac performance with increasing infarct sizes such that normal cardiac outputs can be maintained with Frank-Starling compensation and increased heart rate for acute infarcts no larger than 41% of the ventricular surface. The relationship between infarct stiffness and cardiac function is found to be complex and dependent on both infarct size and end-diastolic pressure, although moderately stiff subacute infarcts are associated with better function than extensible acute infarcts. Also, calculations of extensions and stresses suggest considerable disruption of the border zone contraction pattern, as well as elevated border zone systolic stresses.

Morphometric study of early postnatal development in the left and right ventricular myocardium of the rat. I. Hypertrophy, hyperplasia, and binucleation of myocytes.

Abstract: We reviewed the absolute amd differential growths of the myocyte populations in the left(L) and right (R) ventricular myocardium morphometrically from 1 to 5 days and from 5 to 11 days after birth. From 1 to 11 days hypertrophy of the average myocyte in the ventricles was (L) 2.7- and (R)2.4-fold, and myocyte proliferation was (L) 2.0- and (R) 1.2-fold. Mean cell volume, cell length, and percent binucleation of cardiac myocytes were similar in both ventricles at 1, 5, and 11 days of age. During this period, average myocyte length increased 2-fold (12 sarcomere lengths), and the percentage of binucleate myocytes increased approximately from 2.7 to 17 to 48%. Myocyte hypertrophy from 1 to 5 days resulted mainly from an increase in the volume of cytoplasm per nucleus and from 5 to 11 days from the accumulation of binucleate cells. No differences were observed in the characteristics of epicardial and endocardial myocytes in either ventricle up to 11 days of age. The 61% greater proliferation of myocytes in the left ventricle was the principal basis for the development of a 2-fold difference in ventricular weight gains: (L) 6.2- and (R) 3.4-fold. No increase in right ventricular midwall thickness was observed, in contrast to a 2.7-fold increase of the left ventricle. It was concluded that, asa result of the circulatory changes occurring shortly after birth, right ventricular growth is analogous to eccentric hypertrophy, whereas left ventricular growth represents a combination of eccentric and concentric hypertrophy. Circ Res 46: 495-502, 1980

Computer simulation of arrhythmias in a network of coupled excitable elements.

Abstract: Arrhythmias were simulated in sheets or cables, consisting of coupled excitable elements, which were characterized by a simple regenerative mechanism. The geometry of the network, the amount of coupling among individual elements, and the properties of the elements relating to excitability, automaticity, and duration of the refractory period could be adjusted arbitrarily in an interactive computer program. When a critical amount of coupling was present between automatic and non-automatic cells, sustained repetitive activity could be initiated and stopped by stimulation of the elements. Using this mechanism, it also was possible to evoke reciprocal activity in a one-dimensional cable. In uniform sheets of coupled elements, circus movement of the activation front could be evoked. The presence of an obstacle or dispersion of the refractory periods of the elements was not a prerequisite for the initiation of circus movements. The vortex of circus movements in the homogeneous sheets consisted of elements which were inactivated by depolarizing currents from the circulating wavefront. In sheets of sufficient size, multiple vortices could be present.

The dependence of ultrasonic attenuation and backscatter on collagen content in dog and rabbit hearts.

Abstract: To determine whether collagen content may be a determinant of the ultrasonic attenua-tion and backscatter in myocardium and to identify factors in the measurement of the backscatter coefficient needed for ultrasonic characterization of myocardium in vivo, two series of experiments were conducted. In the first series, the ultrasonic attenuation was measured from 110 regions in 18 dogs studied 2, 4, and 6 weeks after coronary occlusion. The increased slope of the attenuation of regions within zones of infarction correlated closely with regional content of collagen as determined from concentration of hydroxyproline in canine hearts (0.90 in hearts studied at 6 weeks and 0.77 and 0.73 in those studied 4 and 2 weeks after occlusion). In a second series of experiments in which hearts from 21 rabbits were studied from 5 to 7 weeks after coronary occlusion, ultrasonic attenuation, ultrasonic backscatter, and content of collagen within zones of regions of infarction were increased significantly compared to nonischemic regions from the same rabbits (P < 0.001 in each case). To determine whether the increased ultrasonic backscatter depended on the content of intact collagen in regions of infarction, isolated rabbit hearts were perfused with modified Krebs-Henseleit solution and collagenase resulting in a significant reduction of the backscatter coefficient (P < 0.05). Since regional ultrasonic backscatter in the heart appears to be influenced markedly by the regional content of intact collagen, characterization of tissue with reflected ultrasound may permit noninvasive estimation of replacement of myocardium by collagen in vivo facilitating diagnosis and evaluation of the evolution of conditions such as ischemic heart disease and cardiomyopathy. Circ Res 47: 49-58, 1980

Ryanodine alteration of the contractile state of rat ventricular myocardium. Comparison with dog, cat, and rabbit ventricular tissues.

