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

Function and Role of Histamine H1 Receptor in the Mammalian Heart

Histamine can change the force of cardiac contraction and alter the beating rate in mammals, including humans. However, striking species and regional differences have been observed. Depending on the species and the cardiac region (atrium versus ventricle) studied, the contractile, chronotropic, dromotropic, and bathmotropic effects of histamine vary. Histamine is present and is produced in the mammalian heart. Thus, histamine may exert autocrine or paracrine effects in the mammalian heart. Histamine uses at least four heptahelical receptors: H₁, H₂, H₃ and H₄. Depending on the species and region studied, cardiomyocytes express only histamine H₁ or only histamine H₂ receptors or both. These receptors are not necessarily functional concerning contractility. We have considerable knowledge of the cardiac expression and function of histamine H₂ receptors. In contrast, we have a poor understanding of the cardiac role of the histamine H₁ receptor. Therefore, we address the structure, signal transduction, and expressional regulation of the histamine H₁ receptor with an eye on its cardiac role. We point out signal transduction and the role of the histamine H₁ receptor in various animal species. This review aims to identify gaps in our knowledge of cardiac histamine H₁ receptors. We highlight where the published research shows disagreements and requires a new approach. Moreover, we show that diseases alter the expression and functional effects of histamine H₁ receptors in the heart. We found that antidepressive drugs and neuroleptic drugs might act as antagonists of cardiac histamine H₁ receptors, and believe that histamine H₁ receptors in the heart might be attractive targets for drug therapy. The authors believe that a better understanding of the role of histamine H₁ receptors in the human heart might be clinically relevant for improving drug therapy.

The Roles of Cardiovascular H2-Histamine Receptors Under Normal and Pathophysiological Conditions

This review addresses pharmacological, structural and functional relationships among H₂-histamine receptors and H₁-histamine receptors in the mammalian heart. The role of both receptors in the regulation of force and rhythm, including their electrophysiological effects on the mammalian heart, will then be discussed in context. The potential clinical role of cardiac H₂-histamine-receptors in cardiac diseases will be examined. The use of H₂-histamine receptor agonists to acutely increase the force of contraction will be discussed. Special attention will be paid to the potential role of cardiac H₂-histamine receptors in the genesis of cardiac arrhythmias. Moreover, novel findings on the putative role of H₂-histamine receptor antagonists in treating chronic heart failure in animal models and patients will be reviewed. Some limitations in our biochemical understanding of the cardiac role of H₂-histamine receptors will be discussed. Recommendations for further basic and translational research on cardiac H₂-histamine receptors will be offered. We will speculate whether new knowledge might lead to novel roles of H₂-histamine receptors in cardiac disease and whether cardiomyocyte specific H₂-histamine receptor agonists and antagonists should be developed.

Histamine receptors in heart failure

The biogenic amine, histamine, is found predominantly in mast cells, as well as specific histaminergic neurons. Histamine exerts its many and varied actions via four G-protein-coupled receptors numbered one through four. Histamine has multiple effects on cardiac physiology, mainly via the histamine 1 and 2 receptors, which on a simplified level have opposing effects on heart rate, force of contraction, and coronary vasculature function. In heart failure, the actions of the histamine receptors are complex, the histamine 1 receptor appears to have detrimental actions predominantly in the coronary vasculature, while the histamine 2 receptor mediates adverse effects on cardiac remodeling via actions on cardiomyocytes, fibroblasts, and even endothelial cells. Conversely, there is growing evidence that the histamine 3 receptor exerts protective actions when activated. Little is known about the histamine 4 receptor in heart failure. Targeting histamine receptors as a therapeutic approach for heart failure is an important area of investigation given the over-the-counter access to many compounds targeting these receptors, and thus the relatively straight forward possibility of drug repurposing. In this review, we briefly describe histamine receptor signaling and the actions of each histamine receptor in normal cardiac physiology, before describing in more detail the known role of each histamine receptor in adverse cardiac remodeling and heart failure. This includes information from both clinical studies and experimental animal models. It is the goal of this review article to bring more focus to the possibility of targeting histamine receptors as therapy for heart failure.

Human histamine H2 receptors can initiate cardiac arrhythmias in a transgenic mouse

Histamine is known to lead to arrhythmias in the human heart. A mouse model to mimic these effects has hitherto not been available but might be useful to study the mechanism(s) of H₂-histamine receptor-induced arrhythmias and may support the search for new antiarrhythmic drugs. In order to establish such a model in mice, we studied here the incidence of cardiac arrhythmias under basal and under stimulated conditions in atrial and ventricular preparations from mice that overexpressed the human H₂-histamine receptors in a cardiac-specific way (H₂-TG) in comparison with their wild-type (WT) littermate controls. We had shown before that histamine exerted concentration and time-dependent positive inotropic and positive chronotropic effects only in cardiac preparations from H₂-TG and not from WT. We noted under basal conditions (no drug addition) that right atrial preparations from H₂-TG exhibited more spontaneous arrhythmias than right atrial preparations from WT. These arrhythmias in H₂-TG could be blocked by the H₂-histamine receptor antagonist cimetidine. In a similar fashion, histamine and dimaprit (an agonist at H₂ and not H₁-histamine receptors) more often induced arrhythmias in right atrial preparations from H₂-TG than from WT. To understand better the signal transduction mechanism(s) involved in these arrhythmias, we studied partially depolarized left atrial preparations. In these preparations, a positive inotropic effect of histamine was still present in the additional presence of 44 mM potassium ions (used to block sodium channels) in H₂-TG but not WT and this positive inotropic effect could be blocked by cimetidine and this is consistent with the involvement of calcium ion channels in the contractile and thus might mediate also the arrhythmogenic effects of histamine in H₂-TG. However, compounds reported to release histamine from cells and thereby leading to arrhythmias in humans, namely morphine, ketamine, and fentanyl, failed to induce a more pronounced positive inotropic effect in atrial preparations from H₂-TG compared to WT, arguing against an involvement of histamine release in their proarrhythmic side effects in patients. Measuring left ventricular contractility in isolated retrogradely perfused hearts (Langendorff mode), we detected under basal conditions (no drug application) more spontaneous arrhythmias in hearts from H₂-TG than from WT. In summary, we noted that overexpression of human H₂-histamine receptors in a novel transgenic animal model can lead to arrhythmias. We suggest that this model might be useful to understand the mechanism(s) of histamine-induced cardiac arrhythmias in humans better in a molecular way and may be of value to screen novel antiarrhythmic drugs.

