Histamine receptors in the heart and coronary vessels of rabbits

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.

Endothelium-independent contractile and relaxant responses to histamine in the rabbit aorta and common carotid, mesenteric, renal, and femoral arteries

1. The role of the vascular endothelium in the relaxant and contractile responses to histamine of the isolated rabbit aorta; common carotid, mesenteric, renal, and femoral arteries; as well as receptor types mediating these responses were analyzed. 2. Histamine (10(-8) to 10(-4) mol/l) contracted resting rings and caused a further concentration-dependent contraction of rings of the arteries precontracted by phenylephrine. 3. Pyrilamine abolished the contractile response to histamine in resting rings of the arteries, whereas it reversed that response into a concentration-dependent relaxant response in precontracted rings of the arteries. The relaxant effect of histamine was abolished by metiamide, but it was not affected by sotalol and atropine. Moreover, in control experiments, the phenylephrine-induced contractions and acetylcholine-induced relaxations were not changed by pyrilamine and metiamide, respectively. 4. Endothelial removal did not influence the contractile and relaxant responses of the arteries to histamine. 5. These findings indicate that, in the isolated rabbit aorta and common carotid, mesenteric, renal, and femoral arteries, the contractile effect of histamine resulting from the activation of H1 receptors overcomes its relaxant effect resulting from the activation of H2 receptors. The effects of histamine are neither mediated nor modulated by the endothelial cells.

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.

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

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

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.

Different mechanisms of action of histamine in isolated arteries of the dog

1 In helically-cut strips of dog coronary, superior mesenteric, right gastro-epiploic and renal arteries contracted with prostaglandin F2 alpha (PGF 2 alpha), histamine produced a dose-related relaxation, while, in contrast, the amine caused only a contraction of cerebral arterial strips. 2 The contractile response of cerebral arteries to histamine was attenuated by chlorpheniramine (10(-6)M) but was unaffected by cimetidine (10(-5)M). Relaxant responses to histamine of coronary and renal arteries were significantly attenuated by treatment with cimetidine and to a similar extent by combined treatment with cimetidine and chlorpheniramine. Chlorpheniramine alone was ineffective. 3 In mesenteric and gastro-epiploic arteries, relaxant responses to histamine were attenuated only slightly by cimetidine. Chlorpheniramine slowed the development of histamine-induced relaxations but did not alter the magnitude of the relaxations. Combined treatment with these H1- and H2- antagonists attenuated the histamine-induced relaxations to an appreciably greater extent than treatment with cimetidine alone. 4 It may be concluded that the cerebroarterial contraction induced by histamine is mediated through H1-receptors and the relaxations of coronary and renal arteries induced by histamine are mediated through H2-receptors. It appears that H1- and H2-receptors interact with each other to produce potentiation of histamine-induced relaxations of mesenteric and gastro-epiploic arteries.

Effect of impromidine (SK&F 92676) on the isolated papillary muscle of the guinea-pig

Impromidine was found to have a positive inotropic effect on the isolated papillary muscle from the guinea-pig. The dose-response curve to impromidine was shifted to the right by cimetidine and ranitidine. Impromidine was 35 times more potent than histamine but with a maximal response of only 81% that obtained with histamine. This difference, which was statistically significant (P < 0.005), suggested that impromidine acts as a partial agonist at the histamine H2-receptors of the papillary muscle as has been observed in other tissues.