Abstract: To test the hypothesis that ryanodine inhibits the release of contractile Ca2+ from intracellular stores, we compared the contractile responses by rabbit, cat, dog, and rat papillary muscles to ryanodine. Results of cumulative ryanodine concentration (10−9 to 10−4 M) response studies indicate the following order of sensitivity to ryanodine: rat > dog = cat > rabbit which mimics the relative dependence of these species on intracellular Ca2+ for force development. In the presence of 2.5 mM [Ca2+]$, cumulative additions of ryanodine or a single exposure to 10−4 M concentration produced biphasic contractile responses in rabbit, cat, and dog, but not rat ventricular muscle. The elevation of [Ca2+]o to 5 RIM either antagonized the expression of ryanodine's negative inotropic effect or promoted the positive effect of this agent in all species tested. Ryanodine did produce a biphasic change in contractility in the presence of 2.5 mM [Ca2+]o in K+-depolarized, isoproterenol-restored rat papillary muscles. In addition, prior exposure of rat myocardium to ryanodine (2.5 mM [Ca2+]o) was similar to a decreased [Ca2+]o in that it permitted inotropic agents, such as increased stimulation rates, hyperos- molality, and ouabain to produce positive contractile responses from this tissue. In contrast, the positive response by rat cardiac muscle to paired electrical stimulation is prevented by ryanodine. Ryanodine also accelerated the rest-decay of force development in rat myocardium, suggesting that it increased the rate of loss of calcium from contractile-dependent Ca2+ stores. The results are consistent with ryanodine effecting a decreased availability of intracellular contractile Ca2+, perhaps through a diminishment of its release. Circ Res 46: 332-343, 1980

Comparison of the effects of regional ischemia, hypoxia, hyperkalemia, and acidosis on intracellular and extracellular potentials and metabolism in the isolated porcine heart.

Abstract: DC electrograms and transmembrane potentials were recorded from isolated perfused pig hearts.Regional ischemia was produced by clamping the left anterior dascending artery (LOD), and after a reperfusion period, regional hypoxic, glucose-free solutions (with or without acidification, or high K+) or with normoxic high K+ solutions. In transmural biosies, nucleotides, lactate, and K+ were determined. During ischemia, resting potential decrease (T-Q depression), action potential amplitude and upstroke velocity decrease, and local activation is markedly delayed (S-T elevation, late intrinsic deflection, high R wave). A high K+ concentration, up to 13 mM, decrease resting potential (T-Q depression) and shortens the action potential (positive T wave) but has minor effect on amplitude (no S-T elevation) and activation (no delay). Hypoxin (Poz=7 mm Hg, no glucose) causes a moderate decrease in resting potential, marked action potential shortening, and some of loss of amplitude but no or only minor delay in activation (slight T-Q depression and S-T elevation, positive T waves). Acidle perfusate does not influence changes fin transm,embrane potential during hypoxia. Potentials of similar configuration to those seen during ischemia could be obtained by LAD perfusion with hypoxic, glucose-free, high K+(10 mM), acidic (pH 6.8) solutions. Most surprising was improvement of potentials after 20 minutes of perfusion, like that seen during maintained LAD occlusion. The time course of metabolic changes was the same in hypoxia and ischemia. Results indicate (1) that there is no direct relationship between metabolic and electrical changes, (2)that electrical changes during ischemia are caused by a combination of lack of perfusion (hypoxia, no substrate) and lack of washout (hyperkalemia, acidosis), and (3) that action potentils of ischemic cells are more “depressed” than those of normoxic cells, at similar reduced levels of resting membrane potential. Circ Res 46: 634-646, 1980.

Effects of sympathetic and vagal nerves on recovery properties of the endocardium and epicardium of the canine left ventricle.