Fluorescent H2 Receptor Squaramide-Type Antagonists: Synthesis, Characterization, and Applications

Fluorescence labeled ligands have been gaining importance as molecular tools, enabling receptor-ligand-binding studies by various fluorescence-based techniques. Aiming at red-emitting fluorescent ligands for the hH₂R, a series of squaramides labeled with pyridinium or cyanine fluorophores (19-27) was synthesized and characterized. The highest hH₂R affinities in radioligand competition binding assays were obtained in the case of pyridinium labeled antagonists 19-21 (pK _i: 7.71-7.76) and cyanine labeled antagonists 23 and 25 (pK _i: 7.67, 7.11). These fluorescent ligands proved to be useful tools for binding studies (saturation and competition binding as well as kinetic experiments), using confocal microscopy, flow cytometry, and high content imaging. Saturation binding experiments revealed pK _d values comparable to the pK _i values. The fluorescent probes 21, 23, and 25 could be used to localize H₂ receptors in HEK cells and to determine the binding affinities of unlabeled compounds.

Involvement of tyrosine phosphorylation in the positive inotropic effect produced by H(1)-receptors with histamine in guinea-pig left atrium

We investigated the effect of stimulation of H(1)-receptors with histamine on protein tyrosine phosphorylation levels in guinea-pig left atrium and evaluated the influences of tyrosine kinase inhibitors on the positive inotropic effect mediated by H(1)-receptors in this tissue. Histamine induced an increase in tyrosine phosphorylation in four main clusters of proteins with apparent molecular weights of 25, 35, 65 and 150 kDa. Tyrosine phosphorylation of these proteins attained a peak around 2 - 3 min following histamine stimulation and then declined to or below basal levels. Histamine-induced protein tyrosine phosphorylation was antagonized by the H(1)-receptor antagonists mepyramine (1 microM) and chlorpheniramine (1 microM), but not by the H(2)-receptor antagonist cimetidine (10 microM). The positive inotropic effect of histamine was depressed in a concentration-dependent manner by the tyrosine kinase inhibitors tyrphostin A25 (50 to 100 microM) and genistein (10 to 50 microM) but not by the inactive genistein analogue daidzein (50 microM). The positive inotropic effect of isoprenaline was unchanged by tyrphostin A25 and genistein. At a concentration of 1 microM histamine produced a dual-component positive inotropic response composed of an initial increasing phase and a second and late developing, greater positive inotropic phase. Treatment with tyrphostin A25 (100 microM) and genistein (50 microM), but not daidzein (50 microM), significantly attenuated the two components of the inotropic response, although genistein suppressed the initial component more markedly than the late component. We conclude that increased protein tyrosine phosphorylation may play an important role in initiating at least some part of the positive inotropic effect of H(1)-receptor stimulation in guinea-pig left atrium.

Diazepam potentiates the positive inotropic effects of histamine and forskolin in guinea-pig papillary muscles

There have been diverse reports on the effects of diazepam on cardiac contractility. The purpose of this study was to examine whether diazepam modifies the inotropic response elicited by histamine on an isolated guinea-pig papillary muscle. The responses of electrically driven papillary muscle to histamine and cyclic AMP-related inotropic agents were recorded in the absence and in the presence of diazepam. Histamine and forskolin, which directly stimulate adenylate cyclase, significantly increased the contractile force in the papillary muscle in a concentration-dependent manner. A histaminergic H2-receptor antagonist, cimetidine, but not a H1-receptor antagonist, diphenhydramine, at 10 microM produced a rightward shift in the concentration-response curve for histamine. Diazepam (10 microM) shifted the concentration-response curve for histamine and forskolin to the left by 1.8 and 1.6 times, respectively. Neither a central type (fulmazenil) nor a peripheral type (PK11195) of benzodiazepine receptor antagonist modified the effect of diazepam on the histaminergic-evoked contraction. Phosphodiesterase blockade by 3-isobutyl-1-methylxanthine shifted the concentration-dependent curve for histamine to the left. A combination of 3-isobutyl-1-methylxanthine also produced a leftward shift of the curve. However, there was no significant difference between the 3-isobutyl-1-methylxanthine only group and the combination group. These results indicate that diazepam potentiates the positive inotropic effect produced by histamine, probably mediated via an increase in cyclic AMP levels induced by histamine.

Histamine H1-receptor-mediated increase in the Ca2+ transient without a change in the Ca2+ current in electrically stimulated guinea-pig atrial myocytes

The effects of histamine on the intracellular Ca2+ concentration ([Ca2+]i), action potential and membrane currents were assessed in single atrial myocytes prepared from guinea-pigs. Histamine caused a concentration-dependent increase in the [Ca2+]i transient in indol/AM loaded myocytes when stimulated electrically at 0.5 Hz. However, the maximum increase in [Ca2+]i transient produced by histamine was less than 50% of that elicited by isoprenaline. The histamine-induced increase in [Ca2+]i transient was significantly inhibited by chlorpheniramine, but not by cimetidine. Pretreatment with nifedipine nearly completely suppressed the histamine-induced increase in [Ca2+]i transient. Cyclopiazonic acid did not affect the histamine response. In the whole-cell current-clamp mode of the patch-clamp method, both histamine and isoprenaline prolonged action potential duration (APD) in atrial myocytes. In the presence of Co2+ or nifedipine, the isoprenaline-induced APD prolongation was abolished and an APD shortening effect was manifested, while histamine still increased APD. The APD prolongation elicited by histamine was reversed by chlorpheniramine. In the voltage-clamp mode, the histamine-sensitive membrane current was inwardly rectifying and reversed close to the calculated value of the K+ equilibrium potential. Histamine had no apparent effect on L-type Ca2+ current, in contrast to the pronounced effect of isoprenaline. These results indicate that in guinea-pig atrial myocytes stimulation of H1-receptors with histamine does not directly activate Ca2+ channels but causes an elevation of [Ca2+]i transient by increasing Ca2+ influx through the channels during the prolonged repolarization of action potentials resulting from inhibition of the outward K+ current.

