The role of H1 and H2-receptors in the coronary vascular response to histamine of isolated perfused hearts of guinea-pigs and rabbits

Role of cellular acidosis in production of nitric oxide in canine ischemic myocardium

We tested the hypothesis that cellular acidosis modulates the production of nitric oxide (NO) in ischemic hearts. In canine hearts, we decreased coronary blood flow (CBF) to one third of the control by reduction of coronary perfusion pressure (105+/-3 to 41+/-5 mmHg), and thereafter we maintained CBF constant (89.8+/-1.6 to 30.0+/-0.5 ml/100 g/min) with an intracoronary administration of either saline, atropine, rauwolscine, HOE140, 8-sulfophenyltheophylline (8SPT), NaHCO3, or HOE642 (the inhibitor of Na+/H+ exchange). The cardiac NO levels defined as the differences of the nitrate and nitrite levels between coronary venous and arterial blood increased in the saline administration (2.9+/-0.2 to 12.7+/-1.7 micromol/l), and the extents of increases were identical in the condition of either saline, atropine, rauwolscine, HOE140 or 8SPT administration. In the condition with either NaHCO3 or HOE642, the increases in the cardiac NO levels were blunted (4.5+/-0.7 and 4.8+/-0.4 micromol/l, respectively). Cyclic GMP content of epicardial coronary artery in the ischemic area increased, which was also attenuated by either NaHCO3 or HOE642. We confirmed the acidosis-induced NO production in a more severe ischemic myocardium, and also showed that cellular acidosis produced by infusion of HCl increased NO production in non-ischemic myocardium. We conclude that cellular acidosis and subsequent activation of Na+/H+ exchanges modulate production of endogenous NO in canine ischemic myocardium.

Computer-aided video angiometry in isolated rabbit hearts: a new method assessing epicardial coronary selectivity

The clinical value of coronary vasodilators in antianginal therapy depends on the ratio of their epicardial versus resistance coronary vessel actions. The coronary flow measured in standard isolated heart preparations, however, does not allow any conclusion about the function of epicardial vessels. Thus, we established a new technique assessing the epicardial coronary diameter directly by video angiometry. Pictures from the cardiac surface were taken by a videocamera mounted on a long-distance microscope. The video signal was digitized for computer-aided evaluation. An area of interest (AOI) was laid over the vascular section to be measured. The gray values of the pixels across the epicardial vessel were registered, and a mean curve of distribution was obtained by averaging the gray values from all video lines within the AOI. The inner epicardial coronary diameter resulted from the distance between the points of inflection of this mean curve of distribution. Experiments with NO-vasodilators and adenosine showed that epicardial coronary arteries of isolated perfused rabbit hearts have no appreciable tone. Pretreatment of the hearts with a combination of histamine (10[-6] mol/l), cimetidine (10[-5] mol/l), and adenosine (10[-7] mol/1), however, caused a marked contraction of the conductive vessels. NO-donors selectively dilated epicardial vessels in such pretreated hearts whereas adenosine increased both epicardial coronary diameter and coronary flow, with only a slight tendency toward preferential action on resistance vessels in low concentrations. Simultaneous registration of coronary flow and epicardial coronary diameter in isolated rabbit hearts pretreated with a spasmogenic drug combination (histamine, cimetidine, and adenosine) may be a feasible method assessing epicardial selectivity of coronary vasodilators.

Relevance of mediators to cardiac parameters of isolated anaphylactic guinea-pig heart

The release of histamine, eicosanoids and catecholamines were measured after induction of anaphylaxis in isolated guinea-pig hearts. The concentration-time profile of these mediators was compared with changes of cardiac parameters. The histamine and catecholamine levels of the coronary effluent were determined at 10 s intervals; thromboxane and prostacyclin levels at 60 s intervals. The release of histamine and norepinephrine were maximum between 20 and 30 s after the antigen challenge and decreased rapidly within 60 s. Thromboxane and prostacyclin increased to a maximum after 3 min and declined slowly within 10 min. The rise in histamine release was correlated with tachycardia. The release of thromboxane was correlated with the increase of coronary perfusion pressure. Cimetidine inhibited the tachycardia and clemastine reduced bradyarrhythmia. The inhibition of lipoxygenase and cyclooxygenase also reduced the rise in the perfusion pressure. These data suggest that different mediators are time-dependently involved in anaphylaxis-induced cardiac changes.

Adenosine is not the sole vasodilator released on exposure of isolated guinea-pig hearts to histamine and isoprenaline

1. The role of adenosine in the control of coronary vessel tone was studied using two guinea-pig hearts in series. Vasodilatory mediator(s) released by the donor heart were assayed by the recipient heart. 2. Adenosine deaminase reduced the activity of vasodilatory mediator(s) released by histamine and isoprenaline by 58.0 +/- 14.4% and 80.5 +/- 5.9% respectively; responses to exogenous adenosine were abolished. 3. Adenosine deaminase added to the donor perfusate supplying the recipient heart caused a significant increase in basal perfusion pressure. 4. Our results suggest adenosine is not the sole mediator of vasodilation in stressed hearts and additionally unstressed hearts appear to release a basal vasodilatory level of adenosine.

