Restoration by histamine of the calcium-dependent electrical and mechanical response in the guinea-pig papillary muscle partially depolarized by potassium

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.

Cardiovascular effects of LAS 30538, a new vascular selective Ca(2+)-channel blocker

A new compound, 1-[2-(2,6-dimethylphenoxy)ethyl]-alpha,alpha-bis-(p-fluorphenyl)-4 -piperidine methanol (LAS 30538), was found to have potent vasodilator effects. Its vasorelaxant activity was demonstrated in rat perfused hindlimbs contracted with 80 mM K+, having an IC50 value of 40 nM. In conscious spontaneously hypertensive rats, LAS 30538 administered orally, caused dose-dependent sustained falls in systolic blood pressure with an ED30 value of 11 mg kg-1. In pithed rats, LAS 30538, strongly inhibited vasoconstriction induced by the alpha 2-adrenoceptor agonist B-HT 933 and the calcium agonist compound Bay K8644 with ED50 values of 4 mg kg-1 p.o. and 1.3 mg kg-1 i.v., respectively. Results from electrophysiological studies carried out using guinea-pig papillary muscles partially depolarized by 22 mM K+ are consistent with LAS 30538 acting as a Ca(2+)-channel blocker. When compared with verapamil, in guinea-pig and rabbit isolated heart preparations, LAS 30538 caused less cardiodepression and bradycardia. The results suggest that LAS 30538 may have some advantages over other Ca(2+)-channel blockers such as verapamil in causing less myocardial depression for a given level of vasodilatation.

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 and abnormal automaticity in barium- and strophanthidin-treated sheep Purkinje fibers

We used intracellular microelectrodes to study the effects of histamine on both normal and abnormal automaticity in sheep Purkinje fibers. Histamine, dimaprit and 4-methylhistamine caused a similar reduction of the action potential duration in driven Purkinje fibers. Histamine (10-6 M) induced spontaneous activity in p previously quiescent preparations more often than did equimolar concentrations of dimaprit and 4-methylhistamine. The effects of histamine on automaticity were enhanced in the presence of barium In fact histamine, at concentrations which were unable to induce automaticity in normal preparations, induced it in the presence of barium. In Purkinje fibers manifesting barium-induced automatic activity, histamine (10-7--10-6M) significantly increased the average number of spontaneous action potentials and shortened the time of their appearance. In the same range of concentrations, histamine dose-dependently increased the iological manifestation of calcium overload. Histamine (10-6--10-4M) increased the OAP amplitude of strophanthidin -treated Purkinje fibers, eventually inducing triggered extrabeats. All these previously described effects were selectively blocked by cimetidine (10-5 M). It is concluded that histamine may induce cardiac arrhythmias under conditions of calcium overload and that this effect may be due to induction or enhancement of oscillatory afterpotentials.

Effect of corticotropin-releasing factor on the electrical and mechanical activities of the guinea-pig ventricular myocardium

1. The effects of the physiological levels of corticotropin-releasing factor (CRF) on isolated guinea-pig ventricular myocardium were studied using a force transducer and standard microelectrode techniques. 2. CRF increased the contractile force of muscles concentration-dependently in normal Tyrode and a high-K+ (27 mM) solution. The positive inotropic effect of CRF was associated with a significant enhancement of the slow action potentials of partially depolarized muscles in high-K+ solution. CRF potentiated the effect of increasing Ca2+ concentration of Tyrode solution. 3. The inotropic effect of CRF was reduced in the presence of diltiazem, and suppressed by phentolamine, metoclopramide, and cimetidine, but was not affected by propranolol and cold condition. 4. It is suggested that an increase in the slow inward Ca2+ current induced by CRF plays an important role in its positive inotropic effect and that its effect differs from that of cardiotonic steroids.