Increases in cyclic AMP levels couple to H1 receptors in atria from autoimmune myocarditis mice

We have previously shown that myocardium from experimental autoimmune myocarditis expresses H1 receptors not present in normal mice heart. ThEA acting via H1 receptors, augments cyclic AMP production in atria from autoimmune myocarditis mice without any effect on atria from control mice. Addition of mepyramine before ThEA caused cyclic AMP levels to fall to a level similar to basal, confirming the H1 receptor participation. Histamine at low concentrations mimicked the ThEA action on H1 receptor-stimulation of cyclic AMP production by autoimmune myocardium. The fact that the inhibition of phospholipase C blocked the cyclic AMP stimulation by ThEA, supports the assumption that this action is secondary to receptor-mediated hydrolysis of phosphoinositides, generating some oxidative metabolites (IP3-DAG), which in turn may be responsible for the cyclic AMP effect. So, the inhibition of protein kinase C and calcium/calmodulin partially prevented the stimulatory action of ThEA on cyclic AMP levels in autoimmune myocardium, suggesting that both pathways are implicated in this effect. Data shows that the stimulation of H1 receptors by specific agonist in atria from autoimmune myocarditis mice, augments the cyclic AMP, requiring the hydrolysis of phosphoinositide cycle. The role of this cyclic AMP augmentation in myocardium from autoimmune myocarditis mice, will provide a basis to assess the role of this second messenger as an important factor in the regulation and/or modulation of the physiological behaviour of the heart in the course of autoimmune myocarditis.

Effect of histamine in autoimmune myocarditis mice

The contractile effect of histamine, as well as the H1 receptor population and H2 receptor-mediated cAMP production, were measured in cardiac tissue from control normal and autoimmune myocarditis mice. Histamine triggered positive chronotropy and negative inotropy at high concentrations in both control and autoimmune auricles, H2 receptors being the most important mediator of these responses. In contrast, in atria from autoimmune myocarditis mice, histamine at lower concentrations caused positive inotropy and negative chronotropy. These effects, not verified in the normal control atria, are mediated by H1 receptors. The expression of H2 and H1 receptors mediating the cardiac response to histamine was evaluated through histamine-stimulated cAMP level and binding of [3H] mephyramine, respectively. Both control and autoimmune myocardium were able to increase cAMP levels, an effect that was inhibited by H2 antagonist drug. The amount of cAMP was significantly higher in control myocardium than in those from autoimmune ones. Saturable binding of [3H] mephyramine occurs in autoimmune myocardium, with distinct high and low affinity binding sites. In control myocardium non-saturable binding was detected. These results suggest that H1 and H2 receptors coexist in heart from autoimmune myocarditis mice, whereas only H2 receptors are present in myocardium from control mice. The presence of H1 receptors in autoimmune myocardium could be an important factor in the regulation of its physiological behaviour.

Histamine receptors mediating a positive inotropic effect in guinea pig and rabbit ventricular myocardium: distribution of the receptors and their possible intracellular coupling processes

The difference in histamine receptor subtypes that are involved in the positive inotropic effect of histamine in guinea pig and rabbit ventricular myocardium was analytically characterized. In guinea pig papillary muscles, the positive inotropic effect of histamine was antagonized by cimetidine but not by mepyramine. The converse was true in rabbit papillary muscles. However, histamine evoked a positive inotropic effect through H1- and H2-receptors after blockade of H2- and H1-receptors in guinea pig and rabbit papillary muscles, respectively. Adenylate cyclase was significantly activated by histamine via H2-receptors in guinea pig but not in rabbit myocardial ventricular membranes. Accumulation of [3H]inositol monophosphate in ventricular strips prelabeled with myo-[3H]inositol was increased by histamine via H1-receptors to a similar extent in rabbits and guinea pigs. Radioligand binding experiments with [3H]mepyramine and [3H]tiotidine showed an increased number of H1-receptors and a decreased number of H2-receptors in guinea pig compared with rabbit ventricular myocardium. These results suggest that the positive inotropic effects of histamine are dominated by an H1-receptor-mediated effect in rabbits and by an H2-receptor-mediated one in guinea pig ventricular myocardium, and the positive inotropic effect manifested by one subtype apparently restricts the expression of the positive inotropic effect mediated by the other subtype. This species difference is not due to a difference in densities of the receptor subtypes, but may be partly related to a difference in the extents of coupling of H2-receptors to adenylate cyclase.

Changes in the effects of nizatidine and famotidine on cardiac performance after pretreatment with ranitidine

This was an open, randomized study of the cardiovascular effects of the histamine H2 receptor antagonists ranitidine, famotidine, and nizatidine after single oral doses alone or in combination in healthy volunteers. When compared with placebo ranitidine (450 mg) did not have any haemodynamic effects. Nizatidine (300 mg) caused significant falls in heart rate and cardiac output. Famotidine (40 mg) caused significant falls in stroke volume and cardiac output and an increase in pre-ejection period. Pretreatment with ranitidine abolished the haemodynamic effects of nizatidine and caused a time-shift of 2 h in the onset of the cardiovascular effects of famotidine. The difference in the results for nizatidine and famotidine can be explained by the longer half-life of famotidine. Vascular effects are assumed to be responsible for impairment of cardiac performance by famotidine.