Abstract: The purpose of this study was to determine if autonomic nerve interventions exerted quantitatively dissimilar effects on recovery properties of endocardium compared with epicardium. Effective refractory periods (ERP) were measured by the extrastimulus technique in the endocardium and epicardium of the canine left ventricular anterior wall. The basic train and premature stimuli were administered to the endocardium and overlying epicardium via different poles on the same multipolar needle electrode, using cathodal stimuli. Sympathetic augmentation produced via bilateral carotid arterial occlusion or electrical stimulation of right, left, and both sympathetic nerves shortened ERP. Bilateral sympathetic denervation prolonged ERP. The changes in ERP of the endocardium were no different than were changes in the ERP of overlying epicardium. In separate studies, electrical stimulation of the cervical vagi prolonged ERP similarly in epicardium and endocardium. Pacing at slower rates or physostigmine administration potentiated the ERP prolongation in endocardium similar to epicardium. Augmented sympathetic tone produced by carotid occlusion also potentiated prolongation of ERP by vagal stimulation. The percent change in endocardial sites was slightly but significantly less than in epicardial sites. ERP prolongation due to vagal stimulation was attenuated markedly after sympathectomy and abolished with both propranolol and atropine. We conclude that, in the normal anterior left ventricular myocardium of the dog, sympathetic augmentation shortens ERP in epicardial sites equivalent to that in the underlying endocardial sites, that vagal nerve stimulation prolongs ERP in epicardial sites equal to or slightly greater than in the underlying endocardial sites, and that vagal stimulation antagonizes background sympathetic activity. Circ Res 46: 100-110, 1980

Changes in brain adenosine during bicuculline-induced seizures in rats. Effects of hypoxia and altered systemic blood pressure.

Abstract: We analyzed brain tissue in 139 rats for adenosine and its metabolites, inosine and hypoxanthine, during the initial 120 seconds of seizures induced by bicuculline. We also measured ATP, ADP, AMP, phosphocreatine (PCr), and lactate. We divided the rats into four groups by adjustment of their preictal arterial oxygen tension: group I, PaO2 > 200 mm Hg; group II PaO2 = 50 mm Hg; and group III: PaO2 = 100 mm Hg. We treated a fourth group whose PaO2 = 100 mm Hg with phentolamine to block the 44% rise in blood pressure which occurred with the onset of seizures. PaCO2 was maintained between 30 anf 40 mm Hg in all groups. Brain tissue was sampled rapidly after 0, 10, 20, 30, 60, and 120 seconds of seizures by the freeze-blow technique. With normoxia (PaO2 = 100 mm Hg) or hyperoxia (PaO2 > 200 mm Hg), adenosine increased within ten seconds of the onset of seizures and remained elevated even after 120 seconds. Elevations in inosine and hypoxanthine were delayed compared to the increases in adenosine. A reduction in PaO2 (50 mm Hg) or systemic blood pressure during seizures caused a further augmentation in the increase in brain adenosine levels. During the seizure period, transient changes in adenine nucleotides and energy charge were observed, but PCr remained depressed and lactate continued to rise. The rapid and sustained increase in cerebral adenosine levels, temporally paralleling the changes in cerebral blood flow, supports the role for adenosine in the regulation of cerebral blood flow.

Continuous positive-pressure ventilation decreases right and left ventricular end-diastolic volumes in the dog.

Abstract: We investigated the mechanism(s) responsible for the decreased cardiac output during continuous positive-pressure ventilation (CPPV). Seven dogs were anesthetized with chloralose-urethane, intubated, and ventilated using a volume ventilator. We measured heart rate, stroke volume, and the determinants of stroke volume: left and right ventricular end-diastolic volumes, isovolumic and ejection phase indices of myocardial contractility, and pulmonary and systemic arterial pressures. Myocardial blood flow was estimated using radioactive microspheres. Variables were measured during a control period of intermittent positive-pressure ventilation (IPPV), 8-20 minutes after the initiation of CPPV using 12 cm H2O positive end-expiratory pressure (PEEP), and 8-20 minutes after the removal of PEEP. CPPV decreased cardiac output but did not affect total or regional myocardial blood flow or the ratio of subendocardial to subepicardial blood flow. Isovolumic and ejection phase indices of myocardial cointractility, heart rate, and systemic arterial pressure did not change during CPPV. Right and left ventricular end-diastolic and end-systolic volumes decreased markedly during CPPV. We conclude that CPPV decreases cardiac output in accordance with Starling's law by decreasing preload.

Different electrophysiological responses of canine endocardium and epicardium to combined hyperkalemia, hypoxia, and acidosis.