K(+)-channel blockers and coronary vasoconstriction in guinea-pig perfused hearts in-vitro

Glibenclamide, glipizide and phentolamine, three drugs which have been reported to block ATP-dependent potassium channels, increased the coronary perfusion pressure in guinea-pig isolated hearts perfused at constant flow. Blockers of other types of potassium channels, 4-aminopyridine and UK-66,914, did not significantly increase perfusion pressure. Exposing hearts to a single concentration of 3 microM glibenclamide caused a greater degree of vasoconstriction than when this was preceded by lower concentrations. The 3 microM glibenclamide-induced vasoconstriction was reduced by prazosin (1 microM), mepyramine (0.1 microM) and ranitidine (10 microM) but not by a combination of mepyramine and ranitidine or by ritanserin (0.01 microM). These results suggest that a component of the vasoconstriction induced by glibenclamide may result indirectly from the release of vasoactive mediators.

Histamine H1-receptor in heart: unique electrophoretic mobility and autoradiographic localization

Histamine H1-receptors, visualized in the guinea pig heart by autoradiography using [125I]iodobolpyramine as a specific probe, are abundant in the nodal tissue and cardiac vessels but also occur heterogeneously in the myocardium. Following photoaffinity labeling with [125I]iodoazidophenpyramine and electrophoresis, the ligand binding domain of the heart H1-receptor was shown to be present on a major 68-kDa and a less abundant 54- to 58-kDa protein. The 68-kDa protein displayed a molecular size higher in heart than in all other tissues (56 kDa). This indicates the existence of at least two isoforms of the H1-receptor; the cardiac isoform, however, was pharmacologically indistinguishable from the common isoform studied in cerebellar membranes using available ligands. Its distinct electrophoretic properties suggest that the cardiac isoform may have a unique function.

Beneficial effects of alpha 2-adrenoceptor activity on ischemic myocardium during coronary hypoperfusion in dogs

We have previously reported that alpha 2-adrenoceptor stimulation enhances adenosine-induced coronary vasodilation. In the present study, we tested the hypothesis that alpha 2-adrenoceptor activity exerts beneficial effects on myocardial ischemia through augmentation of vasodilatory effects of released adenosine. In open-chest dogs, the left anterior descending coronary artery was perfused through an extracorporeal bypass tube from the carotid artery. Propranolol was infused into the bypass tube to exclude the metabolic effects of norepinephrine. When clonidine (0.24 micrograms/kg/min i.c.) was infused for 10 minutes after reduction of coronary blood flow by partial occlusion of the bypass tube, coronary blood flow was increased by 43% from 27 +/- 1 ml/100 g/min despite no changes in coronary perfusion pressure (38 +/- 5 mm Hg) and a slight decrease in adenosine release. Both fractional shortening and lactate extraction ratio of the perfused area were significantly improved (fractional shortening, 1.8 +/- 1.0 to 10.9 +/- 1.5%, p less than 0.001; lactate extraction ratio, -57.8 +/- 6.5 to -31.9 +/- 2.4%, p less than 0.005). Identical results were observed in the denervated hearts, indicating that the beneficial effect of clonidine is not attributed to the prevention of norepinephrine release from the sympathetic nerve terminals. The beneficial effects of clonidine were prevented by yohimbine, an alpha 2-adrenoceptor blocking agent. An adenosine receptor antagonist, 8-phenyltheophylline, also prevented the beneficial effects of clonidine, indicating that these beneficial effects are mediated by effects of adenosine. Furthermore, the extent of augmentation of coronary flow in the ischemic heart was coincided with that of augmentation of exogenous adenosine-induced hyperemic flow (40%) by clonidine. Production of cyclic AMP in the coronary artery during myocardial ischemia was augmented by clonidine. In 12 other dogs, myocardial ischemia was produced by intracoronary embolization of microspheres (15 microns in diameter). Clonidine enhanced (39%) the hyperemic coronary flow and improved both fractional shortening and lactate extraction ratio. Thus, we conclude that alpha 2-adrenoceptor stimulation can ameliorate myocardial ischemia mainly due to enhancement of vasodilatory effects of adenosine released from the ischemic myocardium.

Effects of promethazine on ischemic and reperfusion arrhythmias in rat heart

The effects of the H1-receptor antagonists promethazine, mepyramine, and chlorpheniramine on ischemic and reperfusion arrhythmias were studied in the isolated perfused rat heart. Promethazine reduced both ischemic and reperfusion arrhythmias (2 x 10(-6)M-7.5 x 10(-6)M). Mepyramine and chlorpheniramine decreased these arrhythmias but at concentrations about 10 times higher. The H2-blockers cimetidine and ranitidine had no antiarrhythmic effect. Promethazine also: (i) increased release of noradrenaline by the heart; and (ii) increased coronary flow in the reperfusion period and in some mildly ischemic zones. It is proposed that promethazine exerts most of its antiarrhythmic effects by a nonspecific mechanism, possibly membrane stabilization; in addition, enhanced coronary flow may play a role.