Electrophysiological characterization of histamine receptor subtypes in sheep cardiac Purkinje fibers

The histamine-receptor-subtype-mediated effects on action potentials of electrically driven and spontaneously active isolated sheep cardiac Purkinje fibers were investigated using H1- and H2-selective agonists and antagonists. In electrically stimulated Purkinje fibers, histamine (3 mumol/l) increased the action potential plateau height, decreased the action potential duration measured at a repolarization level of -60 mV and enhanced the pacemaker activity. These effects were abolished by the H2-selective antagonist cimetidine (30 mumol/l), but were not impaired by the H1-selective antagonist dimetindene (0.3 mumol/l). In spontaneously active Purkinje fibers, histamine (10 mumol/l) increased the spontaneous rate by 24%, the slope of diastolic depolarization by 45% and shortened the duration of the diastole by 32% of the respective control measurements. These effects were blocked by 30 mumol/l cimetidine, but remained unchanged in the presence of 0.3 mumol/l dimetindene. Concentration-response curves of histamine were shifted to the right by approximately 2 logarithmic units in the presence of 30 mumol/l cimetidine, but were not influenced in the presence of 0.3 mumol/l dimetindene. The H2-selective agonist impromidine (0.001-0.3 mumol/l) had similar actions as histamine on spontaneously active Purkinje fibers, while the H1-selective agonist 2-(2-pyridyl-)ethylamine was ineffective. It is concluded that the pronounced stimulatory action of histamine on spontaneous activity in sheep cardiac Purkinje fibers is exclusively mediated by H2 receptors.

Identification and characterization of histamine H1- and H2-receptors in guinea-pig left atrial membranes by [3H]-mepyramine and [3H]-tiotidine binding

1. Histamine receptors in the membranes prepared from guinea-pig left atria were characterized with [3H]-mepyramine and [3H]-tiotidine binding. 2. The binding of the H1-antagonist, [3H]-mepyramine, was saturable and of high affinity with a maximum binding capacity of 307 +/- 27 fmol mg-1 protein (n = 14) and with an equilibrium dissociation constant (KD) of 1.5 +/- 0.2 nM (n = 14). The binding was rapid and readily reversible. 3. The competition curve for [3H]-mepyramine binding by histamine was biphasic and revealed high and low affinity states of binding. The addition of 5'-guanylylimidodiphosphate (GppNHp) (100 microM) converted this heterogeneous binding into homogeneous binding of low affinity. 4. The competition curves of H1-antagonists with [3H]-mepyramine had Hill coefficients not significantly different from unity, consistent with competition with [3H]-mepyramine at a single site. GppNHp did not shift the competition curves. 5. Dissociation constants for H1-antagonists determined from inhibition of [3H]-mepyramine binding correlated well with the constants derived from inhibition of the positive inotropic response of guinea-pig left atria to histamine. 6. The H2-antagonist, [3H]-tiotidine, labelled an apparently homogeneous population of recognition sites with a maximum binding capacity of 41 +/- 8 fmol mg-1 protein (n = 6) and a KD of 10.8 +/- 1.2 nM (n = 6). 7. Although histamine competed for [3H]-tiotidine binding in a concentration-dependent manner, the curve was monophasic and was not shifted by GppNHp. 8. It is concluded that both H1- and H2-receptors exist in guinea-pig left atria. H1-receptors probably couple to intracellular effector(s) through a guanine nucleotide-dependent transducing mechanism. On the other hand, H2-receptors seem unlikely to be linked to guanine nucleotide regulatory proteins in guineapig left atria, which may explain the failure of histamine to cause an increase in cyclic AMP in spite of the presence of H2-receptors.

Characterization of histamine receptors modulating inotropic and biochemical activities in rabbit left atria

The experiments were performed to identify histamine H1- and H2-receptors in rabbit left atrium and to characterize the pharmacological properties mediated by the respective subtypes of histamine receptors. High-affinity saturable binding to the left atrial membranes was obtained for [3H]mepyramine, yielding a maximum binding capacity (Bmax) of 96 fmol/mg of protein and an equilibrium dissociation constant (KD) of 3.8 nM and also for [3H]tiotidine, yielding a Bmax of 126 fmol/mg of protein and a KD of 14.7 nM. In isolated left atrium, histamine produced a concentration-dependent positive inotropic effect, an effect which was competitively antagonized by cimetidine but not altered by chlorpheniramine. Schild analysis showed that the pA2 value for cimetidine was 6.55 and the slope was not significantly different from unity. An excellent correlation was found between the increase in force of contraction and cyclic AMP in the presence of histamine, suggesting that the positive inotropic effect of histamine in rabbit left atrium is dependent on an increased level of intracellular cyclic AMP through stimulation of histamine H2-receptors. Histamine also produced concentration-dependent stimulation of phosphoinositide hydrolysis as measured by [3H]inositol monophosphate accumulation. The phosphoinositide response to histamine was blocked by chlorpheniramine and mepyramine but not by cimetidine. The data indicate that histamine H1-receptors, in addition to histamine H2-receptors, are present in the rabbit left atrium. Although this tissue lacks an inotropic response to histamine H1-receptor stimulation, the histamine H1-receptors interact with histamine to mediate the stimulation of phosphoinositide hydrolysis.

Responses to histamine and selective H2-receptor agonists in lung parenchymal strips from normal and sensitized guinea-pigs

Histamine produces concentration-dependent contractions of lung parenchyma strips obtained from normal and sensitized guinea-pigs. The responsiveness of the sensitized lung strips to histamine was significantly increased compared to normal tissues. Clemizole (0.1 microM) was equally effective as an H1-antagonist in normal (dose-ratio 9.12) and sensitized (dose-ratio 9.77) tissues. The concentration-response curves to histamine were displaced to the left by cimetidine (0.1 microM to 0.1 mM) with similar dose-ratios in normal and sensitized tissues. Cimetidine enhanced maximal responses to histamine only in normal lung strips. The effects of submaximal equieffective concentrations of histamine were augmented to the same extent by cimetidine (0.1 mM) in normal and sensitized tissues. The responses to histamine were not modified by indomethacin (5 microM). The responsiveness and sensitivity of sensitized lung strips to isoprenaline, impromidine, 4-methyl-histamine and dimaprit were not different from those of normal tissues. Cimetidine yielded, as antagonist of dimaprit, similar pA2 values in normal and sensitized tissues. In conclusion, there is no experimental evidence in favour of the existence of an impairment of H2-receptor activity in sensitized airways. Hyperreactivity to histamine is probably due to differences between normal and sensitized tissues with respect to Ca2+ entry and/or intracellular Ca2+ release in response to H1-receptor activation.

