Histamine receptors in the cardiovascular

Effects of enkephalins on perfusion pressure in isolated hindlimb preparations

A series of studies were conducted to determine the effects of leucine-(leu-) enkephalin and methionine-(met-) enkephalin on perfusion pressure. These experiments utilized isolated perfused femoral arterial preparations in pentobarbital-anesthetized cats. The enkephalins were administered intraarterially into the femoral artery and changes in perfusion pressure recorded. Leu-enkephalin in doses of 1 microgram to 320 micrograms produced significant dose-dependent decreases in perfusion pressure (4.0 +/- 1.3% with 1 microgram to 19.1 +/- 2.1% with 320 microgram). Similar declines in perfusion pressure (5.2 +/- 2.4% with 1 microgram to 21.7 +/- 4.1% with 320 micrograms) were observed following the administration of met-enkephalin. Pretreatment with naloxone (3 mg/kg) antagonized the effects of both enkephalins. Diphenhydramine (2 mg/kg) effectively antagonized the leu-enkephalin elicited decline in perfusion pressure but blocked the effects of met-enkephalin only at lower agonist doses. Propranolol treatment (4 mg/kg) did not alter the pressure responses to either enkephalin. The results of the study show that intraarterially administered enkephalins exert a vasodilatory effect on vasculature in skeletal muscle which may be direct, indirect or both. The differential antagonism of the effects of the two enkephalins suggest that the two opioids act through different receptors or multiple receptors.

The effects of cimetidine on histamine atrial chronotropic receptor activity

Clinically useful histamine antagonists have been reported to be specific for H1- or H2-receptors. However, data is accumulating that the specificity of those agents may be relative. This study was undertaken in an attempt to clarify this point. Histamine stimulates the chronotropic responses of rabbit atria by acting on both H1- and H2-receptors. Cimetidine (H2-antagonist) and diphenhydramine (H1-antagonist) both cause some inhibition of the histamine response, but there is no evidence of competitive blockade with these antagonists given separately or in combination. 4-Methylhistamine (H2-agonist) stimulates chronotropic activity, and both H1- and H2-antagonists reduce this response to some degree. 2-Methylhistamine (H1-agonist) is a less effective agonist (perhaps rabbit atria contain fewer H1-receptors), but the response is also decreased by either H1-or H2-antagonists. These two "specific" agonists were combined to stimulate the histamine responses, and the antagonists were given simultaneously. The results were qualitatively similar to those obtained with histamine. The question remains: (1) are these agonists and antagonists specific or (2) are the rabbit atrial chronotropic receptors not typical of the H1- and H2-type receptor?

Pharmacological characterisation of cardiovascular histamine receptors in man in vivo

Data from pharmacological studies carried out in healthy subjects using systemic histamine or impromidine and their antagonists are reviewed. Exogenous histamine by rapid injection appears to stimulate only H1-receptors. Chlorpheniramine alone antagonised the responses to histamine. The effects of cardiovascular H2-receptor stimulation are demonstrated best by a sustained and large dose of histamine given by infusion. If it be considered desirable to antagonise all the cardiovascular responses to endogenous histamine, the available pharmacological data in man suggest this would be achieved best by a combination of an H1-and H2-receptor antagonist.

Inotropic and electrophysiological effects of histamine on human ventricular heart muscle

1. The effects of histamine were investigated on mechanical and electrophysiological parameters in isolated electrically driven human ventricular papillary muscles. The effects of cimetidine and propranolol on histamine responses were also investigated. 2. The effects of histamine were compared with those of noradrenaline, isoprenaline, dimaprit, a selective H2-receptor agonist, and a cyclic AMP derivative, 8-(4-chlorphenylthio) cyclic AMP. 3. The effects of histamine and dimaprit and the effects of cimetidine on histamine responses were also investigated in guinea-pig right ventricular papillary muscles in order to allow a comparison with human papillary muscles. 4. In human papillary muscles, histamine caused concentration-dependent increases in the force of contraction and reductions in both time-to-peak tension and time-to-half-maximal relaxation. Histamine simultaneously caused distinct changes in the action potential configuration with increases in the height and duration of the plateau phase and an increase in the over-all action potential duration. 5. Noradrenaline and isoprenaline produced similar responses to histamine, as did 8-(4-chlorphenylthio) cyclic AMP, consistent with the view that the effects of histamine as well as the beta-adrenoceptor agonists on human ventricle, were associated with cyclic AMP mediated increases in calcium-dependent slow inward current.

