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

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.

Efficacy of a selective histamine H2 receptor agonist, dimaprit, in experimental models of endotoxin shock and hepatitis in mice

Dimaprit, a selective histamine H2 receptor agonist, was examined in experimental models of endotoxin shock and hepatitis in mice. Injection of lipopolysaccharide (8 mg/kg i.v.) into Balb/c mice resulted in an elevation of plasma tumor necrosis factor-alpha (TNF-alpha), reaching the maximal level at 1 h post-lipopolysaccharide (1147 U/ml). Oral administration of dimaprit 200 mg/kg, 1 h prior to lipopolysaccharide challenge, inhibited the increase in plasma TNF-alpha by 71% and also the survival rate was increased to 62.5% from 8.3% in the disease control. In a mouse hepatitis model, simultaneous injection of galactosamine (700 mg/kg i.v.) and lipopolysaccharide (3 micrograms/kg i.v.) into Balb/c mice caused an increase in plasma TNF-alpha, peaking at 1 h, followed by an elevation of L-alanine aminotransferase (E.C.2.6.1.2) activity at 4 h onward. Oral administration of dimaprit 200 mg/kg, 1 h prior to galactosamine and lipopolysaccharide, reduced the increase in plasma TNF-alpha by 99% and L-alanine aminotransferase by 82%. In vitro, dimaprit dose dependently inhibited the production of TNF-alpha in mouse peritoneal macrophages and human peripheral blood monocytes stimulated with lipopolysaccharide with IC50 values of 1 microM. The decrease in TNF-alpha production by dimaprit was reversed by cimetidine, a histamine H2 receptor antagonist. Dimaprit dose dependently suppressed TNF-alpha mRNA in human peripheral blood monocytes. These results suggest that activation of the histamine H2 receptor downregulates the production of TNF-alpha, and that histamine may be an important regulator in pathological conditions in which TNF-alpha plays an important role.

Histamine release during paediatric cardiopulmonary bypass

Histamine release is part of the general inflammatory response and occurs during surgery and cardiopulmonary bypass (CPB) in adults. Few data are available for children. Histamine release was studied in 23 children undergoing CPB with standard anaesthetic and CPB techniques. Blood sampling was performed in relation to specific anaesthetic and surgical events, e.g., start of CPB, removal of aortic clamps, reventilation of the lungs. Plasma histamine was determined by a single isotope radioenzymatic technique. There was no consistent histamine release in the study population although there was an increase in plasma histamine concentration in some subjects after initiation of CPB (P < 0.05) and on removal of the aortic cross-clamp (P < 0.05). No correlation was demonstrated between histamine concentration and systolic arterial pressure, temperature, duration of CPB or cross-clamp time. Histamine concentration was positively correlated with heart rate.

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.

Histamine enhances hypoxia-induced ventricular arrhythmias in isolated rat hearts

1. The effects of hypoxia, histamine-receptor agonist perfusion, and their combination on cardiac rhythm were studied in isolated rat hearts. 2. While hypoxia induced a high incidence of ventricular tachycardia or fibrillation, only a few preparations developed ventricular arrhythmias in response to perfusion with high concentration of histamine, 2-pyridylethylamine or impromidine. 3. The times of onset of hypoxia-induced ventricular arrhythmias were significantly shortened by perfusion with either histamine, 2-pyridylethylamine or impromidine. The accelerated occurrence of hypoxia-induced ventricular arrhythmias by histamine was significantly abolished by pretreatment with either diphenhydramine or cimetidine. 4. The results indicate that hypoxia and histamine can increase ventricular vulnerability of the rat heart to each other. It is also suggested that the arrhythmogenic actions of histamine in hypoxic rat hearts are mediated by both histamine H1-and H2-receptors.

Cardiovascular effects of H2-receptor antagonists

The type II histamine receptor antagonists, cimetidine and ranitidine, widely used in treatment of peptic ulcer disease have been reported to cause bradycardia. To evaluate the cardiovascular effects of H2 antagonists nineteen healthy volunteers were entered into a double-blind crossover comparison of cimetidine 300 mg qid, ranitidine 150 mg bid, and placebo. Subjects ingested study medicine for 7 days prior to being tested by the Bruce Exercise Test. Heart rate, blood pressure, oxygen consumption, expiratory volume, and fractional expiration of CO2 and O2 were measured at rest, exercise and recovery. A plasma sample for determination of cimetidine and ranitidine levels were obtained prior to the exercise period. Multivariate analysis and paired t test revealed no significant differences for the cardiovascular or pulmonary variables. However, in 5 subjects, the heart rate at 25% maximum VO2 was depressed 8% (P less than or equal to 0.03). This effect in a small percentage of the population suggests that further studies are needed to determine if subpopulations are affected.

Analysis of histamine as a hair-cell transmitter in the lateral line of Xenopus laevis

The actions of histamine and histamine antagonists on afferent nerve activity were investigated in the lateral line of Xenopus laevis. Histamine (0.002-2.0 mM) had no effect on spontaneous activity or excitatory responses to water motion. In contrast, pyrilamine, an H1 receptor antagonist, suppressed spontaneous activity beginning at 0.01-0.05 mM. Below 0.3 mM the suppression was often preceded by a small excitatory response and responses to high (24-30 dB re threshold), but not low (0-18 dB) levels of water motion were selectively suppressed. Higher concentrations (0.3-2.0 mM) abolished spontaneous activity and suppressed responses at all levels of water motion. Cimetidine, an H2 receptor antagonist, had similar actions but was one-tenth as potent as pyrilamine. Tetrodotoxin (0.001-0.1 microM), which blocks voltage-sensitive Na+ channels, mimicked the suppressive effects of the histamine antagonists. Histamine (2.0 mM) failed to block the actions of pyrilamine (0.1 mM) indicating its effects are mediated through a mechanism other than histamine receptors. In addition, pyrilamine (0.05-0.1 mM) non-selectively suppressed excitation to exogenously applied L-glutamate (1.0-2.0 mM), L-aspartate (1.0-2.0 mM), kainate (0.005-0.01 mM), and quisqualate (0.002-0.005 mM) and altered responses to N-methyl-D-aspartate (0.5-1.0 mM). The results are inconsistent with histamine being a transmitter in the Xenopus lateral line and reveal that the actions of histamine antagonists are nonspecific, possibly due, in part, to blockade of voltage-sensitive Na+ channels.

Ranitidine-induced chest pain

A 45-year-old woman with no history of heart disease twice experienced chest pain after consuming a dose of ranitidine. The chest pain, which lasted about one hour, was substernal, left of midline, dull, and pounding. H2-receptors are present in cardiovascular tissues. Although several studies have not noted an effect of ranitidine on cardiac indices there have been case reports indicating a cardiac effect. There are no reports of chest pain associated with H2-blocker ingestion; however, both bradycardia and hypotension (reported effects) might cause chest pain. A discussion of the possible mechanisms is presented.