Showing papers in "British Journal of Pharmacology in 1983"

The dissociative anaesthetics, ketamine and phencyclidine, selectively reduce excitation of central mammalian neurones by N-methyl-aspartate

Abstract: 1The interaction of two dissociative anaesthetics, ketamine and phencyclidine, with the responses of spinal neurones to the electrophoretic administration of amino acids and acetylcholine was studied in decerebrate or pentobarbitone-anaesthetized cats and rats. 2Both ketamine and phencyclidine selectively blocked excitation by N-methyl-aspartate (NMA) with little effect on excitation by quisqualate and kainate. 3Ketamine reduced responses to L-aspartate somewhat more than those of l-glutamate; the sensitivity of responses to these two putative transmitters was between that to NMA on one hand and that to quisqualate or kainate on the other. 4On Renshaw cells, ketamine and phencyclidine reduced responses to acetylcholine less than those to NMA but more than those to quisqualate or kainate. Dorsal root-evoked synaptic excitation of Renshaw cells was reduced to a greater extent than that following ventral root excitation. 5Intravenous ketamine, 2.5–20 mg/kg, and phencyclidine, 0.2–0.5 mg/kg, also selectively blocked excitation of neurones by NMA. 6Ketamine showed no consistent or selective effect on inhibition of spinal neurones by electrophoretically administered glycine or γ-aminobutyricacid (GABA). 7The results suggest that reduction of synaptic excitation mediated via NMA receptors contributes to the anaesthetic/analgesic properties of these two dissociative anaesthetics.

The effects of nicotine on locomotor activity in non-tolerant and tolerant rats.

Abstract: 1--Rats were tested for locomotor activity in photocell cages, for 80 min starting immediately after subcutaneous injection of (-)-nicotine bitartrate or 0.9% w/v NaCl solution (saline). In non-tolerant subjects, nicotine (0.1 to 0.4 mg/kg base) depressed activity and induced ataxia in the first 20 min, but increased activity later in the session; these actions were dose-dependent. 2--Tolerance was studied by comparing rats given nicotine (0.4 mg/kg s.c.) every day with control rats given saline instead. Each week, every subject was tested once with nicotine (0.4 mg/kg) and once with saline. With daily or even weekly injections of nicotine, the initial depressant action of the drug was replaced by a dose-dependent stimulant action which occurred throughout the session. In these tolerant animals, little ataxia was seen except when a larger dose of 0.8 mg/kg was given. Tolerance to the depressant action of nicotine persisted for at least 3 weeks. 3--In non-tolerant subjects, mecamylamine (0.5, 1.0 mg/kg s.c.) prevented the initial depressant action of nicotine (0.4 mg/kg). In tolerant rats, the locomotor stimulant action of nicotine (0.4 mg/kg) was prevented by mecamylamine (0.1, 0.32, 1.0 mg/kg s.c.) in a dose-related way; the quaternary ganglion blocker, hexamethonium (0.2, 1.0, 5.0 mg/kg s.c.) had little or no such effect. Neither mecamylamine nor hexamethonium altered activity when given alone. 4--It is suggested that a few treatments with nicotine can unmask a stimulant action of the drug, probably of central origin, which possibly reflects a stimulation of nicotine receptors.

Studies on the antinociceptive action of α-agonist drugs and their interactions with opioid mechanisms

Abstract: 1 A modified abdominal constriction test, whereby the drugs used are injected intraperitoneally when the writhing response is maximal, has been used to study the antinociceptive activity of various sympathomimetic drugs. Of those tested, clonidine was the most potent, with an ID50 value in the nanomolar range. (-)-Isoprenaline, (-)-adrenaline and (-)-noradrenaline were only a little less potent. Phenylephrine, the least potent, had only about one-sixtieth of the activity of clonidine. 2 The antinociceptive action appears to occur within the peritoneum, since it was apparent almost immediately after the drugs were injected and was produced by doses far smaller than were effective by the subcutaneous route. 3 α-Adrenoceptors appear to be involved in the reaction, since noradrenaline showed stereospecificity, and the α-adrenoceptor antagonists phentolamine and piperoxan both shifted the dose-response curves of the α-adrenoceptor agonist drugs to the right, usually parallel to the control curves. 4 The high antinociceptive potency of clonidine and oxymetazoline, indicate the importance of α2-adrenoceptors and this was supported by the finding that piperoxan was a more effective antagonist than phentolamine. The moderate potency of phenylephrine suggests that α1-adrenoceptors may also be involved, although the selective α1-antagonist, prazosin, did not antagonize noradrenaline and had antinociceptive activity of its own. 5 β-Adrenoceptors also appear to be involved in the antinociceptive response, since propranalol antagonized the effect of isoprenaline, but not that of clonidine. 6 Piperoxan was a very effective antagonist of morphine, while phentolamine had a weaker action. Naloxone had little action against the α-adrenoceptor agonists. 7 Mice pretreated with clonidine or oxymetazoline but not noradrenaline showed a very great cross-tolerance to morphine. Morphine pretreatment caused marked desensitization of itself, but little cross-tolerance to clonidine or oxymetazoline. 8 It is suggested that sensory nerves in the mouse peritoneum have α2- and β-adrenoceptors on their terminals, and possibly α1-receptors also. It is possible that when activated by the appropriate agonists they depress the generation of pain impulses. There is an interaction between the α-adrenoceptors and opioid receptors in the mouse peritoneum.