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)

On the mechanism of histamine induced enhancement of the cardiac Ca2+ current

In guinea pig ventricular myocytes, the effect of histamine on the slow Ca2+ current (ICa) was studied and the following results were obtained: (1) Superfusion of cells with histamine resulted in a dose-dependent enhancement of the amplitude of ICa. The threshold concentration of histamine was 10(-8) M, half maximal increase occurred at 3 X 10(-7) M and maximal enhancement (about 3-4-fold) at 5 X 10(-6) M. (2) The histamine effect was greatly reduced by the H2 antagonist cimetidine (10(-5) M) but only slightly by the H1 antagonist diphenhydramine (10(-5) M). (3) Effects of isoprenaline (ISP) and histamine at maximal effective concentrations on ICa were not additive, suggesting that both agents use the same intracellular pathway. Intracellular infusion of a blocker of the cAMP-dependent protein kinase, Rp-cAMPS (10(-4) M), prevented the histamine effect. (4) The involvement of GTP-dependent transducer proteins was studied by cell dialysis with several GTP derivatives. Intracellular application of the stable GDP-analogue, GDP-beta-S, reduced the histamine effect on ICa, whereas the stable GTP analogue, GTP-gamma-S, mimicked the histamine effect.

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)

Synergistic interaction between histamine and ouabain on ventricular fibrillation threshold

The interaction between histamine and ouabain on the ventricular fibrillation threshold (VFT) was studied in the isolated, Langendorff-perfused heart of the guinea-pig. When administered separately, histamine and ouabain each reduced the VFT in a concentration-related manner with ED50 values of 84.3 and 250 nM, respectively. When perfused in combination the effects of these two drugs on VFT were significantly greater than the sum of their individual contributions. Mepyramine and cimetidine both antagonized the fibrillatory effects of the histamine-ouabain combination. Neither antagonist affected the fibrillation concentration-response curve for ouabain. These results suggest that both classes of histamine receptor may participate in the histamine-ouabain interaction.

Effect of Ni2+ on the multiphasic positive inotropic responses to histamine mediated by H1-receptors in left atria of guinea pigs

Effects of calcium antagonists and Ni2+ on the positive inotropic responses to histamine mediated by H1-receptors were investigated in left atria of guinea pigs. The preparations were electrically driven at 0.5 Hz in Krebs-Henseleit solution at a temperature of 30 degrees C unless stated otherwise. Histamine in low concentrations of 10 and 100 nmol/1 produced a monophasic positive inotropic effect, whereas in concentrations higher than 1 mumol/1 it exerted a multiphasic inotropic response composed of an initial increasing phase (initial component) and a second and late developing, greater positive inotropic phase (second component). These two components were sometimes separated by a transient decrease in developed tension. Both positive inotropic components caused by 3 mumol/1 histamine were inhibited by the H1-antagonists mepyramine, D-chlorpheniramine and diphenhydramine in a concentration-dependent manner, but not by the H2-antagonist cimetidine. Pretreatment with Ni2+ in concentrations of 0.2 and 0.5 mmol/1 preferentially suppressed the second component of the inotropic effect produced by 3 mumol/1 histamine without significantly affecting the initial component. On the other hand, the monophasic positive inotropic effect of 30 nmol/1 histamine was almost unaffected by this cation. The concentration-response curves for the positive inotropic effect of histamine were hardly influenced by Ni2+ in the range of lower concentrations of histamine, but significantly suppressed in the range of higher concentrations of the agonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological effects of platelet-activating factor (PAF-acether) in guinea-pig papillary muscles

The effects of PAF-acether (10(-11) to 10(-7) M) were studied on the electrical and mechanical activity of guinea-pig papillary muscles. At 10(-11) M PAF-acether did not modify the amplitude and Vmax of the upstroke or the resting membrane potential. At higher concentrations PAF-acether produced a dose-dependent increase in the amplitude and Vmax of the upstroke, shortened the action potential duration and hyperpolarized the resting membrane potential. These effects were accompanied by a biphasic effect on ventricular contractile force. The shortening of the APD was inhibited in muscles pretreated with tetraethylammonium or verapamil. In papillary muscles depolarized by 27 mM K Tyrode solution PAF-acether induced slow action potentials which were blocked by verapamil. PAF-acether produced a dose-dependent increase in amplitude and Vmax of the upstroke on the slow action potentials elicited by isoproterenol, prolonged the action potential duration and hyperpolarized the resting membrane potential. These results suggest that in guinea-pig papillary muscles PAF-acether increased Ca influx via the slow inward current.