Differential effects of histamine mediated by histamine H1- and H2-receptors on contractility, spontaneous rate and cyclic nucleotides in the rabbit heart

The effects of histamine on the contractile force, spontaneous rate of contraction, and cyclic AMP and cyclic GMP content were investigated in isolated rabbit cardiac preparations. Histamine had a positive inotropic effect in the left atrium and papillary muscle, and a positive chronotropic effect in the right atrium. Both effects were produced in a concentration-dependent manner. Impromidine also induced the same effect in the left and right atrium as histamine did. The effects produced by histamine and impromidine were antagonized by cimetidine and tiotidine. On the other hand, the positive inotropic response of papillary muscle to histamine was antagonized by mepyramine and chlorpheniramine and was mimicked by 2-(2-pyridyl)ethylamine. Impromidine at a high concentration induced a small increase in the contractile force, an effect which was antagonized by cimetidine. Histamine significantly increased the cyclic AMP levels in both atria but not in papillary muscles. The increase in cyclic AMP was abolished by cimetidine. Histamine also increased cyclic GMP levels in all of the preparations. The increase in cyclic GMP was abolished by chlorpheniramine. The results suggest that both H1- and H2-receptors exist in all parts of the rabbit heart. However, the positive inotropic and chronotropic effects induced by histamine in left and right atrium are mediated predominantly via H2-receptors, whereas the positive inotropic effect in papillary muscle is predominantly mediated via H1 receptors.

Indirect evidence for a role of phosphatidylinositol turnover in the cardiac response to H1-receptor stimulation

The influence of lithium on the positive inotropic effect of the H1-agonist 2-pyridyl-ethylamine (PEA) and of the H2-receptor agonist 4-methylhistamine was studied in isolated guinea-pig ventricular strips electrically stimulated at 1 Hz. Lithium (1-10 mM) was devoid of any effect on cardiac contraction; the positive inotropic effect of 4-methylhistamine was unaffected in the presence of 10 mM lithium. On the other hand, lithium (1-10 mM) dose-dependently shifted the dose-inotropic effect curve for PEA to the right; an antagonistic effect, qualitatively similar to that of lithium, was induced by the myoinositol antagonist 2-2'-anhydro-2-C-hydroxymethyl-myoinositol, at a concentration of 100 microM. Moreover the antagonistic effect of the higher lithium concentration (10 mM) was almost completely prevented in preparations superfused with 10 mM myoinositol. Since it is known that lithium is able to reduce the cellular availability of myoinositol by an interference with the phosphatidylinositol (PI) cycle, these results suggest that the H1-receptor-mediated increase in contractility may be linked to an increased turnover of PI, while the H2-receptor-mediated one is not.

Effects of Ca2+ channel antagonists and ryanodine on H1-receptor mediated electromechanical response to histamine in guinea-pig left atria

Effects of organic Ca2+ channel antagonists, Ni2+ and ryanodine on the electrophysiological and positive inotropic responses to histamine were examined in isolated guinea-pig left atria. Histamine increased force of contraction, prolonged action potential duration (APD) and hyperpolarized the membrane in a concentration-dependent manner. Histamine at a concentration of 1 mumol/l produced a dual-component positive inotropic response composed of an initial increasing phase (initial component) and a second and late developing, greater positive inotropic phase (second component), whereas causing monophasic changes in APD and resting potential. The electrophysiological and dual-component positive inotropic effects induced by histamine were antagonized by chlorpheniramine (1 mumol/l) but not by cimetidine (10 mumol/l), indicating that both effects are exclusively mediated by H1-receptors. The positive inotropic response to 1 mumol/l histamine was changed by the pretreatment with nifedipine (1 mumol/l) and nisoldipine (1 mumol/l). In the presence of these dihydropyridines, the second component was almost completely abolished, while the initial component was hardly affected. On the other hand, verapamil (3 mumol/l) and diltiazem (10 mumol/l) failed to modify the multiphasic inotropic response to histamine. None of the Ca2+ channel antagonists affected the histamine-induced APD prolongation. In the presence of Ni2+ at a concentration of 0.3 mmol/l, at which it produced no negative inotropic action, the second component of the positive inotropic effect of histamine was specifically suppressed whereas the histamine-induced APD prolongation was unaffected. Preferential attenuation of the second component was also observed in the presence of 30 nmol/l ryanodine.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of leukotrienes and histamine by the isolated anaphylactic heart

The purpose of this study was to demonstrate the ability of heart tissue to release the mediators of anaphylaxis after antigenic challenges. Guinea pigs were sensitized with ovalbumin. Hearts were excised, perfused in a langendorff apparatus, and challenged with a bolus injection of ovalbumin. Analysis of the perfusates demonstrated the presence of histamine as determined by radioenzymatic assay. Histamine release was observed to be maximum after 2 min (8 +/- 1 nmol) of perfusion, then decreased to baseline level. The heart also released LTB4, LTC4, LTD4, and LTE4 as determined by high performance liquid chromatography and bioassays. The release of LTC4 occurred rapidly, reaching maximum after 2 min (4.2 +/- 1 pmol) and then returned to baseline level. Although the release of LTD4 paralleled the release of LTC4, it reached a maximum after 5 min (7.7 +/- 2 pmol). LTE4 was detected after 10 min and was undetectable after 15 min. Maximum release of LTB4 was observed after 5-10 min (15 +/- 3 pmol) and was no longer detectable after 15 min. These results indicate that the isolated sensitized heart undergoing antigenic challenge releases leukotrienes and histamine suggesting the cardiac anaphylaxis might occur by the locally released mediators.