Studies on RX 781094: a selective, potent and specific antagonist of α2-adrenoceptors

Abstract: 1 The selectivity and specificity of RX 781094 [2-(2-(1,4 benzodioxanyl))2-imidazoline HCl] for alpha-adrenoceptors have been examined in peripheral tissues. 2 In isolated tissue experiments RX 781094 was a competitive antagonist at prejunctional alpha 2-adrenoceptors situated on the sympathetic nerve terminals of the rat (pA2 = 8.56) and mouse (pA2 = 7.93) vas deferens and on the parasympathetic nerve terminals of the guinea-pig ileum (pA2 = 8.55). 3 Although RX 781094 was also a competitive antagonist at the postjunctional alpha 1-adrenoceptors of the rat anococcygeus muscle (pA2 = 6.10) its affinity for these receptors was markedly less than that displayed for prejunctional sites. From pA2 values obtained in the rat vas deferens and anococcygeus muscle the calculated alpha 2/alpha 1-adrenoceptor selectivity ratio for RX 781094 was 288. 4 The rank order of alpha 2/alpha 1-adrenoceptor selectivities for the antagonists studied was RX 781094 greater than RS 21361 greater than yohimbine greater than piperoxan greater than phentolamine greater than WB 4101 greater than prazosin. 5 RX 781094 had extremely low affinity for beta-adrenoceptors, histamine receptors, cholinoceptors, 5-hydroxytryptamine and opiate receptors in vitro. 6 In pithed rats, intravenous administration of RX 781094 antagonized the prejunctional alpha 2-adrenoceptor agonist effects of clonidine and guanabenz on electrically-induced contractions of the vas deferens and anococcygeus muscle respectively. 7 In the vas deferens the rank order of alpha 2-adrenoceptor antagonist potencies was RX 781094 greater than phentolamine greater than piperoxan greater than yohimbine greater than RS 21361 greater than WB 4101. Only RX 781094, yohimbine and RS 21361 were active against guanabenz in the anococcygeus muscle. 8 In the pithed rat, RX 781094 preferentially antagonized the pressor responses evoked by postjunctional alpha 2-adrenoceptor activation by UK 14,304 although higher doses also inhibited the effects of phenylephrine and cirazoline at postjunctional alpha 1-adrenoceptors. 9 RX 781094 had little effect on the cardiovascular responses to 5-hydroxytryptramine, angiotensin II, histamine, acetylcholine and isoprenaline in pithed rats and rats anaesthetized with pentobarbitone. 10 These results demonstrate that RX 781094 is a potent and selective alpha 2-adrenoceptor antagonist with a high degree of specificity for these receptors.

Responses of human skin to intradermal injection of leukotrienes C4, D4 and B4

Abstract: The ability of intradermally injected leukotrienes C4 (LTC4), LTD4 and LTB4 to produce inflammatory changes in human skin alone and in combination with prostaglandin E2 (PGE2) has been investigated. LTC4 and D4 (0.012-0.38 nmol) caused dose-related erythema and wealing. No evidence of synergism between PGE2 and LTC4 or LTD4 was detected, although only single dose combinations were studied. LTB4 (0.15-1.5 nmol) caused areas of induration which persisted for more than 4 h and which showed perivascular neutrophil infiltrates on histological examination. Only slight synergism between PGE2 and LTB4 was found. It was concluded that these pro-inflammatory properties of LTC4, LTD4 and LTB4 are consistent with their proposed roles as mediators of inflammation in the skin and other tissues.

Characterization of the locomotor stimulant action of nicotine in tolerant rats

Abstract: Tests of locomotor activity (photocell cages) were used to investigate the development of tolerance to nicotine in rats. Repeated exposure to the apparatus did not influence the rate at which tolerance was acquired. Comparisons of (+)-nicotine (0.4-1.6 mg kg-1, s.c.) and (-)-nicotine (0.1-0.4 mg kg-1, s.c.) in tolerant rats showed that the (-)-isomer was at least ten times more potent in stimulating motor activity. Subcutaneous pretreatment with mecamylamine (1.0 mg kg-1) completely prevented the locomotor stimulant action of nicotine in tolerant rats, whereas chlorisondamine (0.01 or 0.1 mg kg-1 s.c.) only partially reduced it. When mecamylamine was given after an injection of nicotine, the locomotor stimulant action of nicotine was blocked, and nicotine actually reduced activity. A single intraventricular dose of chlorisondamine (2 micrograms) blocked the stimulant actions of nicotine for the duration of the experiment (23-24 days).

GABAA and GABAB receptor-mediated effects in guinea-pig ileum.