Abstract: To determine whether canine epicardium and endocardium show intrinsically different electrophysiological responses to metabolic alterations that occur during acute myocardial ischemia in vivo, endocardial, epicardial, papillary muscle tip, and Purkinje fibers (PF) were superfused in vitro with Tyrode's solution containing 8.0 DM KC1 at a pH of 6.85 and a Po2 < 50 mm Hg. During the initial 10 minutes of superfusion with altered Tyrode's solution, reduction of action potential (AP) amplitude and dV/dtmax and prolongation of activation times were greater in epicardium than in endocardium or in PF, despite similar changes in resting membrane potential. After superfusion for 15 minutes, only 3 of 18 epicardial and 5 of 16 papillary muscle cells were excitable, whereas 14 of 16 endocardial muscle cells and 13 of 13 PF still were responsive. Membrane responsiveness at takeoff potentials < — 65mV was lower in epicardium and papillary muscle than in endocardium during superfusion with normal or altered Tyrode's solution. The effects produced by the initial 15 minutes of superfusion with altered Tyrode's solution were partially reversed in endocardium and PF but not in epicardium or papillary muscle, during a subsequent 20to 60-minute period of continued superfusion. Tetrodotoxin (1'1'X, 5 x 10" M) depressed AP amplitude and dV/dtm., more in epicardium than in endocardium. TTX had only a small effect on the upstroke of PF AP, but shortened AP duration by 30-40%. Verapamil (2 x 10~ M) had equivalent effects on endocardium and epicardium. These data indicate that excitability is more easily depressed by a combination of hyperkalemia, hypoxia, and acidosis, and by TTX, in epicardium and papillary muscle tip than in endocardium and PF. Responsiveness of endocardial muscle during exposure to altered Tyrode's solution or TTX may be enhanced by contact with PF. drc Res 46: 814-825, 1980

Effects of tetrodotoxin, lidocaine, verapamil, and AHR-2666 on Ouabain-induced delayed afterdepolarizations in canine Purkinje fibers.

Abstract: We used standard microelectrode techniques to record delayed afterdepolarizations (DAD) induced by ouabain (2 x 10−7 M) in isolated canine Purkinje fibers (PF) and studied the response of DAD to the fast Na+ channel blocker, tetrodotoxin (TTX, 1 mg/liter); the slow channel blocker, verapamil (verap, 1 mg/liter); the putative Ca2+ blocker, AHR-2666 (AHR, 45 mg/liter); and lidocaine (lido, 4 mg/liter), which increases steady state outward current and decreases background inward current. PF were driven at cycle lengths of 1000-200 msec. Ouabain superfusion for 30 minutes induced DAD with amplitudes of 17.0 ± 1.5 (mean ± SE) mV at a cycle length of 200 msec. TTX, verap, AHR, and lido all depressed DAD amplitude (P < 0.05). To intercompare the effects of the drugs, graphs were constructed relating DAD amplitude to basic cycle length, and the relative magnitude of effects of the drugs on DAD amplitude at all cycle lengths was tested using a nested analysis of variance. The effects of verap and AHR were equivalent, and both decreased DAD amplitude more at short (to 37% of ouabain control) than at long (to 76%) cycle lengths (P< 0.05). Lido had a different effect and decreased DAD nearly equivalently at short (to 64%) and long (to 75%) cycle lengths. The actions of TTX were intermediate between-and significantly different from-those of the other drugs (P < 0.05). AHR and verap appear to act similarly, by modifying the current responsible for DAD, whereas lido appears to act by a different mechanism, perhaps by increasing steady state outward current. The actions of TTX may be a result of its effect on the transient inward current or on a background current carried by Na+. CircRes 46: 117-124, 1980

Ionic currents during hypoxia in voltage-clamped cat ventricular muscle.

Abstract: To explore the mechanisms underlying the shortening of the cardiac action potential in hypoxia, we studied the effect of hypoxia on the ionic currents in cat papillary and trabecular muscles using the single sucrose gap-voltage clamp technique. For potentials positive to -70 mV, hypoxia induces an increase in time-independent outward current. The changes in the tail current suggest that time-dependent outward current is not increased but, rather, reduced. Because the time course of ik remains unchanged, we concluded that the shortening of the action potential is not a result of a change in the time-dependent outward current. In the potential range of the plateau, the amplitude of the slow inward current is not affected by hpoxia. Its time constant of inactivation appears slightly decreased. The prolongation of the action potential by epinephrine during hypoxia is accompanied by an increase in the slow inward current. As a result of these studies, we conclude that the shortening of the cardiac action potential in the early stage of hypoxia results from an increase in K+ outward background current.

Nonlinearities of the human carotid baroreceptor-cardiac reflex.