Effects of histamine on coronary hemodynamics in humans: role of H1 and H2 receptors

To evaluate whether histamine exerts a direct effect on coronary hemodynamics in humans, and to investigate the role played by H1 and H2 receptors in this response, intracoronary saline solution or histamine (4 micrograms) was administered in 10 patients with normal coronary arteries during diagnostic cardiac catheterization. Histamine injection was repeated after intravenous cimetidine (400 mg) and diphenhydramine (10 mg). The electrocardiogram, arterial pressure and thermodilution coronary blood flow were continuously monitored during and for 40 seconds after each injection. Immediately after histamine injection there was a significant increase in coronary blood flow (65 +/- 6%) and a decrease in coronary vascular resistance (-40 +/- 3%) (both p less than 0.001), with minor changes in the RR interval and the mean arterial pressure. H2 receptor blockade with cimetidine did not affect these changes, while H1 receptor blockade with diphenhydramine significantly reduced the histamine-induced increase in coronary blood flow and the decrease in coronary vascular resistance (26 +/- 6%, p less than 0.005 and -18 +/- 5%, p less than 0.001, respectively). Twenty to 30 seconds after histamine injection, a significant decrease in mean arterial pressure (-17 +/- 2%, p less than 0.001) and in the RR interval (-4 +/- 1%, p less than 0.01) was observed. These changes persisted after H2 receptor blockade with cimetidine, but were completely abolished after H1 receptor blockade with diphenhydramine. In each case coronary and systemic hemodynamics returned to normal within 40 seconds of the injection.(ABSTRACT TRUNCATED AT 250 WORDS)

Coronary artery spasm induced in miniature swine: angiographic evidence and relation to coronary atherosclerosis

In a swine model of coronary artery spasm, the pathogenetic role of coronary atherosclerosis was examined. Following endothelial balloon denudation of the left circumflex coronary artery (LCX), male miniature swine were fed a laboratory chow diet containing 2% cholesterol. Although there was no difference in the extent of coronary vasoconstrictive response to histamine, serotonin, and ergonovine between the left anterior descending coronary artery (LAD) and the LCX before the denudation, a constrictive response was significantly augmented along the denuded portion of the LCX 1 and 3 months after the denudation. Augmented vasoconstrictive responses to phenylephrine were never evidenced. Histamine was the most potent vasoactive agent, and coronary artery spasm was provoked repeatedly by intracoronary or intravenous administration of histamine in the presence or absence of cimetidine. The spasm was provoked only in the denuded portion of the LCX, the same area which was angiographically normal before the occurrence of the spasm. Histologically, atherosclerotic changes were predominant along the denuded portion of the LCX. Topologic correlation was suggested between the site of the spasm and the site of coronary atherosclerosis. It is concluded that in this swine model of coronary artery spasm, atherosclerotic changes may be an important causative factor, in terms of an activation of multiple receptor-operated calcium channels in the coronary artery.

The effect of calcium on cardiac anaphylaxis in guinea-pig Langendorff heart preparations

This study was designed to determine the effects of different calcium concentrations on the perfused isolated guinea-pig heart preparation subjected to cardiac anaphylaxis. Following challenge both physiological and biochemical effects were determined on hearts from guinea-pigs previously sensitized to ovalbumin. Perfusion media containing either 1,2.54 or 5 mM of calcium was used. In comparison to nonsensitized controls challenged to ovalbumin, challenged sensitized hearts (CSH) perfused with 1 mM Ca2+ showed an initial increase in dF/dt, a prolonged rise in H.R. and depressed coronary flow. Raising the calcium concentration to either 2.54 or 5 mM in CSH preparations resulted in a marked increase in the release of lactate dehydrogenase (LDH) into the coronary effluent and depressed coronary flow. Perfusing CSH preparations with increasingly higher calcium concentrations more often produced severe tachyarrhythmias and fibrillation. The highest level of histamine released into the coronary effluent occurred immediately following challenge and then declined exponentially over the next 20 min. Both challenge and the administration of histamine induced an immediate but transient increase in H.R., a rise in dF/dt, and LDH release. The infusion of histamine produced an increase in coronary flow, but on porcine tubular coronary arterial segments only a direct constricting effect was obtained. The prior administration of cimetidine (10(-5) M) attenuated the rise in LDH and dF/dt in CSH and nonsensitized preparations infused with histamine (3 micrograms). However, although cimetidine did not affect the decreased coronary flow in CSH preparations, it initially attenuated the rise in coronary flow in preparations infused with histamine. These results suggest that calcium enhances the likelihood of tachyarrhythmias in cardiac anaphylaxis. The release of LDH in histamine-infused preparations and those CSH preparations perfused with 2.54 and 5 mM calcium-containing media also suggests the possibility that calcium enhances the damaging effects on the myocardial cell in cardiac anaphylaxis.