Abstract: 1 The effects of γ-aminobutyric acid (GABA) and related substances were examined in guinea-pig ileum longitudinal muscle. 2 GABA at doses ranging from 10-7 M to 3 × 10-6 M elicited a relaxation while at higher doses (3 × 10-6 M — 10-4 M), as previously described, it caused a contraction followed by relaxation. 3 GABA-induced relaxation was bicuculline-insensitive, was mimicked by (-)-baclofen but not by homotaurine and muscimol. The effect of baclofen was stereospecific. GABA- and (-)-baclofen-induced relaxations were dose-dependent and their ED50 values were similar. A specific cross-desensitization occurred between GABA and (-)-baclofen. 4 The bicuculline-insensitive relaxation induced by GABA and (-)-baclofen was prevented by tetrodotoxin and hyoscine but not by phentolamine plus propranolol, naloxone or theophylline. 5 In preparations in which the muscle tone was raised by histamine or prostaglandin F2α, GABA and (-)-baclofen induced relaxation to the same extent as before increasing the tone. If the tone was raised by DMPP, a greater bicuculline-insensitive relaxation occurred. 6 Contraction caused by GABA was bicuculline-sensitive and was mimicked by homotaurine and muscimol. Contraction was dose-dependent and muscimol was about three times more potent than GABA or homotaurine. A specific cross-desensitization occurred between the contractile effects of GABA and those of homotaurine or muscimol. 7 Bicuculline competitively antagonized the contractile effects of GABA, homotaurine and muscimol and gave closely similar pA2 values. The slope of the Schild plot for the above drugs was near 1, confirming the competitive nature of the antagonism. 8 The bicuculline-sensitive contraction induced by GABA, homotaurine and muscimol was abolished by tetrodotoxin and was non-competitively antagonized by hyoscine, while it was unaffected by hexamethonium, mepyramine and methysergide. 9 It is concluded that two receptors mediate the GABA effects in guinea-pig ileum: a bicuculline-sensitive GABAA receptor, which elicits contraction through an excitatory action on cholinergic post-ganglionic neurones; and a bicuculline-insensitive GABAB receptor which causes relaxation through an inhibitory presynaptic action on cholinergic post-ganglionic neurones. We confirm that GABA, homotaurine and muscimol are GABAA agonists, while GABA and (-)-baclofen are GABAB agonists.

Endothelium-dependent relaxation of the pig aorta: relationship to stimulation of 86Rb efflux from isolated endothelial cells.

Abstract: Bradykinin, adenosine triphosphate (ATP) and acetylcholine each relaxed histamine-contracted strips of pig aorta in a dose-dependent manner. These relaxations were abolished when the endothelium was removed. Relaxation induced by ATP was mimicked by adenosine diphosphate (ADP) but adenosine monophosphate (AMP) and adenosine were about 120 times less potent. Relaxation induced by acetylcholine was antagonized by atropine in a competitive manner, and carbachol induced the same degree of relaxation as acetylcholine, but was about 10 times less potent. The calcium ionophore, A23187, also induced a dose-dependent relaxation of pig aortic strips provided the endothelium was present, suggesting that a rise in the level of ionized calcium within the endothelial cells is one means by which vascular smooth muscle relaxation can be triggered. Bradykinin, ATP, ADP, AMP, adenosine and A23187 each induced a dose-dependent increase in 86Rb efflux from preloaded pig aortic endothelial cells. The dose-response curves for stimulation of 86Rb efflux and for endothelium-dependent relaxation were similar for each individual compound. ADP was equipotent with ATP, but AMP and adenosine were about 120 times less potent. Neither acetylcholine nor carbachol, in concentrations that induce endothelium-dependent relaxation, had any effect on 86Rb efflux from isolated aortic endothelial cells. Lanthanum, which blocks calcium influx, abolished the increases in 86Rb efflux induced by bradykinin and ATP, and the calcium ionophore A23187 was the most effective stimulant of 86Rb efflux, suggesting that the potassium transport induced by these agents is calcium-activated. It is concluded that endothelial responses to bradykinin and ATP can be assessed by monitoring 86Rb efflux, which probably reflects a calcium-activated efflux of potassium associated with the endothelium-dependent vascular relaxation induced by these agents. This pathway is apparently not involved in endothelial responses to acetylcholine.

Vascular reactivity in diabetes mellitus: role of the endothelial cell.

Abstract: The response to vasoactive agents of microvessels in situ and large arteries in vitro was compared in normal and alloxan-diabetic rats. Noradrenaline was equally effective in evoking a constrictor response of mesenteric microvessels in normal and diabetic animals. The constrictor response to a standard amount of noradrenaline in such vessels was fully antagonized by acetylcholine or papaverine, the minimum effective doses being equivalent in normal and diabetic animals. In contrast, the minimum doses of histamine or bradykinin, effective in normal animals, had to be increased about 20 fold to be active in diabetic animals. Increased osmolarity of extracellular fluid caused a significant and equivalent increase in latency of the vasoconstrictor response of microvessels to noradrenaline in normal and diabetic animals. Concentration-effect curves, constructed from the response of isolated aortae to noradrenaline, were similar in normal and diabetic animals, provided the endothelium was removed. Diabetes only affected preparations in which the endothelium was left intact. In these, the median effective concentrations of noradrenaline were greatly increased in comparison with normal values. Precontracted aortae from normal and diabetic animals were equally relaxed by acetylcholine and histamine, provided the endothelium was left intact. Loss of the relaxant response of the preparations in all groups of animals was observed following removal of endothelial cells. It is suggested that different mechanisms may be involved in the effects of vasodilator agents on large arteries in vitro or small vessels in situ. Histamine and bradykinin which are potent permeability-increasing factors, may antagonize the vasoconstrictor response of microvessels to noradrenaline through an action on endothelial cells with increased vascular permeability and temporary changes in composition of extracellular fluid. The reactive process of endothelial cells to permeability factors was affected by diabetes mellitus. However, the response of microvessels to acetylcholine and papaverine which are devoid of permeability-increasing properties, was not influenced by diabetes.

Compared myocardial and vascular effects of captopril and dihydralazine during hypertension development in spontaneously hypertensive rats

Abstract: When administered to young spontaneously hypertensive rats (SHRs), dihydralazine (25 mg kg-1, daily) and captopril (100 mg kg-1, daily) prevent with the same efficacy genetic hypertension development (GHD). Dihydralazine treatment increased vascular mesenteric compliance, as shown by a significant decrease in the stiffness of the vessels (-27%), and induced slight reductions in contractility (-12%) and in wall to lumen (W/L) ratio (-15%). After treatment withdrawal, all these parameters returned to control values within 7 weeks, as did blood pressure. Captopril treatment also strongly increased the mesenteric vessels compliance, vessel stiffness being decreased by 16%, and reduced their contractility (-15%) and their W/L ratio (-30%). These effects as well as those exerted on blood pressure persisted up to 7 weeks after treatment ceased although there was a slight trend to a progressive reduction in the intensity of both phenomena. These experiments show that captopril but not dihydralazine has a long-lasting effect in opposing the functional and morphological vascular alterations occurring during GHD in SHRs and this phenomenon probably contributes to a large extent to the sustained preventive effects of the drug against GHD.