Effects of histamine on the circulatory system

Experiments have been made in anaesthetised cats and dogs and in healthy, human volunteers to compare the changes in blood pressure and heart rate during systemic administration of histamine. Histamine, 1 x 10(-9) to 1 x 10(-7) mol/kg/min, lowered blood pressure in a similar dose-dependent fashion in all three species. In man and the cat this was accompanied by clear dose-dependent tachycardia whereas in the dog heart rate changes were minimal. Pharmacological analysis of the depressor responses to histamine in all three species and the reduction in total peripheral resistance in the cat and dog showed that the immediate responses to histamine in all three species involved H1-receptors and that sustained responses involved H2-receptors. Abolition of responses to histamine throughout infusions required H1-and H2-receptor blockade. Histamine antagonists, used in doses which cause abolition of cardiovascular responses to large doses of histamine, do not cause any significant change in the resting cardiovascular system.

Effects of histamine type 2 receptor stimulation on myocardial function in normal subjects

Myocardial histamine(H)2 receptor stimulation has been studied in six normal men. Since histamine is a potent vasodilator, the haemodynamic effects of histamine infusion were compared with those of nitroprusside at equihypotensive doses, to identify changes in myocardial contractility attributable to vasodilatation. After H1 receptor blockade with mepyramine, subjects received, in single blind crossover fashion, either histamine alone and with the H2 receptor antagonist cimetidine, or nitroprusside alone and with cimetidine. Echocardiographic left ventricular dimensions, plasma catecholamines, blood pressure, and heart rate were measured. The rise in catecholamines suggested similar baroreflex activation by both histamine and nitroprusside. Echo ejection phase indices did not alter significantly after nitroprusside, but histamine caused an increase in percentage fractional shortening from 38.2 +/- 4.1 to 53.5 +/- 3-6% and in mean fibre shortening velocity from 1.31 +/- 0.19 to 1.99 +/- 0.22 cm/s. These changes were both greatly reduced by cimetidine and suggest that H2 receptor stimulation in man causes a direct positive inotropic response.

H1- and H2-receptors in the guinea-pig heart: an electrophysiological study

The mechanical and electrophysiological effects of 2-pyridylethylamine (PEA) and 4-methylhistamine (4MeH) in different sections of guinea-pig heart were examined. 4MeH produced a dose-dependent increase in contractility in the right ventricle and the right atrium, and a decrease in functional refractory period (FRP) in all the sections studied; the action potential duration was decreased and the plateau phase was usually heightened in both atria and the ventricle. These effects were consistently antagonized by cimetidine. PEA-induced changes in contractility, FRP and the action potential profile were studied in the presence of cimetidine. Triprolidine antagonized PEA effects on FRP and the action potential profile only in the left atrium. The results obtained are discussed in terms of the functional role of both H1 and H2 receptors in the various guinea-pig heart sections.

Peripheral hypotensive and central hypertensive effects of cimetidine

Rapid intravenous (i.v.) injections of high doses (16-128 mumol/kg) of cimetidine induced a short-lasting (5-15 min) hypotension in anaesthetized rats. Diastolic pressure was reduced more than systolic pressure, suggesting vasodilatation. Heart rate was not affected. Diphenhydramine pretreatment (100 mumol/kg i.v.) did not antagonize the hypotensive effect of cimetidine. However, in the presence of diphenhydramine, cimetidine induced bradycardia. Intracerebroventricular administration of cimetidine or metiamide (2 mumol/rat) increased the blood pressure and heart rate. It is concluded that the hypotension after i.v. cimetidine is mediated by peripheral mechanisms. Since diphenhydramine pretreatment had no antagonistic effect cimetidine-induced hypotension could not be due to indirect H1-receptor stimulation caused by histamine liberation.

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.