The effects of histamine on contraction frequency, sodium influx, and cyclic AMP in cultured rat heart cells

Histamine has been shown to have both positive inotropic and chronotropic effects. To evaluate the chronotropic effects, spontaneously contracting monolayers of cultured rat myocardial cells were treated with histamine, 10(-7) M-10(-4) M. This resulted in a dose-dependent increase in contraction frequency reaching a maximum in 10(-5) M histamine. Contraction frequency (mean +/- SEM) increased from a control of 121 +/- 5 contractions per minute to 153 +/- 4.5, 181 +/- 9, 212 +/- 4, and 216 +/- 1 in 10(-7) M, 10(-6) M, 10(-5) M, and 10(-4) M histamine, respectively (for each n = 10, p less than 0.001). The effect was time-dependent, taking 30 minutes to develop fully. Changes in contraction frequency were accompanied by parallel dose- and time-dependent increases in the verapamil-sensitive sodium influx. Verapamil-sensitive sodium influx (pmol/cm2/sec) increased from a control of 10.45 +/- 1.44 (mean +/- SEM) to 24.34 +/- 2.41 and 32.57 +/- 2.35 at 10- and 30-minute treatment with 10(-6) M histamine (n = 5, p less than 0.001). These data fit the previously described relation between verapamil-sensitive sodium influx and contraction frequency in these cells. Cimetidine (10(-4) M) but not diphenhydramine (10(-4) M) abolished both the contraction frequency and sodium influx response to histamine. Subsequent studies showed a dose- and time-dependent elevation of cyclic adenosine monophosphate (cAMP) with histamine treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of competitive antagonism when this property occurs as part of a pharmacological resultant

In this paper, pharmacological resultant is defined as the net effect of a single compound resulting from the simultaneous expression of two or more specific actions. The principles of concentration-ratio analysis are extended to develop a method for detecting and quantifying competitive antagonism when this property is a component of a pharmacological resultant. The method is general to the extent that it allows analysis of competitive antagonism in combination with all types of post-receptor intervention. Essentially it depends on the altered expression of competition by a reference antagonist. It incorporates tests for validating its application and it is independent of agonist concentration-effect curve shape: in these respects the method is analogous to Schild plot-analysis of simple competition. The methodology for the practical application of the analysis is exemplified by studying the net effect of a combination of a phosphodiesterase inhibitor (isobutylmethylxanthine) and histamine H2-receptor antagonist (metiamide) on histamine-stimulated tachycardia in guinea-pig, isolated, right atrium. Cimetidine was used as the reference antagonist. The equation used in this analysis is similar in form to one recently described by Hughes & Mackay (1985) to elucidate the situation when competitive antagonism occurs in combination with functional interactions. The relation between their method and the present analysis is discussed.

Electrophysiological characterization of histamine receptor subtypes in mammalian heart preparations

Histamine-induced electrophysiological effects have been investigated in guinea-pig left atria, papillary muscles and rabbit AV-nodal preparations by means of intracellular recording of action potentials, slow responses in the presence of 27 mmol/l (K+)o and voltage clamp experiments. Differentiation of the H-receptor subtypes was performed by the use of the H2-selective agonists dimaprit and impromidine and the H1- and H2-selective antagonists dimetindene and cimetidine, respectively. The following results were obtained: Histamine and the H2-agonists dimaprit and impromidine show similar actions on electrophysiological parameters of ventricular myocardium. Histamine at concentrations less than 1 mumol/l leads to a small increase in APD30 and APD90, but to a marked decrease at concentrations greater than or equal to 1 mumol/l, whereas Vmax, resting potential and amplitude remain nearly unchanged. The effects on ADP are completely blocked by cimetidine and not changed by dimetindene. Changes in action potential may be explained by an increased in slow inward current and outward currents as shown by voltage clamp experiments. In left atria histamine increases APD30 and APD90, whereas there is only a minor increase in amplitude with no changes in Vmax and resting potential. These effects are completely reversed by the H1-antagonist dimetindene but not by cimetidine. IBMX decreases APD90 and does not potentiate the action of histamine. Vmax of slow responses is increased in left atria by stimulation of H1-receptors and in papillary muscles by stimulation of H2-receptors. The results suggest that stimulation of atrial H1-receptors directly causes an increase in Ca-channel conductance.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain histamine response to stress in 12 month old rats

The regional brain histamine regulation in response to stress was investigated in 12 month old Sprague-Dawley male rats. Air blast exposure (15 min) induced significant (26.5%) elevation in hypothalamic HA level; midbrain and cortical HA concentrations were not affected. Histamine methyltransferase activity was not altered by stress in any of the brain regions investigated. Plasma corticosterone levels of stressed rats were significantly elevated (6.5 fold). Hence, the response of hypothalamic HA to stress is still evident in 12 month old rats.

Electrophysiological actions of histamine and H1-, H2-receptor antagonists in cardiac tissue

Electrophysiological investigations of histamine in different cardiac tissues have led to the following results: Histamine and the H2-agonists dimaprit and impromidine show similar actions on electrophysiological parameters of ventricular myocardium (especially a decrease in action potential duration), which are completely blocked by cimetidine and enhanced by the phosphodiesterase inhibitor 1-methyl,3-isobutylxanthine (IBMX). These effects may be explained by an increase in cellular cAMP leading to an increase in slow inward current and outward currents as shown by voltage clamp experiments. Histamine in contrast to IBMX increases action potential duration at 90% repolarization (APD90) in atria. Histamine effects in atrial myocardium are completely reversed by the H1-antagonist dimetindene. Stimulation of atrial H1-receptors is suggested to directly cause an increase in Ca-channel conductance independent of intracellular cAMP content. Histamine reduces AH-interval, increases V max of NH-cells and may induce AV-node arrhythmias (at concentrations greater than or equal to 3 mumol/l). These effects remain unchanged by dimetindene, but are reversed by cimetidine. The results indicate that histamine increases AV-nodal conduction via H2-receptors. Unspecific membrane actions of cimetidine are not observed up to 100 mumol/l. Dimetindene increases action potential duration (APD) in left atria and decreases Vmax at concentrations greater than or equal to 10 mumol/l. However, H1-antagonistic actions of dimetindene are already observed at concentrations 1,000 to 10,000 times lower (pA2-values 8.39-9.12) so that unspecific membrane actions are suggested not to occur on a therapeutic dose level.

On the presence of H1-receptors in various sections of guinea-pig heart: a correlation between binding and functional studies

The binding of 3H-mepyramine in different sections of guinea-pig heart was examined. 3H-mepyramine binds to a single class of binding sites to guinea-pig ventricular membranes and to right atrial suspension with an apparent dissociation constant (Kd) of 4.35 nM and 14.90 nM respectively. When treated as those obtained from the right atrium, the left atrial suspensions do not seem to bind 3H-mepyramine specifically.