Altered calcium concentration and rabbit atrial H1-receptors

Rabbit atria were incubated in various concentrations of calcium (0.9, 1.8 and 3.6 mM) and then homogenized and histamine H1-receptors analysed by radioligand binding using (3H)-pyrilamine. Only atria incubated in 3.6 mM calcium showed an alteration in (3H)-pyrilamine binding. In these tissues the Kd of H1-receptors for (3H)-pyrilamine was increased suggesting a decrease in the affinity of the H1-receptor. That this decreased affinity is due to altered calcium concentrations was demonstrated by showing that no change in histamines' chronotropic effects were produced by decreasing the sodium concentration to produce the same calcium/sodium ratio compared to high calcium alone. We conclude that calcium has effects directly on H1-receptor affinity and appears to produce a decrease in the ability of histamine to bind.

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.

Stimulation of slow action potentials in guinea pig papillary muscle cells by intracellular injection of cAMP, Gpp(NH)p, and cholera toxin

To test the hypothesis that intracellular cAMP has a regulatory role in cardiac slow channel function, intracellular pressure injections of cAMP and adenylate cyclase activators, Gpp(NH)p and cholera toxin, were carried out. Guinea pig papillary muscles were depolarized to about -45 mV by superfusion with 22 mM K+-Tyrode's solution to inactivate the fast Na+ channels. Induction of slow action potentials or enhancement of ongoing slow action potentials was observed in about 70% of all cells in which a successful intracellular injection of the testing compounds was obtained. The slow AP is highly dependent on slow inward current and is known to be enhanced by catecholamines. The effect of the injected cyclic nucleotides and related compounds occurred within 3 minutes after starting the injection, whereas superfusion with these compounds (dibutyryl cAMP was used in place of cAMP) required 10-30 minutes to show an effect. This difference is attributed to the intracellular injection of the compound. The effect on stimulating slow action potentials persisted (greater than 5 minutes) after termination of the application of either Gpp(NH)p or cholera toxin, indicating the long-lasting nature of their action. The effect of the cAMP injections decayed within 1 minute. Intracellular injection of 5'-AMP was without effect. These results support the view that a causal relationship exists between intracellular cAMP level and slow channel function. Phosphorylation of a protein constituent of the slow channel by a cAMP-dependent protein kinase may be involved.

Dysrhythmias caused by histamine release in guinea pig and human hearts

Histamine is released into the systemic circulation during anaphylaxis, by drugs and by surgical procedures. Studies in animal models have conclusively demonstrated that released cardiac histamine is a major mediator of arrhythmias that occur during anaphylaxis and following the administration of histamine-releasing drugs. Several lines of evidence suggest a similar arrhythmogenic role for cardiac histamine in humans: (1) The human heart is rich in histamine; (2) cardiac histamine can be readily released from human heart in vitro by therapeutic concentrations of drugs; (3) histamine has potent arrhythmogenic effects on the human heart in vitro. Arrhythmogenic effects of histamine include enhancement of normal automaticity, induction of abnormal automaticity, induction of triggered tachyarrhythmias, depression of atrioventricular conduction, and increase in the vulnerability of the ventricles to fibrillation. A combination of H1 and H2 antihistamines is needed to block the arrhythmogenic effects of histamine. Certain arrhythmogenic effects of histamine (e.g. induction of slow responses and delayed afterdepolarizations) can also be blocked by drugs which inhibit the influx of cations through slow channels. In contrast, the commonly-used drug digitalis potentiates the arrhythmogenic effects of histamine. We propose that histamine release produced by drugs and surgical procedures may be an overlooked factor in fatal cardiac arrhythmias. Experimental studies suggest that selective pharmacological methods can be developed to block the arrhythmogenic effects of histamine.

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.