Effects of histamine on the resting and stimulation-induced release of [3H]-noradrenaline in guinea-pig isolated atria

1 The sympathetic transmitter stores of guinea-pig isolated atria were labelled with [3H]-noradrenaline. The effects of histamine (0.3 to 100 mumol/l) on resting and stimulation-induced (S-I, 2 Hz for 10 s) release of radioactivity were investigated. 2 Histamine, in low concentrations (0.3 and 1 mumol/l) had no effect on resting release but inhibited S-I release of radioactivity. The inhibition was abolished by the H2-receptor antagonist, cimetidine (10 mumol/l) and also by the H1-receptor antagonist, mepyramine (1 mumol/l). 3 The inhibitory actions of histamine on S-I release were not due to indirect effects involving alpha-adrenoceptors, beta-adrenoceptors, muscarinic cholinoceptors or prostaglandin synthesis. 4 Histamine in a high concentration (100 mumol/l) increased the resting and S-I release of radioactivity. The increase in resting release was abolished by the neuronal uptake blocking drug cocaine (30 mumol/l) but the increase in S-I release was only partially blocked by cocaine.

Histamine provocation of clinical coronary artery spasm: implications concerning pathogenesis of variant angina pectoris

Twelve patients with nonexertional chest pain and nonobstructive fixed coronary disease (less than 50% luminal diameter narrowing) were given histamine to investigate the potential role (coronary artery H1 receptor agonism) of the endogenous agent in producing coronary artery spasm (CAS). Histamine, at intravenous dose of 0.5 to 1.0 microgram/kg/min, provoked CAS in four patients. In six patients neither histamine nor ergonovine provoked spasm, and these patients were considered by chronic follow-up evaluation to have noncardiac etiology for their chest pain syndrome. In one patient CAS was provoked with ergonovine but not by histamine, and one ergonovine-positive patient had an equivocally positive histamine result. Pretreatment with cimetidine (H2 receptor antagonism) was necessary to avoid unpleasant side effects of histamine. Thus these observations indicate that histamine should be included among the specific agents capable of inducing CAS and provide new insight concerning the mechanism(s) causing variant angina pectoris.

Histamine release from human right atrium

Histamine release has been detected biologically after exposure of human right atrial biopsy samples to compound 48/80. Human right atrial samples contain large quantities of histamine, 1035 +/- 65 ng/g fresh weight. Exposure of untreated electrically paced atrial strips to compound 84/80, 300 micro/ml, caused an increase in the force of atrial contraction similar to that caused by histamine. Pretreatment of strips with cimetidine, 3.16 x 10(-5) M, prevented the inotropic response to compound 48/80. It is concluded that mast cell degranulation in human atrial tissue can cause sufficient release of histamine to modify the function of the heart.

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.

The effects of chronic oral cimetidine therapy on the cardiovascular system in man

1 The demonstration of histamine (H2) receptors in the cardiovascular system in man and the widespread use of the specific H2 receptor antagonist cimetidine for the treatment of peptic ulcer has necessitated assessment of the cardiovascular effects of this drug during oral therapy in man. 2 Consequently, nineteen patients with a history of peptic ulcer but in symptomatic remission underwent a double-blind crossover trial of oral cimetidine v placebo, each treatment period lasting 4 weeks. No significant difference in various cardiac parameters at rest (heart rate, blood pressure, ECG) or following a maximal treadmill exercise test (time-into-protocol, maximum heart rate and blood pressure) was found between the groups. Similarly, cimetidine produced no change in the incidence of ventricular ectopics or in heart rate on 24 h ambulatory monitoring and had no effect on left ventricular volume or contractility as measured by echocardiography. 3 Thus oral administration of cimetidine in a dose of 400 mg four times daily was associated with no demonstrable cardiovascular changes. In addition, since this dose is thought compatible with cardiovascular H2 receptor blockade this study favours the lack of a physiological role for these receptors in man.

Vascular studies with histamine in vitro

In vitro studies to analyse the pharmacology of histamine-induced dilatation of resistance vessels in rat hindquarters have been made. Histamine caused dose-dependent dilatation of resistance vessels over the concentration range 10(-9) to 10)-6) mol. Responses to histamine were antagonized by cimetidine but not my mepyramine. Dimaprit also caused vasodilatation. Responses to dimaprit were inhibited by cimetidine 10(-6) to 10(-5) M. A pA2 of 6.43 (6.11 - 6.75, 95% confidence limits) was calculated for cimetidine in the resistance vessels of the hindquarters.