Characterization and partial purification of 'renocortins': two polypeptides formed in renal cells causing the anti-phospholipase-like action of glucocorticoids.

Abstract: 1 Anti-inflammatory steroids reduce prostaglandin E2 (PGE2) synthesis in rat renomedullary interstitial cells in culture by inhibiting the release of arachidonic acid from membranous phospholipid stores, exhibiting antiphospholipase-like properties. 2 After treatment of the cells with dexamethasone 10(-6)M, these cells release a protein in the supernatant. 3 This supernatant is able to inhibit PGE2 secretion in untreated cells and to inhibit phospholipase A2 activity in an in vitro system. 4 Using chromatofocusing separation, we showed that two distinct proteins exist with isoelectric points of 5.8 and 8.3. 5 Using gel permeation separation, we showed that two proteins exist with apparent molecular weights of 15,000 and 30,000 daltons. 6 We conclude that, in renal cells in culture, anti-inflammatory steroids induce the synthesis and the release of two polypeptides which we have named 'Renocortins' (induced by corticoids in renal cells) causing the antiphospholipase-like action of glucocorticoids. 7 Our results are in good agreement with others, but as renal cells are not directly involved in the inflammatory process, we suggest that this steroid-induced phenomenon is not solely involved in the inflammatory reaction but is of more general physiological relevance.

Calcium antagonist and the peripheral circulation: differences and similarities between PY 108-068, nicardipine, verapamil and diltiazem.

Abstract: 1 The effects of two dihydropyridines, PY 108–068 (PY) and nicardipine (N), and two other calcium antagonists, verapamil (V) and diltiazem (D), on regional blood flow were measured in open-chest cats, anaesthetized with chloralose-urethane. 2 Each substance was infused at 3 different dose rates, each for 10 min. The total doses given were 5 plus 10 plus 35 (total of 50) μg/kg for PY, 10 plus 20 plus 70 (total of 100) μg/kg for N and 100 plus 200 plus 700 (total of 1000) μg/kg for V and D. 3 All substances lowered blood pressure and increased total peripheral conductance. Heart rate was lowered only by V, D and PY. Cardiac output was markedly increased only by the diyhydroypridine derivatives; D had small and V almost no effects. 4 All substances increased coronary flow and redistributed it in favour of the subepicardial layer. All substances also increased blood flow to the brain. The effects of verapamil were comparatively small. 5 Skeletal muscle flow was increased strongly by the two dihydropyridine derivatives. D and V had negligible effects. 6 Blood flow to stomach and small intestine was only slightly increased. Flow to the kidneys increased slightly in diltiazem-treated animals but did not change with all other treatments. Flow to the liver, the adrenals, and the spleen remained unchanged or showed a tendency to decrease. 7 The organ conductances which reflect the active changes in vascular tone better than blood flow values, showed that there was a tendency towards vasodilatation even in most organs where blood flow tended to decrease. 8 Results obtained in an earlier series of experiments with nifedipine were very similar to those described here for N, except that nifedipine was about twice as potent. 9 Calcium antagonists were thus neither general peripheral vasodilators nor did they show a uniform pattern of preferential sites of action. The most important common features were increases in coronary and cerebral blood flow and the most important differences the divergent effects of the dihydropyridines on one side and V and D on the other side on skeletal muscle flow. The size of this vascular bed may help to explain why dihydropyridines appear to be particularly potent as peripheral vasodilators.

The source of thromboxane and prostaglandins in experimental inflammation

Abstract: Although cyclo-oxygenase products have been detected at inflammatory sites the tissue of origin remains uncertain. Inflammatory exudates were collected from rats 4, 6, 8, 12 or 24 h after subcutaneous implantation of carrageenin-impregnated sponges. Concentrations of the cyclo-oxygenase products prostaglandin E2 (PGE2), 6-oxo-PGF1 alpha and thromboxane B2 (TXB2) in inflammatory exudates and serum (obtained from blood clotted at 37 degrees C) were measured by specific radioimmunoassays. TXB2 concentrations in exudates increased to about 100 ng ml-1 at 8 h but decreased to less than 20 ng ml-1 after 24 h. PGE2 concentrations increased from 4-12 h and remained between 80 and 120 ng ml-1 from 12-24 h. 6-oxo-PGF1 alpha had the same time course as that of PGE2 but concentrations were approximately one third of PGE2 values. TXB2 concentrations in serum from thrombocytopaenic rats were less than 5% of control values. Thrombocytopaenia did not affect TXB2, PGE2 or 6-oxo-PGF1 alpha concentrations or total leukocyte numbers in inflammatory exudates. Methotrexate-induced neutropaenia did not affect serum TXB2 concentrations but cyclo-oxygenase products (including TXB2) in 6 h inflammatory exudates were reduced by 60-95%. Colchicine (1.0 mg kg-1 s.c.) prevented leukocyte accumulation in sponge exudates and this was accompanied by a reduction in TXB2, PGE2 and 6-oxo-PGF1 alpha concentrations at 6 h. These results indicate that platelets are the source of TXB2 in clotting blood but do not contribute to cyclo-oxygenase activity in experimental inflammation. The results also suggest that migrating leukocytes are the major source of thromboxane and to a lesser degree prostaglandins in acute 6 h inflammatory exudates.