A marked H1-receptor-mediated vasodilator effect of histamine in the isolated perfused rat heart

Infusion of histamine into the coronary circulation of the isolated perfused rat heart produced a marked and concentration-dependent fall in perfusion pressure. A similar effect was obtained with 2-pyridylethylamine, a histamine H1-receptor agonist. Impromidine, a potent H2-receptor agonist, however, was found to be ineffective. Mepyramine but not cimetidine competitively inhibited the fall in perfusion pressure induced by histamine and 2-pyridylethylamine. Propranolol, a non-selective beta-adrenoceptor blocker, failed to abolish the coronary vasodilator effect of both histamine and 2-pyridylethylamine. From these results it was concluded that although the rat is known to be insensitive to histamine, this amine has a marked H1-mediated vasodilator effect in the isolated perfused coronary vasculature of this species.

Coronary artery spasm induced in atherosclerotic miniature swine

Angiographically demonstrable coronary artery spasm could be provoked repeatedly by giving intracoronary or intravenous injections of histamine to miniature swine with experimentally induced atherosclerotic lesions of the coronary artery. The spasm induced in this way subsided either spontaneously or after the administration of nitroglycerin and was prevented by a calcium antagonist or an agent that blocks histamine H1 receptors. This model, which suggests that atherosclerotic changes may be one of the primary factors in the occurrence of coronary artery spasm, should facilitate studies on the pathogenesis of this condition.

Effects of histamine and the histamine antagonists mepyramine and cimetidine on human coronary arteries in vitro

The effects of histamine have been studied on human isolated coronary artery preparations taken from hearts ranging in age from 9 to 73 years. Histamine in large concentrations (100 microM) contracted arteries which were without tone or spontaneous activity and sometimes induced rhythmic contractile activity. If spontaneous rhythmic activity was present it was enhanced by histamine. The contractile effects of histamine were inhibited by mepyramine but not by cimetidine. Arteries which were contracted by depolarization responded with relaxation to histamine concentrations lower than those required to evoke a contraction; arteries from younger hearts were more sensitive than those from older hearts. Mepyramine potentiated the maximal relaxant effect of histamine in arteries from hearts of all ages but cimetidine had very little effect. In the presence of mepyramine, cimetidine antagonized the relaxant effect of histamine, shifting the concentration-effect curve to the right. It is concluded that human coronary arteries contain both H1- and H2-type receptors, the H1-receptors mediating contraction. The relaxant effects of histamine can only be inhibited by a combination of both H1- and H2-receptor antagonists.

Inhibition of cardiac sympathetic neurotransmission by histamine in the dog is mediated by H1-receptors

1 The role of histamine H1- and H2-receptors in mediating prejunctional inhibition of cardiac sympathetic neurotransmission and histamine-induced coronary vasodilatation were investigated in perfused dog hearts in situ. 2 Intra-arterial injections of histamine into the right coronary artery during the resting state caused slightly positive chronotropic responses in doses larger than 1 microgram. 3 Histamine in doses of 0.1 to 10 micrograms into the right coronary artery reduced the tachycardia resulting from electrical stimulation of the cardiac sympathetic nerves. 4 Intra-coronary infusions of chlorpheniramine (300 micrograms/min) significantly reduced the histamine-induced depression of cardiac nerve stimulation. The effects of cimetidine (300 micrograms/min) and metiamide (300 micrograms/min) were less pronounced. 5 Histamine (1 to 10 micrograms) further increased heart rate resulting from the continuous intra-coronary infusion of noradrenaline (1 or 3 micrograms/min). 6 Intra-arterial injections of histamine (0.1 to 10 micrograms) caused an increase in coronary blood flow in a dose-dependent manner. This was partially inhibited by intra-coronary infusion of chlorpheniramine (10 to 300 micrograms/min) and by cimetidine (10 to 300 micrograms/min). The combination of both drugs (10 to 100 micrograms/min of each) caused a larger inhibition. 7 The present results suggest that the histamine-induced depression of heart rate during cardiac sympathetic nerve stimulation is due to a prejunctional effect mediated mainly by H1-receptors. Histamine-induced coronary vasodilatation in the dog is mediated both by H1- and H2-receptors.

Protective effects of glucagon during the anaphylactic response in guinea-pig isolated heart