The involvement of histamine receptor subtypes in gastric acid secretion and mucosal blood flow in the anaesthetized rabbit

1. The involvement of histamine receptor subtypes in the gastric acid secretory and mucosal blood flow responses of the pentobarbitone-anaesthetized rabbit to histamine were investigated. 2. Histamine produced a dose-related increase in gastric acid secretion and mucosal blood flow. Maximal effects were stimulated further by mepyramine and inhibited by metiamide. Mepyramine had no effect on methacholine- or impromidine-stimulated gastric acid secretion or mucosal blood flow. 3. Impromidine also produced a dose-related increase in gastric acid secretion and mucosal blood flow. The maximal effect produced depended on the method of administration. The H1-receptor agonist 2-TEA inhibited impromidine-stimulated blood flow but not gastric acid secretion. 4. It is concluded that, in addition to stimulating acid secretion by an action on H2-receptors, histamine also exerts an inhibitory effect which is mediated by H1-receptors and that this inhibitory effect operates via reduced blood flow resulting from vasoconstriction in the stomach.

Histamine H1- and H2-receptors are differentially and spatially distributed in cerebral vessels

Receptor sites for neurotransmitters may be spatially oriented within cerebral vascular walls. The direction from which neurohumoral stimuli arise (e.g., perivascular or intravascular) and the location and type of receptor activated may therefore determine the nature of vascular response. I review a series of studies that examined cerebrovascular responses to histamine and suggest that histamine receptors are differentially and spatially organized in two profiles within the cerebral circulation. A transmural distribution is suggested from the following results: increases in permeability of the blood-brain barrier (endothelial cells) to intra-arterial infusion of histamine were mediated by H2-receptors; increases in blood flow to intra-arterial infusion of histamine occurred only after the blood-brain barrier was disrupted and were the result of stimulation of both H1- and H2-receptors. These responses probably occur within inner layers of arterial smooth muscle; dilatation of pial arterioles to local microapplication of histamine and its receptor agonists indicates that H2-receptors are the predominant type in outer layers of arterial smooth muscle. A segmental profile of histamine receptors within the cerebrovascular bed is suggested as follows: since both H1- and H2-receptors could mediate dilatation of arterioles and arteries, it may be concluded that both types of receptor are present in resistance vessels; in the capillary bed, H2-receptors are the predominant type; capacitance vessels (pial veins) did not respond to perivascular application of histamine or its agonists. These studies suggest that receptors for histamine may be sparsely populated or absent in cerebral venous smooth muscle.

Electrophysiologic properties of cimetidine in man

Severe bradycardia has been reported with the use of cimetidine. This observation has been explained by histamines' known action in increasing the contraction rate of the spontaneously beating right atrium, strips of ventricular muscle, and whole heart of guinea pigs. Histamine can also produce cardiac arrhythmias in animals. There is evidence for H2 receptors in the heart. Black and his associates have shown that the characteristic effect of histamine on isolated guinea pig atrium could be selectively blocked by anti-H2 drugs. Thus cimetidine probably can produce a sinus bradycardia by blocking the H2-receptor site in the heart. Recently Engel and Luck reported that 300 mgs of intravenously administered cimetidine did not affect heart rate, sinus node recovery time, or sinoatrial conduction time. These workers concluded that there is insufficient evidence to demonstrate that cimetidine caused appreciable sinus node dysfunction in man. In view of the disparity in these reports, we investigated the effects of 300 mgs of intravenously administered cimetidine on the human conduction system.

Some in vitro and in vivo actions of the new histamine H2-receptor antagonist, ranitidine

1 Ranitidine has been investigated as an antagonist of the H2-receptor-mediated responses to histamine of guniea-pig atrium and rat uterus in vitro and as an inhibitor of gastric acid secretion in the rat. 2 Ranitidine competitively antagonized histamine-induced increases in contraction frequency of the guinea-pig isolated rat atrium. Ranitidine had a pA2 of 7.2 and was 7.9 and 4.5 times more potent than metiamide and cimetidine respectively. 3 Ranitidine competitively antagonized histamine-induced relaxations of the rat isolated uterine horn. Ranitidine had a pA2 of 6.95 and was 3.6 and 5.9 times more potent than metiamide and cimetidine respectively. 4 Ranitidine, even at high concentrations, did not affect responses of the guinea-pig isolated atrium or rat isolated uterus to (-)-isoprenaline. Similarly it was without effect on either histamine or bethanechol-induced contractions of guinea-pig isolated ileum. 5 Ranitidine inhibited histamine- and pentagastrin-induced gastric acid secretion in the perfused stomach preparation of the anaesthetized rat. Ranitidine was 5.2 and 7.0 times more potent on a molar basis than metiamide and cimetidine respectively, as an inhibitor of histamine-induced gastric acid secretion. 6 It is concluded that ranitidine is a potent, competitive and selective antagonist of histamine at H2-receptor sites and and effective inhibitor of gastric acid secretion in vivo.