Separation of adrenergic and non-adrenergic contractions to field stimulation in the rat vas deferens

Abstract: Adrenergic and 'non-adrenergic' nerve-induced contractions in rat vas deferens were separated pharmacologically. Responses to single stimuli comprised two components, an alpha-noradrenergic component (IIs), dominant in the epididymal portion, and a 'non-adrenergic' component (Is), dominant in the prostatic portion. Is but not IIs was blocked by nifedipine. A combination of adrenergic blockade and nifedipine virtually abolished all components. After cocaine, a third component (IIIs) emerged which was abolished by either adrenergic blockade or nifedipine. The response to trains of stimuli consisted of 'twitch' and 'secondary' components. This biphasic time course was modified by adrenergic blockade or nifedipine to reveal the time course of the 'non-adrenergic' and adrenergic components, respectively: these did not correspond to the 'twitch' and 'secondary' components. A combination of adrenergic blockade and nifedipine virtually abolished the whole response. Prejunctional alpha 2-adrenoceptor-mediated inhibition of the contractile responses could be blocked by selective alpha 2-adrenoceptor antagonists. The adrenergic contractile response demonstrated this 'feed-back' even on the second pulse at 0.5 Hz. Endogenous inhibition of the 'non-adrenergic' contraction required higher frequencies or enhancement of the extracellular concentration of noradrenaline by blockade of its neuronal uptake. Contractile responses to exogenous noradrenaline were abolished by nifedipine, at a concentration that did not affect the adrenergic (IIs) neurotransmission. These results reinforce the view that part of the motor transmission in rat vas deferens is non-adrenergic and allow the disentanglement of the various postjunctional and prejunctional elements contributing to the complex response to a train of stimuli.

Characterization of adenosine receptors in isolated cerebral arteries of cat.

Abstract: The effect of some adenosine analogues and xanthine derivatives were studied on isolated cerebral arteries from cats. The adenosine analogues caused an almost complete relaxation of cerebral arteries contracted by prostaglandin F2 alpha (PGF2 alpha, 30 microM). The order of potency was: 5-N-ethylcarboxamide adenosine (NECA) greater than 2-chloroadenosine greater than adenosine greater than L-N6-phenylisopropyl adenosine (L-PIA). The analogue D-PIA was very weak and its maximum effect was small. NECA and L-PIA enhanced [3H]-cyclic AMP accumulation in [3H]-adenine labelled feline pial vessels with similar absolute and relative potency to their relaxant effects. The relaxant effects of adenosine and of NECA were competitively antagonized by 8-phenyl-theophylline (pA2 = 6.5). The effect of theophylline and enprofylline could not be tested in higher concentrations than 30 or 10 microM because they affected the vessels directly. At these concentrations they were essentially inactive as adenosine antagonists. The non-xanthine phosphodiesterase inhibitor rolipram (0.1 and 100 microM) caused a slight but non-significant potentiation of the relaxant effect of adenosine. The results are compatible with the opinion that adenosine relaxes cerebral vessels by an action on adenosine A2-receptors. The effect may be linked to adenylate cyclase and can be antagonized by 8-phenyl-theophylline.

Evidence for an Ax-adenosine receptor in the guinea-pig atrium

Abstract: 1 The purpose of this study was to determine whether the adenosine receptor that mediates a decrease in the force of contraction of the guinea-pig atrium is of the A1- or A2-sub-type. 2 Concentration-response curves to adenosine and a number of 5'- and N6-substituted analogues were constructed and the order of potency of the purines was: 5'-N-cyclopropylcarboxamide adenosine (NCPCA) = 5'-N-ethylcarboxamide adenosine (NECA) greater than N6cyclohexyladenosine (CHA) greater than L-N6-phenylisopropyl adenosine (L-PIA) = 2-chloroadenosine- greater than adenosine greater than D-N6-phenylisopropyl adenosine (D-PIA). 3 The difference in potency between the stereoisomers D- and L-PIA was over 100 fold. 4 The adenosine transport inhibitor, dipyridamole, potentiated submaximal responses to adenosine but had no significant effect on those evoked by the other purines. 5 Theophylline antagonized responses evoked by all purines, and with D-PIA revealed a positive inotropic effect that was abolished by atenolol. 6 The results indicate the existence of an adenosine A1-receptor in the guinea-pig atrium.

Effect of steroids on β‐adrenoceptor‐mediated relaxation of pig bronchus

Abstract: 1--Progesterone, testosterone (40 microM), cortisol and cortisol hemisuccinate (80 microM) caused 6-8 fold potentiations of (+/-)-isoprenaline (Iso)-induced relaxations of pig bronchus while several other steroids caused smaller potentiations or had no effect. 2--17 beta-Oestradiol (40 microM) increased the potency of Iso, (-)-adrenaline (Adr) and (-)-noradrenaline (NA) by 10.6, 2.3 and 2.6 fold respectively but had no significant effect on the potency of fenoterol (Fen). 3--Inhibition of catechol-O-methyl transferase (COMT) with U-0521 (30 microM) caused a 6 fold increase in the potency of Iso but failed to alter the potency of Adr, NA or Fen. The extraneuronal uptake inhibitor normetanephrine (50 microM) caused significant 2 fold increases in the potency of Iso and Adr but did not potentiate the responses to NA or Fen. 4--In preparations where the potency of Iso had already been increased by U-0521 (30 microM) or by normetanephrine, 17 beta-oestradiol produced no significant further increase in potency. These results indicate that steroid-induced increases in the potency of catecholamines in pig bronchus can be explained in terms of inhibition of COMT or extraneuronal uptake or both.