1 Cardiac anaphylaxis and the effects of glucagon pretreatment were studied in guinea-pig isolated hearts actively sensitized to ovalbumin.2 Antigen challenge of the sensitized hearts markedly increased creatine phosphokinase (CPK) activity in the coronary venous effluent. Control values of CPK release from the hearts before challenge were 3.56 +/- 0.15 mu min(-1) mg(-1). In the first 10 min following challenge, CPK release remained stable at increased levels which ranged between 4.88 +/- 0.20 to 5.39 +/- 0.38 mu min(-1) mg(-1). There was no correlation between immunologically released histamine and CPK release.3 Pretreatment of the hearts with glucagon (0.15 mumol l(-1)) exerted a pronounced anti-arrhythmic activity, reducing the conduction arrhythmias and completely preventing automaticity arrhythmias which normally occurred following ovalbumin challenge.4 Anaphylactic histamine release was reduced significantly in the presence of glucagon. The percentage inhibition of histamine release from glucagon pretreated hearts, during the first 10 min after challenge, ranged between 58% and 94% of that from hearts similarly challenged in the absence of glucagon.5 Glucagon significantly elevated sinoatrial nodal automaticity, enhanced atrioventricular conduction, improved coronary flow and reduced contractile force during anaphylaxis. It appears that these effects are caused both by modulating anaphylactic histamine release and by influencing the effects of the released histamine.6 CPK release from the anaphylactic hearts was significantly inhibited in the presence of glucagon. The average percentage inhibition of CPK activity during the first 10 min after challenge ranged between 42% and 98%.7 The findings from this study provide experimental evidence for protective effects of glucagon pretreatment during cardiac anaphylaxis.

Some characteristics of the inotropic effects of histamine H1- and H2-receptor agonists in comparison with those of alpha- and beta-adrenoceptor agonists

The positive inotropic effects of 2-pyridyl-ethylamine (PEA) and of 4-methylhistamine (4MeH) were studied in isolated guinea-pig ventricular strips electrically stimulated at a rate of 60 and 150/min. The increase in contractile tension induced by PEA (10(-7)-3 X 10(-4) M) in the presence of cimetidine (10(-5) M) was associated with a slight increase in time to peak tension and with a lengthening of the relaxation phase; the positive inotropic effect of PEA was significantly higher at a frequency of 60/min than at 150/min. Conversely, the inotropic response to 4MeH (10(-8)-3 X 10(-6) M) was not frequency dependent, and was associated with an evident decrease in relaxation time. Moreover, 4MeH consistently antagonized, in dose-dependent manner, the negative inotropic effects induced by the calcium antagonistic drug D600 and by lowering calcium concentration in the medium, and was able to restore the contractility abolished by treatment of preparations with a high K+ medium. On the other hand PEA, in the presence of cimetidine, scarcely antagonized the negative inotropic effects induced either by D600 or by low calcium solution, and was unable to restore the contractility of K+-depolarized preparations. The characteristics of the inotropic response of the H1-receptor agonist were very similar to those of the alpha-adrenoceptor agonist phenylephrine. This observation suggests that a common mechanism is probably involved in the inotropic effects mediated by H1 and by alpha receptors, and that this mechanism does not include a stimulation of the calcium transmembrane influx.

Histamine H1- and H2-receptors in the gastric vasculature of the rabbit

The histamine receptors in the rabbit blood perfused gastric vasculature were analysed pharmacologically. Histamine elicited a monophasic increase in perfusion pressure which was antagonized by mepyramine and enhanced by metiamide. The maximum observed response was enhanced by metiamide to that produced by a specific H1-receptor agonist. It is concluded that the gastric vasculature responds to histamine with an H1-receptor mediated vasoconstriction and an H2-receptor mediated dilatation. In this preparation the H1-effect predominates in response to injection of histamine.

Further characterization of the guinea pig left atrial tension response to histamine by use of selective agonists

1. The responses of guinea pig left atrial tension and right atrial rate to histamine receptor agonists and histamine were compared. 2. Single doses of histamine and another non-selective agonist, N,N-dimethylhistamine, produced biophasic inotropic responses, with an initial increase in tension and a secondary sustained tension increase separated by a negative inotropic component. 3. Selective H1-receptor agonists -- 2-methylhistamine, 3-methylhistamine and 2-pyridylethylamine (2-PEA) -- also exhibited biphasic inotropic responses. 2-PEA did so only after blockade with propranolol or in atria from reserpine-pretreated animals, indicating an additional release of endogenous catecholamines which masked the biophasic response. 4. Selective H2-receptor agonists -- 4-methylhistamine and dimaprit -- failed to produce biphasic responses except at high concentrations only in the case of 4-methylhistamine. 5. The biophasic responses were converted to monophasic responses by mepyramine. This was a result of antagonism of the initial positive component and the secondary negative component which were therefore mediated via H1-receptors. The production of a biphasic inotropic response therefore depends upon these two components which were exhibited preferentially by the H1-receptor-selective agonists.