Cerebral circulation and histamine: 1. Participation of vascular H1- and H2-receptors in vasodilatory responses to carotid arterial infusion

We examined the cerebral circulatory effects of intra-carotid infusion of histamine or its receptor agonists in anesthetized rats. Cerebral blood flow was measured by two methods: an intracarotid 133Xe clearance technique and tissue sampling after systemic administration of the diffusible tracer, [14C]iodoantipyrine. Brain metabolic responses were estimated by the 2-deoxyglucose method and tissue sampling. Intracarotid infusion of histamine when the blood-brain barrier was intact did not increase cerebral blood flow. Following disruption of the blood-brain barrier by carotid injection of hypertonic urea, histamine evoked dose-dependent increases (133Xe clearance method) in cerebral blood flow to a maximum of 50% (20 μg min−1 kg−1); histamine produced increases in blood flow to areas supplied by the internal carotid artery, e.g., thalamus and parietal cortex ([14C]iodoantipyrine method). Both classes of histamine receptors (H1 and H2) participated in mediating increases in cerebral blood flow after blood-brain barrier opening. Mepyramine (H1-antagonist) and metiamide (H2-antagonist) attenuated blood flow responses to histamine infusion (133Xe clearance method); metiamide was the more effective blocking agent. Pyridylethylamine (H1-agonist) and dimaprit (H2-agonist) both caused increases in regional cerebral blood flow ([14C]iodoantipyrine method). Histamine infusion after blood-brain barrier opening did not increase cerebral glucose consumption. We suggest that increases in cerebral blood flow produced by histamine are the result of stimulation of vascular H1- and H2-receptors, rather than a secondary response to metabolic activation.

Cerebral circulation and histamine: 2. Responses of pial veins and arterioles to receptor agonists

H2-receptors predominantly mediate pial arteriolar dilatation in response to histamine, but the reaction of pial veins to histamine has not been clearly identified. In anesthetized cats, we examined responses of pial veins and arterioles to perivascular microapplication of histamine and specific histamine H1 and H2 receptor agonists. Arterioles were very sensitive to the H2-receptor agonist impromidine, with significant dilatation (+16%) occurring at concentrations as low as 10(-10) M. Arteriolar responses to H1 receptor stimulation by 2.2-pyridylethylamine were small, even at high concentrations. The order of potency and maximum dilatations found for the receptor agonists were: H2 (43%) greater than histamine (28%) greater than H1 (17%). By contrast, pial veins did not respond to histamine or the receptor agonists. The results indicate that pial venomotor activity to histamine is negligible, and suggest a sparse distribution of histamine receptors on the outer surfaces of pial veins.

Gastric and cardiac H-2 receptor effects of dimaprit and N'(Me),5(Me)histamine in conscious dogs

Two new H-2 agonists 5,N'-dimethyl histamine (DMH), an imidazole analog, and dimaprit, a non-imidazole compound, and histamine (H), a mixed H-1, H-2 agonist, were given to four conscious gastric fistula dogs. Gastric acid was stimulated dose responsively. Dimaprit stimulated a 20% greater maximum output of acid, and more pepsin than the other agents, but inhibited pepsin secretion as doses of greater than 0.63 mumol/kg-h. Heart rate was increased dose responsively by all drugs to approximately 200 beats/min with ED50 of 0.17:0.42:0.80 mumol/kg-h (H:DMH:dimaprit). Normalized dose responses showed that histamine was equipotent on acid and heart rate (ED50 0.15 vs. 0.14 respectively) while DMH (ED50 0.21 vs. 0.40) and dimaprit (ED50 0.27 vs. 0.85) stimulated heart rate less effectively than acid. Histamine was more effective at reducing blood pressure, with approximate ED50 of 0.28:0.60:0.96 for H, DMH and dimaprit respectively. The results indicate considerable heterogeneity of histamine responses for different actions mediated by the H-2 receptors, as well as differences for any one action between H-2 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.

Histamine H1- and H2- receptor antagonists reduce histamine-induced increases in vascular permeability and oedema formation in cat skeletal muscle

Intra-arterial fusions of histamine cause vasodilation of resistance vessels, extravasation of albumin and oedema in cat skeletal muscle. Treatment with either mepyramine or metiamide significantly reduced the extravasation of albumin and rate of oedema formation. The doses of mepyramine and metiamide used did not modify the vascular responses to bradykinin.