The actions of Paf-acether (platelet-activating factor) on guinea-pig isolated heart preparations.

Abstract: Paf-acether (platelet-activating factor) is a phospholipid capable of stimulating platelets to release their granular contents and cause platelet aggregation. When Paf-acether was administered to isolated heart preparations from normal guinea-pigs there was a significant concentration-dependent reduction in coronary flow and contractile force. The high concentration of Paf-acether was equally effective in reducing these cardiac parameters in the presence of atropine. The non-acetylated Paf-acether analogue, 2-lyso Paf-acether, the enantiomer, and a closely related phospholipid 1, alpha-lysophosphatidylcholine palmitoyl, did not affect coronary flow and contractile force, indicating the specificity of Paf-acether. These data demonstrate a potent effect of Paf-acether on cardiac function. Whether or not these effects are direct or mediated through generation of endogenous mediators remains to be established.

Adenosine inhibits and potentiates IgE-dependent histamine release from human basophils by an A2-receptor mediated mechanism.

Abstract: Adenosine added to human basophils before anti-IgE challenge inhibited histamine release, whereas addition after challenge potentiated release. Peak responses for the two effects occurred 15 min before and after challenge respectively. The effects of adenosine on histamine secretion were dose-related over concentration ranges of 1-100 microM for inhibition and 0.01-1 microM for potentiation. The capacity of adenosine to inhibit and potentiate histamine secretion was inversely related to the strength of immunological challenge. The ability of theophylline (50 microM) to inhibit and dipyridamole (1 microM) to enhance slightly adenosine-induced responses, and the differing pharmacological effect of 2',5'-dideoxyadenosine suggested that adenosine's effects on basophil histamine secretion were mediated by stimulation of cell surface adenosine receptors. The order of potency of adenosine and its analogues L- and D- N6-phenylisopropyladenosine (PIA) and 5'-N-ethylcarboxamideadenosine (NECA) in inhibiting and potentiating IgE-dependent histamine release from basophils indicated that both responses were mediated by stimulation of the adenosine A2-receptor subtype. The capacity of adenosine to cause a transient increase of cyclic AMP levels in 40-70% basophil-enriched leucocytes confirmed the association between stimulation of A2-receptors and activation of adenylate cyclase.

Ligand binding to thromboxane receptors on human platelets: correlation with biological activity.

Abstract: The preparation of enantiomerically pure [3H]-15 (S) 9, 11-epoxymethano PGH2 (a thromboxane A2-like agonist) has enabled the binding of ligands to the thromboxane receptor of the human platelet to be studied. The binding of the radio-ligand to washed human platelets has 3 components. One component is not displaceable by 'cold' 9, 11-epoxymethano PGH2 and its concentration-binding plot is roughly linear. The other 2 components are displaceable and saturable, and the larger of the two, which is sensitive to the stereochemistry of the C15 secondary alcohol, appears to represent the thromboxane receptor. About 1700 15(S)9, 11-epoxymethano PGH2 molecules are specifically bound to a single platelet and 50% of this binding is achieved with a concentration of 75 nM. Displacement of [3H]-15(S)9, 11-epoxymethano PGH2 is effected by (a) TXA2 and PGH2 and a number of bicyclic stable analogues (e.g. 9,11-azo PGH2), all of which produce irreversible aggregation of human platelets; (b) analogues of PGF2 alpha with potent thromboxane-like activity (e.g. ICI 79939); (c) compounds with partial agonist activity on the platelet thromboxane system (e.g. CTA2); (d) Thromboxane/endoperoxide analogues which specifically antagonize thromboxane-like actions on the human platelet (e.g. PTA2 and EP 045). Displacement is not achieved with the natural prostaglandins PGE2, PGD2 and PGF2 alpha. Neither the thromboxane-synthetase inhibitor dazoxiben nor R(+)-trimethoquinol have high displacing activity. The correlation of radio-ligand displacement with the biological activity of the competing ligands is discussed in relation to the nature of the thromboxane receptor on the human platelet.

Opiate activation of potassium conductance inhibits calcium action potentials in rat locus coeruleus neurones.

Abstract: Opiates act on mu-receptors to increase the potassium conductance of rat locus coeruleus neurones Opiates also depress the rate of rise and peak amplitude of calcium action potentials in these cells The action of opiates on calcium action potentials was prevented by two procedures which blocked the opiate-induced potassium current, intracellular caesium and extracellular barium This indicates that the opiate reduction in calcium entry is secondary to an increased potassium current

Substance P inhibits the M-current in bullfrog sympathetic neurones.

Abstract: Substance P (SP, 2.5-10 microM) was applied by rapid bath perfusion to bullfrog lumbar sympathetic neurones in vitro, voltage-clamped through a single micro-electrode. In unclamped cells, SP produced a depolarization accompanied by an increase in apparent input resistance. Under voltage-clamp a voltage-dependent inward current was induced by SP, during which the time-dependent relaxations induced by square voltage commands were inhibited. It is concluded that SP inhibits the M-current (IM), a species of voltage-dependent K+-current, and that IM-inhibition was the primary cause of the inward current and membrane depolarization in the cells tested.

Responsiveness of platelets and coronary arteries from different species to synthetic thromboxane and prostaglandin endoperoxide analogues.