Uptake and metabolism of histamine in guinea-pig isolated atria and their relationship to the pharmacological responses

1. The uptake of radiolabelled 14C-histamine into guinea-pig isolated left atria was determined. 2. Mepyramine up to 10(-7) M had no significant effect upon the uptake of total radioactivity. Metiamide (10(-4) M) produced a small but not significant inhibition of uptake. SK & F 91581 (10(-4) M) and amodiaquine (10(-4) M) both significantly inhibited uptake by 48.2 +/- 4.2 and 56.7 +/- 1.2% respectively. 3. The metabolism profiles of radiolabelled histamine after 5 and 30 min incubation were examined by thin-layer chromatography. 4. SK & F 91581 (10(-4) M) and amodiaquine (10(-4) M) reduced the proportion of N tau-methylhistamine and acid metabolites after a 30 min incubation. The levels of unmetabolized histamine were elevated. This may explain their inhibitory action upon histamine uptake. 5. The effects of mepyramine, metiamide, SK & F 91581 and amodiaquine were then examined upon the positive inotropic and chronotropic responses of isolated left and right atria to histamine. 6. Mepyramine (10(-7) M) inhibited the positive inotropic responses only (H1), whereas metiamide (10(-4) M) inhibited the positive chronotropic responses only (H2). SK & F 91581 (10(-4) M) had no effect upon these responses and amodiaquine (10(-5) M) depressed the maxima of the rate response dose-response curves. 7. The relationship between uptake and metabolism of histamine and its pharmacological response is discussed.

A question of the specificity of rabbit atrial chronotropic histamine receptors and agents which affect their activity

The histamine chronotropic response of rabbit atria appears to be controlled by both H1 and H2 receptors and can be blocked in part by either metiamide (an H2 antagonist) or diphenhydramine (an H1 antagonist), while both 2- and 4-methylhistamine (H1 and H2 agonists, respectively) stimulated the chronotropic response. At low agonist concentrations, the simultaneous presence of both H1 and H2 blockers results in considerably less inhibition than could be expected from calculations of individual inhibition data, suggesting that some sites behave as if they have both H1 and H2 properties. Additional compounds were tested for specific action on H1 and H2 receptors: 2-(2-pyridyl) ethylamine, reported to be an H1 agonist, appears to stimulate rabbit atria by releasing norepinephrine and guinea pig atria by releasing both norepinephrine and histamine; while dimaprit, reported to be an H2 agonist, may stimulate histamine receptors directly but has a nonspecific depressant action on rabbit atria which interferes with its use as an agonist in this species.

Chronotropic effect of histamine on cultured neonatal rat heart cells

The positive chronotropic effect (PCE) of histamine in cultured neonatal rat heart cells was monitored using a microscopic method as well as an electro-optically recording device. The action potential frequency was also measured (by means of microelectrodes). An increase in PCE was noted when histamine (from 1 X 10(-6) M to 1 X 10(-5) M) was added to the cells. However, higher concentrations (from 1 X 10(-5) M to 1 X 10(-4) M) were less effective. The PCE of histamine was reduced by pretreating the cells with antihistaminic drugs. H1-blocking agents (promethazine and mepyramine) were more potent than H2-blocking drugs (metiamide and cimetidine). In addition, the PCE of histamine was abolished when the cells were in presence of high K+ medium (26 mEq) but contraction and action potential amplitudes were increased. Our results demonstrate that these cultures respond to histamine and that this response is abolished by antihistaminic drugs thus suggesting the H1 and/or H2 receptors may be present in the neonatal rat heart cell cultures.

Influence of contrast media osmolality on isolated rabbit heart performance

A low osmolality contrast medium (Iopamidol) caused less cardiac disturbance on isolated rabbit heart than a high osmolality medium (Urografin). The same effects were induced by glucose solutions iso-osmolal with the two media. After perfusion with Propranolol (1 microgram/ml) and Cimetidine (1 microgram/ml) the observed effect was unaltered.

Chronotropic and dromotropic effects of histamine on the canine heart

The actions of 2-methylhistamine (H1 agonists), 4-methylhistamine (H2 agonist), and histamine were studied by selective perfusion of the sinus node artery and atrioventricular node artery in 75 dogs anesthetized with pentobarbital sodium. 2-Methylhistamine and histamine had variable and inconsistent effects on the sinus rate. 4-Methylhistamine (100 microgram/ml) produced acceleration of the sinus rate from 158 +/- 4 to 173 +/- 5 beats per minute (P less than 0.05) when perfused via the sinus node artery. The effects of the histamine agonists on atrioventricular junctional rhythms were similar to the effects on sinus rhythm. The response of the sinus node to vagal stimulation was attenuated by selective perfusion with histamine; however, the direct negatively chronotropic action of acetylcholine was not affected by histamine. Neither 2-methylhistamine nor 4-methylhistamine affected the response of the sinus node to vagal stimulations. Both 4-methylhistamine and histamine (but not 2-methylhistamine) attenuated (P less than 0.05) the response of the sinus node to stimulation of the right stellate ganglion. The positively chronotropic effects of directly perfused norepinephrine were unaffected by histamine or 4-methylhistamine. These results suggest a neural depressing action of histamine on autonomic efferent fibers. In the atrioventricular junction, both histamine and 2-methylhistamine (but not 4-methylhistamine) had negatively dromotropic effects. Cimetidine (an H2 antagonist) had no significant direct effects on the sinus rate or atrioventricular conduction and failed to prevent the acceleration of the sinus rate produced by local perfusion with 4-methylhistamine.