Effect of cyclic AMP, db-cyclic AMP and phosphodiesterase inhibitors on histamine inhibition of the contractile response of the mouse vas deferens

Histamine inhibited the contractile response of the mouse vas deferens to various frequencies of stimulation, and the inhibition was inversely related to the frequency of stimulation. This effect of histamine was mimicked by cyclic AMP, db-cyclic AMP and various phosphodiesterase inhibitors (IBMX, aminophylline and theophylline). Histamine-produced inhibition, but not that produced by the other compounds, was blocked by cimetidine. The low concentrations (10-100 microM) of various phosphodiesterase inhibitors caused inhibition but failed to potentiate the inhibitory response to histamine. The basal cyclic AMP levels of the tissues were unaffected at these concentrations of aminophylline. At higher concentrations (1 and 5 mM), however, aminophylline significantly elevated the basal cyclic AMP levels of the tissues and markedly inhibited the contractile response to various frequencies of stimulation but still failed to enhance or potentiate the inhibitory response to histamine. In fact, the inhibitory response to histamine at these concentrations of aminophylline was reduced. Based on these and earlier [3] findings, it is concluded that, although histamine increases the accumulation of cyclic AMP in the mouse vas deferens and although its inhibitory effect on the preparation can be mimicked by both the cyclic nucleotides and phosphodiesterase inhibitors, the involvement of cyclic AMP in the mediation of its inhibitory response is still unresolved.

Effects of histamine on mechanical performance and biochemical and electrical activity in the heart of monkeys (Macaca fuscata)

The properties of the cardiac effects of histamine on the isolated heart muscles of the Japanese monkey (Macaca fuscata) were investigated. Histamine had a concentration-dependent positive inotropic effect on left atria and papillary muscles and a positive chronotropic effect on right atria. Histamine increased the levels of cyclic AMP and shortened the duration of the action potential in Purkinje fibers, measures of its biochemical and electrophysiological effects. All of these effects of histamine were blocked by 10(-5) M cimetidine. These results indicate that histamine H2-receptors mediate the cardiac effects of histamine on the monkey heart. Histamine (10(-5) M) also restored the action potentials and contractility of K+-depolarized preparations. These effects were inhibited by verapamil (10(-6)-10(-5) M) but not by TTX (10(-5) M), suggesting that, in the monkey heart, histamine may act by increasing the slow inward current.

Modification of isolated atrial chronotropic histamine receptor responses by alterations in the calcium concentration of the bathing medium

Rabbit and guinea pig atrial pairs were tested in 0.9, 1.8, and 3.6 mM calcium Tyrodes using histamine chronotropic dose response curves, before and after treatment with cimetidine or diphenhydramine. Increased calcium levels alone caused an increase in spontaneous rate: the rate of rabbit atria increased 27 beats/min, and that of guinea pig atria 29 beats/min, between the 0.9 and 3.6 mM levels. Both H1-and H2-blockers tested with rabbit atria caused competitive inhibition in 0.9 mM Tyrodes and noncompetitive inhibition in 1.8 mM Tyrodes. In the in 3.6 mM medium, before blockade, the concentration of histamine needed to cause a half maximum increase in rate doubled, strongly suggesting a decrease in histamine's affinity for its receptor. In addition, after blockade, cimetidine acted as a competitive inhibitor, but diphenhydramine did not. In contrast, the guinea pig atria responses were not much affected by the calcium concentration in the bathing medium. Both blockers appeared to act as competitive inhibitors, except in 1.8 mM calcium Tyrodes, in which diphenhydramine acted as a noncompetitive inhibitor. Rabbit atria have both H1- and H2-chronotropic histamine receptors, while guinea pig chronotropic receptors are predominantly of the H2 type. This data suggests that binding to the H1-receptor is in part dependent on calcium while H2-receptor binding appears to be relatively independent of calcium.

Cardiac effects of the new H2-receptor antagonists

A series of new H2-receptor antagonists were tested for their effects on different isolated heart preparations. In the guinea-pig atria and papillary muscle the inhibitory effect on histamine H2-receptors was evaluated. In the perfused rabbit heart and in strips of human atria the effect of the H2-antagonists on the spontaneous or electrically-stimulated contractions was evaluated. In the first two preparations some main quantitative differences were pointed out, tiotidine and compound SKF 93479 being the most potent antagonists, cimetidine, metiamide and ranitidine the less effective. In the rabbit heart and in human atria results were quite different: cimetidine and ranitidine were virtually ineffective up to the maximum concentration tested (3 x 10(-3) M), oxmetidine and compound SKF 93479 had a negative inotropic and chronotropic effect starting from concentrations of 3 x 10(-6)-10(-5) M. On the basis of the behaviour of other compounds endowed with negative cardiac effects (propranolol, anaesthetic-like compounds, verapamil) and of that of compounds capable of counteracting the effect of oxmetidine (increased concentration of calcium ions and isoproterenol) it was hypothesized that oxmetidine may interfere in the transport of calcium ions. Our data emphasize the importance of the different structure of the H2-antagonists in determining non-specific effects absolutely independent of the primary action that is the H2-receptor blockade.

Guinea-pig and rabbit papillary muscles differ in their response to histamine

Guinea-pig and rabbit papillary muscles differ in their response to histamine. 1. Basal developed force (BDF) and maximal developed force (MDF) following isoproterenol and histamine were determined in rabbit and guinea-pig papillary muscles under a variety of experimental conditions. Hypothermia and an increase in the calcium concentration increased the BDF in both tissues. 2. In the guinea-pig MDF increased to a similar extent following both agonists. In the rabbit the MDF response to histamine was much less than the response to isoproterenol. 3. In the guinea-pig papillary muscle, stimulation of both beta- and H2-receptors produces an increase in cyclic AMP. The rabbit papillary muscle contains H1-receptors, stimulation of which has no effect on cyclic AMP. The reduced MDF effect of histamine compared to isoproterenol in the rabbit may be due to the differing biochemical events following beta- or H1-stimulation.

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?