Abstract: Carotid baroreceptors of nine healthy young men and women were stretched or compressed with neck suction or pressure, before and after beta-adrenergic and cholinergic blockade, to evaluate several nonlinearities of sinus node baroreflex responses. Sinus node inhibition was related linearly to the intensity of brief baroreceptor stimuli over a range extending from carotid-distending pressures of about 101 +/- 5 (mean +/- SE) to 160 +/- 6 mm Hg (the subject's average systolic pressure was 108 +/- 2 mm Hg). Sinus node response to sustained (5 seconds) neck suction or pressure were strikingly asymmetrical. Responses were abolished by atropine, or by atropine and propranolol. Propranolol alone augmented sinus node responses to both neck suction and pressure. These results suggest that, in normal human subjects, sinus node responses to abrupt alterations of afferent baroreceptor traffic are nonlinear and are mediated by fluctuations of efferent cholinergic activity. Most of the observed nonlinear behavior of the integrated reflex can be explained on the basis of known properties of afferent and central portions of the baroreflex arc.

Evidence and quantification of the vasopressin arterial pressure control system in the dog.

Abstract: We determined quantitatively the importance of vasopressin (AVP) release in the regulation of arterial pressure during hemorrhagic hypotension in dogs. The recovery of arterial pressure after rapid hemorrhage was studied in 10 dogs with spinal cord destruction below the level of C-l to remove efferent sympathetic nerve activity. Bilateral nephrectomy was used to remove the reninangiotensin system. Results for this group were compared to those for another group of dogs in which the entire central nervous system, including the pituitary gland, were removed surgically. In all groups, arterial pressure was lowered rapidly from an average control value of 106 ± 2 (mean ± SE) to 51 ± 1 mm Hg by hemorrhage. In dogs with spinal cord destruction and bilateral nephrectomy, arterial pressure rose from 51 mm Hg to 89 ± 3 mm Hg in 3 minutes and stabilized at that level over the next 30 minutes, representing a 71% compensation of arterial pressure. Left atrial pressure fell from 3.3 to 0.8 mm Hg during hemorrhage and subsequently rose only 12% during the hemorrhage. Plasma AVP rose during hemorrhage from 19 ± 2 to 75 ± 10 μU/ml. Injection of the competitive AVP inhibitors, [1-deaminopenicillamine, 4-valine]-8-D-arginine-vasopressin (dPVDAVP), completely reversed the effects and returned the compensated pressure to 50 mm Hg. In the complete absence of the central nervous system, arterial pressure compensation averaged only 10 ± 3%. Plasma osmolality, sodium and potassium concentrations, hematocrit, and heart rate were unchanged in all experimental groups. The relationships involved in the AVP-arterial pressure control system were determined quantitatively and analyzed. A systems analysis using experimentally determined values for AVP secretion and metabolism, plasma AVP, and changes in arterial pressure closely predicted the changes observed during hemorrhage. It appears from this study that AVP has potent systemic vasoconstrictor actions enabling it to make a significant contribution in the restoration of arterial pressure during hemorrhage. Circ Res 46: 58-67, 1980

Characteristics of action potentials of hypertrophied myocardium from rats with renal hypertension.

Abstract: We investigated the electrophysiological effects of cardiac hypertrophy induced by renal hypertension in rats by comparing transmembrane action potentials (AP) recorded from the papillary muscles of hypertensive (HBP) and normal (SHAM) rats. No significant difference was found between HBP and SHAM AP with regard to resting membrane potential (RMP), action potential amplitude (AMP), overshoot (OS), or maximum rate of rise of the upstroke. In contrast, the duration of 50% (APD50) and 75% (APD75) of repolarization to the RMP was significantly and consistently longer for HBP AP than for SHAM AP. The mechanism for prolonged HBP AP was investigated by changing extracellular fluid composition and by use of ion channel blockers. The responses of HBP and SHAM AP to various treatments differed in a quantitative rather than qualitative fashion. Exposure to C2+-containing or low-Na+ Tyrode's solution produced a differentially greater decrease in APD50 and APD75 in HBP AP than in SHAM AP. Treatment with D600 also produced differential shortening of HBP AP, but its effect was limited to APD50. In contrast, exposure to Sr2+-containing and TEA-containing Tyrode's solution produced an increase in APD50 and APD75, but the lengthening effect was not differentially greater in HBP than in SHAM AP. Treatment with Ca2+-free Tyrode's solution had little effect on APD in either HBP and SHAM rats. None of the treatments had a significant differential action on RMP or AMP in HBP AP as compared to SHAM AP. Our results show that AP prolongation is a specific and consistent feature of hypertrophied myocardium and that the changes responsible for prolonged HBP AP are quantitative rather than qualitative in nature. The specific differential effects of high Ca2+ concentration ([Ca2+]o) and low Na+ concentration ([Na*]o) on the duration of HBP AP indicate that the membrane sensitivity to these ions is altered in hypertrophied myocardium and that one possible explanation for prolonged HBP AP is slowed inactivation of a Ca2+-inactivated inward current. Circ Res 47: 443-454, 1980