Differentiation of the roles of histamine H1- and H2-receptors in the mediation of the effects of histamine in the isolated working heart of the guinea-pig

1 Differentiation of the roles of histamine H1- and H2-receptors in the mediation of the effects of histamine on the isolated working heart of the guinea-pig was achieved through the use of histamine and selective histamine receptor agonists and antagonists. 2 Histamine over the dose range 10(-9) mol to 10(-6) mol produced dose-related increases in sinus rate, left intraventricular pressure (LVP)max, LVdP/dtmax, coronary flow, aortic flow, total cardiac output and external pressure-volume work. 3 Dimaprit, a selective histamine H2-receptor agonist, produced very similar responses to histamine. 4 2-Pyridylethylamine, a selective histamine H1-receptor agonist, had little effect on cardiac function unless large doses were administered. Such doses produced increases in all measured parameters. 5 Cimetidine, a selective histamine H2-receptor antagonist, antagonized the effects of histamine and dimaprit and some but not all effects of 2-pyridylethylamine. In the presence of cimetidine a decrease in all parameters with the exception of sinus rate was observed with both histamine and 2-pyridylethylamine. 6 The selective histamine H1-receptor antagonist, mepyramine, had little effect on responses to all three agonists. However, the depressant effects observed with histamine and 2-pyridylethylamine in the presence of cimetidine were antagonized by mepyramine. 7 The results indicate the important role of the histamine H2-receptor in the mediation of the gross cardiac effects of histamine and also indicate that histamine H1-receptors can mediate cardiac depression.

Cimetidine: a review of its pharmacological properties and therapeutic efficacy in peptic ulcer disease

Cimetidine is a specific competitive histamine H2-receptor antagonist which effectively inhibits gastric acid secretion and is advocated for the treatment of chronic peptic ulceration, haemorrhage from erosive gastritis, and the control of gastric hypersecretion and peptic ulceration in the Zollinger-Ellison syndrome. Placebo-controlled trials in outpatients have demonstrated its efficacy in promoting the healing of endoscopically diagnosed duodenal ulceration, during a period of 4 to 6 weeks, but its role in the treatment of gastric ulcer is less clear. Preliminary evidence suggests that maintenance therapy with cimetidine reduces the rate of recurrence of duodenal ulcer, but further studies are required to clarify its role in this situation and in the treatment of oesophagitis and acute gastrointestinal haemorrhage. Cimetidine controls the peptic ulceration of Zollinger-Ellison syndrome in most patients when given continuously for up to 2 years. Side-effects have generally been trivial and have very seldom necessitated withdrawal of therapy except in the rare occurrence of gynaecomastia. The haematological abnormalities particularly agranulocytosis, which lead to the withdrawal from clinical use of metiamide, have not been reported with cimetidine, except for 1 case of transient neutropenia. The safety of long-term cimetidine administration has yet to be determined.

Regional vascular resistance and heart rate responses mediated through H1- and H2-histamine receptors in the unanaesthetised rabbit

The effects of histamine infusions (10-100 microgram/kg/min) on heart rate and hindlimb, carotid, mesenteric and renal vascular resistance were investigated in unanaesthetised rabbits after "total" autonomic effector block to abolish reflex effects. Histamine caused a rise in heart rate that was predominately due to stimulation of H2-receptors (blocked by metiamide). Hindlimb and carotid vascular resistance did not change significantly during histamine infusion. However, after blocking H2-receptors with metiamide histamine infusions produced dose-related vasoconstriction in these beds while after H1-receptor block with mepyramine histamine caused dose-related vasodilatation indicating that H1- and H2-RECEPTORS MEDIATED OPPOSITE VASCULAR EFFECTS WHICH WERE OF SIMILAR MAGNITUDE. By contrast, histamine infusion caused vasodilatation in both the mesenteric and renal vasculature before giving antagonists. This dilatation was mediated by both H1- and H2-RECEPTORS AS EITHER RECEPTOR ANTAGONIST ATTENUATED THE RESPONSE. These studies suggest that H1-receptors in the same species mediate vasoconstriction in som- beds and vasodilatation in others while H2-receptors mediate vasodilatation in all the beds studied and also account for most of the increase in heart rate.

The cardiovascular response to dimaprit, a selective histamine H2-receptor agonist

1 The cardiovascular responses to dimaprit (S-(3-dimethylaminopropyl) isothiourea) a selective histamine H2-receptor agonist have been studied in anaesthetized cats, dogs, rats and rabbits. 2 Dimaprit lowered systemic arterial blood pressure in all species whether given by intravenous injection or by continous infusion. Intra-arterial injections or infusions caused vasodilatation in the femoral vascular bed. 3 In the cat, continuous intravenous infusions of dimaprit lowered blood pressure without any significant change in heart rate or cardiac output. The fall in blood pressure was thus due to a decrease in peripheral vascular resistance. 4 Dimaprit, given intravenously, increased coronary and gastric blood flows and there were associated marked reductions in vascular resistance. Small reductions in vascular resistance occurred in skeletal muscle, in the intestine and the kidneys. 5 The histamine H2-receptor antagonists, metiamide and cimetidine, antagonized the depressor and vasodilator responses to dimaprit.