Abstract: 1--Platelet-rich plasma (PRP) from humans, cats, dogs (after addition of 10 microM adrenaline), rabbits and guinea-pigs aggregated in response to sodium arachidonate or 9,11-azo-prostaglandin H2, while PRP obtained from sheep was unresponsive to either agent. 2--The stable thromboxane (Tx) analogues, carbocyclic TxA2 (CTA2) and pinane TxA2 (PTA2) significantly inhibited these aggregatory responses in platelets from humans, dogs and guinea-pigs, while PTA2 but not CTA2 produced significant inhibition in cat platelets. The aggregatory response of PRP from rabbits was not significantly blocked by either analogue. 3--CTA2 and the endoperoxide analogue 9,11-methanoepoxy PGH2 (U-46619) constricted coronary arteries from cats, dogs, rabbits and guinea-pigs, while sheep vessels were unresponsive to either analogue. 4--Vasoconstrictor responses to U-46619 were significantly attenuated by PTA2 in vessels from all species. However, constriction produced by CTA2 was blocked significantly only in vessels from cats, dogs and guinea-pigs. 5--These results demonstrate the species differences which exist in the responsiveness of platelets and coronary arteries to thromboxane and endoperoxide analogues. Furthermore, the results illustrate the importance of species selection in the study of thromboxane antagonists for potential therapeutic use.

β-Adrenoceptors of the human myocardium: determination of β1 and β2 subtypes by radioligand binding

Abstract: beta-Adrenoceptors of the human myocardium were investigated with binding studies using 125iodocyanopindolol (ICYP) as ligand Inhibition of ICYP-binding by betaxolol (a selective beta 1-antagonist) and ICI 118551 (a selective beta 2-blocking drug) resulted in non-linear Scatchard-plots suggesting that both beta-adrenoceptor subtypes are present in human left atrium and left ventricle Computer analysis of the data gave a beta 1/beta 2-adrenoceptor ratio of approximately 65:35 both for left atrium and for left ventricle

Characterization of postsynaptic α‐adrenoceptors in rat aortic strips and portal veins

Abstract: Postsynaptic alpha-adrenoceptors in rat isolated aortic strips and portal veins have been examined using a number of agonist and antagonist drugs which have varying selectivity for alpha 1- and alpha 2-adrenoceptors. In both tissues (-)-noradrenaline [-)-NA), (-)-adrenaline [-) Adr) (-)-alpha-methyl noradrenaline [-)-alpha-Me-NA) and (-)-phenylephrine [-)-PE) were full agonists, while clonidine, oxymetazoline and (2-(2,6-dichlorophenyl)-5,6-dihydroimidazo(2,1,b) thiazole (44,549) were partial agonists. Guanfacine was a full agonist in aortic strips but only a partial agonist in portal veins. In aortic strips, pA2 values for prazosin and yohimbine were not significantly different using (-)-NA, (-)-PE or guanfacine as the agonist, suggesting a single population of alpha-adrenoceptors. The order of potency of the antagonists, prazosin = 2-(beta-(4-hydroxyphenyl)-ethylaminomethyl)-tetralone (BE2254) greater than phentolamine greater than yohimbine greater than rauwolscine, is indicative of an alpha 1-type of receptor. In portal veins, the order of potency of the antagonists was prazosin greater than BE2254 greater than phentolamine greater than yohimbine greater than rauwolscine, again indicating an alpha 1-type of receptor. The mean pA2 value for yohimbine was not significantly different in either tissue. However, mean pA2 values for prazosin, BE-2254 and phentolamine were approximately one order of magnitude lower in portal veins than in aortic strips, suggesting that the receptors in the two tissues may not be identical.

On the specificity and type of receptor involved in carotid body chemoreceptor activation by adenosine in the cat.

Abstract: Experiments were performed on cats anaesthetized with pentobarbitone in which carotid chemoreceptor activity was recorded from the peripheral end of a sectioned carotid sinus nerve. Intracarotid injections of adenosine 5'-triphosphate (ATP) (1-100 micrograms i.c.) caused a dose-related increase in chemosensory discharge which was delayed in onset. The adenosine uptake inhibitor dipyridamole potentiated the chemoexcitatory effects of injected adenosine and ATP. The stable ATP analogue alpha-beta-methylene ATP (10-100 micrograms i.c.) depressed chemoreceptor discharge, which suggests the presence of a P2-purinoceptor in the carotid body, and provides evidence that the chemoexcitatory effect of ATP results from its hydrolysis to adenosine 5'-phosphate (AMP)/adenosine. Adenine, inosine, guanosine, cytidine and uridine had no appreciable effect on chemoreceptor discharge. The adenosine R-site agonists 2'-chloroadenosine and N6-methyladenosine had chemoexcitatory effects which were similar to those of adenosine, whereas the P-site agonist 2'-deoxyadenosine had no appreciable effect on discharge. We conclude that the adenosine receptor in the cat carotid body has some of the characteristics of an R-site receptor according to the classification of Londos & Wolff (1977).

Characteristics of conditioned taste aversion produced by nicotine in rats

Abstract: 1 Nicotine produced conditioned taste aversions in rats which were directly related to the dose of nicotine and to the number of conditioning trials. 2 The tobacco alkaloid (-)-nicotine was four to five times as potent as its stereoisomer, (+)-nicotine. 3 Mecamylamine but not hexamethonium blocked the development of taste aversions produced by nicotine. 4 Mecamylamine did not block the development of taste aversions produced by apomorphine. 5 Prolonged treatment with mecamylamine prior to conditioning did not produce supersensitivity to nicotine.