Inhibition by the tetramine disulphide, benextramine, of cardiac chronotropic histamine H2-receptor-mediated effects

1 Benextramine (N,N1-bis[o-methoxybenzylamino)-n-hexyl]cystamine), which irreversibly blocks alpha-adrenoceptors and does not inhibit the H1-receptor-mediated contractile effect of histamine on guinea-pig isolated ileum, also did not inhibit the H1-receptor-mediated inotropic effect of histamine on guinea-pig isolated atrium. 2 Benextramine irreversibly inhibited the H2-receptor-mediated chronotropic effect of histamine on guinea-pig isolated atrium. 3 Since its combination with the competitive H2-receptor blocking drug cimetidine had a additive blocking effect, benextramine appears to act directly on the chronotropic H2-receptor.

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.

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.

Effect of some new Histamine H2-receptor antagonists on the guinea-pig papillary muscle

Several histamine H2-receptor antagonists (cimetidine, ranitidine, oxmetadine and tiotidine) were tested for their activity on the papillary muscle of the guinea pig stimulated by histamine. All of the compounds exerted a competitive antagonism against histamine the order of potency being tiotidine greater than oxmetidine greater than ranitidine greater than cimetidine. Oxmetidine was the only drug which at high concentrations (10-6 M) decreased in maximum response of histamine probably because of non specific effects of the molecule already described in the literature. As it was expected, the H1-receptor antagonist, mepyramine, exerted a non-competitive antagonism.

The effect of phosphodiesterase inhibitors on guinea-pig cardiac responses to histamine and isoprenaline

(1) The effect of phosphodiesterase inhibitors was studied on right artial rate, left atrial tension and left ventricular papillary muscle tension responses to histamine and isoprenaline. (2) Only responses mediated via beta-adrenoceptors and H2-receptors were potentiated by theophylline. This is proposed to be due to its phosphodiesterase inhibiting properties and therefore indicates the involvement of cAMP in these responses. (3) 3-Isobutyl-1-methylxanthine was approximately 30 times more potent than theophylline in producing leftward shifts of isoprenaline left atrial tension curves. The potentiating effect of papaverine was masked by an opposing depressant action. (4) Left ventricular papillary muscle tension responses to histamine were enhanced by the phosphodiesterase inhibitors confirming the reported involvement of cAMP in the right ventricle. (5) The left artial tension dose-response curves to histamine were not potentiated. Single doses revealed a biphasic response consisting of an H1-receptor mediated component and one resistant to both H1- and H2-receptor anagonists. Neither component was potentiated suggesting no involvement of cAMP.

Studies on histamine H2 receptors coupled to cardiac adenylate cyclase. Effects of guanylnucleotides and structural requirements for agonist activity

In particulate preparations from guinea-pig ventricle, histamine in the concentration range 10(-6)--10(-3) M caused a 3--5fold stimulation of adenylate cyclase activity which was dependent on the presence of GTP. The effects of fourteen analogs of histamine were examined on this cyclase preparation. Five of the compounds studied proved to be partial agonists relative to histamine while nine others had essentially the same intrinsic activity as histamine. The intrinsic activities of the partial agonists were increased by GppNHp to the extent that dimaprit, which was a partial agonist in the presence of GTP, became a full agonist in the presence of GppNHp. The relative potencies of the full agonists as activators of the cyclase were found to correlate with the relative potencies on physiologically defined H2 receptor systems. Activation of the cyclase by histamine, as well as by several of the agonist analogs, including dimaprit and tolazoline, was completely blocked by the H2 antagonist cimetidine, but was not affected by pharmacologically relevant concentrations of the H1 antagonist mepyramine, the beta-blocker alprenolol, or the alpha-blocker phentolamine. The results suggest that all the agonists studied probably interact with a common H2 receptor site on the cardiac muscle cell leading to activation of adenylate cyclase. The accompanying increase in cyclic AMP is presumably responsible for the chronotropic and inotropic effects of histamine and related compounds on cardiac muscle.

Relationship between histamine-induced changes of cyclic AMP and mechanical activity on smooth muscle preparations of the guinea-pig ileum and the rabbit mesenteric artery

On guinea-pig ileum and rabbit mesenteric artery contracted by high potassium (100 mM) histamine produced relaxations which were inhibited by the H2-receptor antagonist metiamide. These results are thus indicative for the role of H2-receptors in mediating relaxation and for H1-receptors in mediating contraction on smooth muscle. Time course studies for the relaxing and cyclic AMP responses to histamine showed that the cyclic AMP increase preceded the H2-receptor mediated relaxation. The cyclic AMP increase in response to histamine was prevented by metiamide, but remained unaffected by mepyramine on both the guinea-pig ileum and the rabbit mesenteric artery. In addition, dose-response curves obtained on the mesenteric artery demonstrated that the H2-receptor mediated depressor responses coincided with cyclic AMP increases. Thus, these results gave clear-cut evidence that cyclic AMP is an intracellular metabolic event only implicit in the H2-receptor mediated relaxation, but not in the H1-receptor mediated contraction on smooth muscle preparations.

Hypothermia-induced potentiation of histamine H2-receptor-mediated relaxation and cyclic AMP increase in the isolated mesenteric artery of the rabbit

On helically cut strips of the rabbit's mesenteric artery, a temperature decrease from 42 degrees C to 25 degrees C reduced the contractile responses to histamine. Metiamide shifted the dose-response curve of the histamine-induced contraction towards higher values at 25 degrees C, but not at 42 degrees C. Furthermore, on arterial strips contracted by phenylephrine histamine evoked a dose-dependent relaxation at 25 degrees C whereas at 42 degrees C only slight relaxing responses to histamine occurred. Metiamide was capable of preventing the relaxation induced by histamine in a competitive manner. At 25 degrees C the relaxation as produced by histamine was accompanied by increases in cyclic AMP which occurred prior to the relaxing effects. Metiamide abolished the cyclic AMP increase in response to histamine. At 42 degrees C histamine was unable to elevate the cyclic AMP content. Thus, it is concluded that a cyclic AMP-mediated relaxation due to stimulation of H2-receptors counteracts the histamine-induced contraction and reduces the contractile responses to histamine at low temperatures. In addition, clear-cut evidence exists from the present study that also on artery smooth muscle the H2-receptor-mediated responses are closely associated to cyclic AMP.