Vasodilator response to histamine: dependence upon the site of administration

Histamine causes vasodilation in part by interacting with histamine H2 receptors. The present study was undertaken to evaluate the difference in vasodilator sensitivity of H2 receptors on the inside compared to the outside of the vessel. In order to induce tone, norepinephrine was administered to an in vitro preparation of rabbit ear artery treated with mepyramine to block H1 receptors. Histamine was then either selectively perfused through the artery or added to the outside of the vessel via the organ bath. The outside of the artery was found to be twice as sensitive to the vasodilator effect of histamine as the inside. These data provide the first evidence for greater extraluminal sensitivity of vascular smooth muscle and suggest that the response to histamine will depend in part on the route the amine takes to reach receptors, e.g., from blood-borne sources or from extraluminal stores.

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.

The dilating effect of histamine on pial arteries of cats and its mediation by H2 receptors

We studied the effect of histamine and H1 or H2 blockers on the diameter of pial arteries (39-227 micron) using microapplication into the perivascular space. Concentration-response curves for histamine showed dilations which started at 10(-7) M and were maximal at 10(-5) and 10(-4) M. The H2 blocker, cimetidine, induced no vascular reaction over the whole concentration range tested (10(-7) to 10(-3) M). The H1 blocker, mepyramine, was not vasoactive in the concentration range from 10(-7) to 5 X 10(-5) M and evoked dilations at higher concentrations. The concentration-response curve for histamine was only slightly displaced by 10(-5) M mepyramine but was significantly shifted to the right by 10(-5) M cimetidine. The dilating effect of histamine could be reduced in a stepwise manner by increasing concentrations of cimetidine. These findings are in accordance with a selective antagonism between histamine and cimetidine at the H2 receptors of smooth muscle cells of pial arteries. The insignificant role of H1 receptors in histamine-induced dilations is supported by the finding that a combination of H1 and H2 blockers resulted in the same reduction of histamine-induced dilation as did the application of the H2 blocker.

Pharmacological analysis of histamine receptors in musculature and vasculature of the dog trachea in situ

1 The role of histamine H1- and H2-receptors in the musculature and vasculature of the dog trachea was investigated in the blood-perfused trachea in situ. 2 Histamine and acetylcholine caused increases in blood flow (tracheal, vasodilatation) and in intraluminal pressure (tracheal constriction) in a dose-dependent manner. Histamine was almost equipotent to acetylcholine in causing tracheal vasodilatation but was about 30 times less potent in causing tracheal constriction. 3 The histamine H2-receptor agonist, dimaprit, caused a dose-dependent increase in tracheal blood flow but failed to cause tracheal constriction. 4 The tracheal constriction produced by histamine was inhibited strongly by diphenhydramine but not modifed by metiamide. The tracheal vasodilatation produced by histamine was antagonized by both diphenhydramine and metiamide; diphenhydramine was more effective than metiamide. 5 It is concluded that in the tracheal musculature, histamine receptors are exclusively of the H1-type and mediate constriction, whereas in the tracheal vasculature, both histamine H1- and H2-receptors mediate vasodilatation but histamine H1-receptors are predominant.

Spasmolytic action of histamine in airway smooth muscle of horse

Histamine, 2-methylhistamine (a specific H1-agonist), 5-HT, PGF2alpha, SRS-A, bradykinin (BK) and carbachol contract bronchial and tracheal smooth muscles of the horse. Isoprenaline, PGE1, E2, dimaprit and 4-methylhistamine (last two = specific H2-agonists) relaxed airways which were partially contracted to carbachol. Mepyramine (a specific H1-antagonist) selectively antagonized contractions to histamine. In the presence of mepyramine, histamine caused relaxation of airways partially contracted to carbachol. Metiamide and burimamide (specific H2-antagonists) specifically antagonized or reversed histamine-induced bronchorelaxation. However the H2-antagonists, indomethacin and propranolol each failed to block histamine-induced relaxations in trachea. Thus, the results of this study show: (i) preponderence of H1-receptors-mediating contractions in horse airways; (ii) presence of H2-receptors-mediating bronchorelaxation and (iii) the existence of an atypical (relaxant) response (resistance to H2-antagonists; indomethacin and propranolol) in the horse trachea.