Adventures and excursions in bioassay: the stepping stones to prostacyclin

Abstract: Physiology has spawned many biological sciences, amongst them my own field of pharmacology. No man has made a more important contribution to the fields of physiology and pharmacology than Sir Henry Dale (1875-1968, Nobel Laureate in Physiology or Medicine in 1936). Dale had a great influence not only on British pharmacology in general but also on my own scientific endeavours. Indeed, I can put forward a strong case for considering myself as one of Dale's scientific grandchildren. My early days as a pharmacologist were influenced not only by Dale himself but also by his own school of colleagues, including Burn, Gaddum and von Euler. It was Burn who taught me the principles and practice of bioassay. Some of Gaddum's first publications were on the development of specific and sensitive methods for biological assay and he maintained a deep interest in this subject for the rest of his life (1). In 1964 he said 'the pharmacologist has been a \"jack of all trades\" borrowing from physiology, biochemistry, pathology, microbiology and statistics but he has developed one technique of his own, and that is the technique of bioassay' (2). Expensive, powerful and sophisticated chemical methods, such as gas chromatography and mass spectrometry, have been developed and perfected for detection and quantification of prostaglandins (PGs) and related substances. One should not forget, however, that starting with the discovery and isolation of prostaglandins by von Euler (3) (see also Bergstrom (4)), biological techniques and bioassay have contributed very substantially to the development of the field. Bioassay has provided crucial information on the role of the lungs in the removal of circulating prostaglandins (5), the participation of prostaglandins in inflammatory reactions (6, 7), the contribution of prostaglandins to the autoregulation and maintenance of blood flow to the kidney (8-10), the inhibitory effect of aspirin-like drugs on the biosynthesis of prostaglandins (11-13), the mediation of pyrogen fever by prostaglandins (14), and the release of rabbit aorta-contracting substance (RCS; now identified as thromboxane A2, TXA2) from lungs during anaphylaxis (15, 16). Moreover in 1976, bioassay made possible the discovery of PGX, now renamed prostacyclin (PGI2), the latest member of the prostaglandin family (17-20). Indeed, it is doubtful whether the biological significance of any of the unstable products of arachidonic acid metabolism would have been recognized without bioassay techniques. With extraordinary simplicity and convenience, by its very nature, bioassay distinguishes between the important biologically active compounds and their closely related but biologically unimportant metabolites. In this review I shall discuss the development of the cascade superfusion bioassay technique and some of the discoveries and concepts which arose from its application, leading up to the discovery and development of prostacyclin. The effects of prostacyclin in man and its clinical assessment (another application of bioassay) will also be discussed.

Inflammatory actions of platelet activating factor (Paf-acether) in guinea-pig skin

Abstract: Cutaneous responses to synthetic platelet activating factor (Paf-acether) have been studied in guinea-pigs by means of radioisotopic marker techniques. Intradermal injection of Paf-acether elicited increased plasma protein extravasation (IPPE) (0.2-200 pmol/site), platelet accumulation (PA) (20-200 pmol/site) and red blood cell accumulation (RBCA) (200 pmol/site), whereas lyso-Paf (up to 2 nmol/site) was inactive in all these respects. Following intradermal injection, the IPPE responses to Paf-acether (2 and 20 pmol/site) were complete within 15 and 30 min respectively, although in response to 200 pmol/site, IPPE was detectable up to 1.5 h. The PA and RBCA responses to Paf-acether (200 pmol/site) were complete within 1 h. IPPE induced by Paf-acether (3 pmol/site) was potentiated by concomitant intradermal injection of a cutaneous vasodilator prostaglandin E2 (PGE2, 1 nmol/site) and inhibited by the beta-adrenoceptor agonist, isoprenaline (4.5 nmol/site) or the alpha-adrenoceptor agonist, phenylephrine (6 nmol/site). Such observations are consistent with Paf-acether effecting increased vessel wall permeability. Intradermal injection of PGE1 (3 nmol/site) significantly reduced PA in response to Paf-acether (200 pmol/site), whilst significantly enhancing IPPE. This dissociation of increased vascular permeability from PA is consistent with Paf-acether eliciting IPPE via a platelet-independent mechanism. These results indicate that a direct effect on vessel wall permeability contributes to the inflammatory response to Paf-acether in guinea-pig skin. It is suggested that Paf-acether is a potential mediator of allergy and inflammation.

Importance of physico-chemical properties in determining the kinetics of the effects of Class I antiarrhythmic drugs on maximum rate of depolarization in guinea-pig ventricle

Abstract: The effects of Class I antiarrhythmic drugs on the maximum rate of depolarization (Vmax) of guinea-pig ventricular action potentials were studied by standard microelectrode techniques. The ability of seven different drugs to depress Vmax in unstimulated tissue ('resting block') was found to correlate poorly with the lipophilicity (log P) of the compounds and only a little better with their molecular weights. Depression of Vmax in stimulated tissue was studied for 11 drugs and found, in all cases, to increase with stimulation frequency ('rate-dependent block'). The rapidity of onset of rate-dependent block (at approximately equipotent concentrations) varied markedly between drugs. It correlated well with molecular weight (r = 0.83; P less than 0.01). The time constant of recovery from rate-dependent block (tau re) also correlated very well with molecular weight (r = 0.94; P less than 0.001) for the seven drugs thus studied. A simplified model for the interaction of Class I drugs with the fast sodium channel is proposed in which the drugs all act as 'inactivation enhancers' (as suggested by other workers) but in which their molecular weight plays a central role in determining the kinetics of this interaction.