H1 - and H2-receptor mediated responses to histamine on contractility and cyclic AMP of atrial and papillary muscles from guinea-pig hearts

On guinea-pig heart we investigated whether cyclic AMP serves as a messenger for H1 - and/or H2-mediated responses to histamine. (1) On papillary muscle histamine elicited positive inotropic responses which were antagonized by burimamide but not by promethazine. The stimulation of H2-receptors was not only associated with an increase in contractility but also with an increase in cAMP. As shown by the time course of effects for 10(-5) M histamine, the maximal increase in cAMP preceded the maximum in contractility. The mechanical and biochemical responses to histamine were potentiated by the phosphodiesterase inhibitor papaverine, but antagonized by burimamide. (2) On the left guinea-pig atrium containing H1-receptors the inotropic response to histamine (10(-5) M) was not accompanied by increases in cAMP at stimulation frequencies of 0.5 and 2 Hz, respectively. In addition, in the presence of papaverine (3 X 10 (-5) M) no change in the cyclic AMP level occurred after application of histamine. Papaverine by itself, however, concomitantly increased contractility and cyclic AMP at a stimulation frequency of 0.5 Hz. In contrast, at 2 Hz papaverine increased only cAMP leaving the contractility unchanged. At this frequency the well-known Ca2+-antagonistic effect comes into prominence, thus masking the positive inotropic effect attributable to the inhibition of the phosphodiesterase. (3) On the right guinea-pig atrium the mediation of the positive chronotropic response to histamine by H2-receptors which is partly involved in the inotropic effect via the frequency-force relationship does not lead to a concomitant increase in cAMP. Also, in the presence of papaverine, histamine had no influence on the cAMP. However, papaverine potentiated the cardioacceleration produced by histamine. Although it is very likely that the cAMP in the sinus node rises, we were not able to detect an increase in cAMP in the whole atrial tissue. From the present results the conclusion can be drawn that the mediation of the inotropic effect due to stimulation of H2-receptors by histamine is associated with an increase of cyclic AMP, whereas that of H1-receptors is not. The view that cAMP may be the second messenger in the chronotropic action of histamine needs further elucidation by experiments on sino-atrial cells.

Effects of the H1-antagonist promethazine and the H2-antagonist burimamide on chronotropic, inotropic and coronary vascular responses to histamine in isolated perfused guinea-pig hearts

On guinea-pig atria part of the inotropic response to histamine is attributable to a concomitant increase of the frequency [7]. Since the chronotropic effect of histamine is mediated by a stimulation of H2-receptors a direct interaction of histamine with H1-receptors a direct interaction of histamine with H1-receptors mediating the inotropic response on heart may be overlooked. For this reason the ability of the H1-antagonist promethazine and the H2-antagonist burimamide to inhibit the positive chronotropic, inotropic and coronary vascular responses to histamine was determined in spontaneously beating and electrically driven perfused guinea-pig hearts. (1) Burimamide produced a competitive blockade of the positive chrono- and inotropic responses to histamine. (2) On the other hand, promethazine in concentrations that had no effect on cardiac function by itself, proved to be ineffective against the positive chrono- and inotropic responses produced by histamine on spontaneously beating and electrically driven heart preparations. (3) The predominant coronary vasodilation observed after infusion with histamine was competitively antagonized by promethazine and burimamide. This blockade was not attributable to an interaction with myocardial H2-receptors mediating increases in heart rate and contractility and was, therefore, direct in nature. (4) Based upon the present study and former investigations [7] the following distribution of different histamine receptors in the guinea-pig heart does exist: H1-receptors are present in the atrial muscle and the coronary vascular bed. H2-receptors are located in the sinus node, the ventricular myocardium and the coronary vessels.

The release of a coronary vasodilator metabolite from the guinea-pig isolated perfused heart stimulated by catecholamines, histamine and electrical pacing and by exposure to anoxia

1 A procedure involving two guinea-pig isolated hearts perfused in series is described for detecting in the recipient heart the release of a possible coronary vasodilator metabolite from the donor heart. 2 Adrenaline and isoprenaline stimulated the rate and force of contraction and produced a multiphasic coronary vascular response, the predominant phase of which was vasodilatation. When the beta-adrenoceptors of the recipient heart were blocked, stimulation of the donor heart by the catecholamines was associated with a coronary vasodilatation of the recipient heart. 3 Histamine stimulated rate and force of contraction and was predominantly coronary vasodilator. After blockade of histamine H1- and H2-receptors in the recipient heart, coronary vasodilatation followed increases in activity of the donor heart in response to histamine. 4 These vasodilator responses of the recipient heart were attributed to the release from the donor heart of a vasodilator metabolite by the increased activity. This is the proposed mechanism for the predominant coronary vasodilator response to catecholamines and histamine. 5 Periods of electrically-paced tachycardia and anoxia of the donor heart also led to the release of vasodilator activity. 6 The possible identity of the metabolite is discussed.