Drugs affecting the calcium-dependent slow depolarization mechanism of the cardiac cell membrane

Transmembrane action potentials were recorded from isolated electrically paced cat and guinea pig left auricles in Tyrode solution and in the presence of 25 mM K+ or tetrodotoxin (TTX). The overshoot (OS) was increased, and the plateau of the action potential (AP) was shortened by epinephrine (5 X 10(-6)M) and by celluline-A (an organic calcium complex obtained from frog skin). The maximum rate of depolarization (Vmax) was enhanced by serotonin (5-HT) (10(-6)M), prostaglandin F2 alpha (2.8 X 10(-6)M), and by celluline-A. After inactivation of the fast sodium channel by TTX (10(-5)M) or by partial depolarization (25 mM K+ Tyrode) of electrically driven cat and guinea pig left auricle, catecholamines (epinephrine and isoproterenol, 4 X 10(-7) to 5 X 10(-6)M), histamine (10(-6) to 10(-5)M), and celluline-A restored the electrical and mechanical activity. Prostaglandins (PGE1, PGF2 alpha, 1.4 X 10(-8)-2.8 X 10(-6)M), 5-HT (10(-6)-5 X 10(-6)M), and 4-aminopyridine (1.5 X 10(-4)M) were ineffective in such conditions. The beta-adrenergic blocking agent pindolol (4 X 10(-7)M) prevented only the effect of catecholamines; the H1 receptor blocker mepyramine (10(-5)M) antagonized only that of histamine, and D-600 (2 X 10(-6)M), a selective Ca channel blocker, prevented the restoring effect of catecholamines, histamine, and celluline-A.

Effect of dantrolene sodium on the transmembrane potentials and contractility of guinea pig atrial myocardium

Dantrolene sodium (3 X 10(-4) M) exerted a biphasic effect on the mechanical and electrical activity of the isolated guinea pig left atrium. In the first phase, the drug increased the contractile force by about 200%, and prolonged the action potential duration. This transient positive inotropic effect was antagonized by D-600, a slow Ca2+ channel inhibitor. In the later phase, the drug gradually decreased the amplitude of contractions, shortened the action potential duration and induced contracture. Dantrolene sodium increased the rate of depolarization and overshoot of Ca2+ -mediated slow action potentials. The results suggest that dantrolene sodium increases the slow inward Ca2+ current, causing a positive inotropic effect in the atrial myocardium. The late negative inotropic effect of the drug could be due to a secondary inhibition of the slow inward Ca2+ current due to an increased intracellular free Ca2+ concentration as a consequence of other actions of the drug.

Differential effects of histamine on left and righ guinea-pig atria

In electrically stimulated strip from right guinea-pig atria histamine produced a positive inotropic effect and it increased the amplitude and duration of calcium-dependent slow action potentials, which suggests an increase in the slow inward current. These effects were apparently mediated by H2-receptors. On strips from left atria histamine produced an H1-receptor mediated positive inotropic effect, without alteration of slow action potentials. The mechanism of this positive inotropic effect remains unknown.

The arrhythmogenic actions of histamine on human atrial fibers

We used standard microelectrode techniques to study the effects of histamine on right atrial tissues from patients undergoing corrective cardiac surgery. In the 10(-6) to 10(-4) M range, histamine increased maximum diastolic potential, action potential amplitude, and automaticity. In some preparations, histamine also induced delayed afterdepolarizations and triggered activity. The potency of histamine in increasing automaticity was about 10 times less than that of epinephrine. Propranolol (2 x 10(-7)M), which abolished the chronotropic effect of epinephrine, did not alter the effect of histamine. Conversely, the effect of histamine but not that of epinephrine was antagonized by cimetidine (3 x 10(-6) to 1 x 10(-5) M). This suggests that H2 receptors mediate the chronotropic effects of histamine on the human heart. The slow channel blocker verapamil (2 x 10(-8) to 2 x 10(-6) M) counteracted the effects of histamine on automaticity, delayed afterdepolarizations, and triggered activity, suggesting that in human atrium histamine may act by increasing slow inward (presumably Ca2+) current. If one considers these arrhythmogenic effects of histamine and the fact that human cardiac tissue contains large amounts of histamine, our experiments lend further support to the concept that histamine release can induce arrhythmias.

Effects of H1- and H2-receptor antagonists on calcium-dependent action potentials produced by histamine in rabbit left auricles depolarized by potassium

After inactivation of the fast sodium channels by partial depolarization (25 mmol/l K+ Tyrode solution) histamine (10(-6)-10(-5) mol/l) restored the electrical and mechanical activity in isolated electrically paced left auricle of rabbit. Transmembrane action potentials restored by histamine in this condition have a nearly normal height of overshoot with a low resting potential and with a very low rate of rise. The AP-restoring effect of histamine was affected neither by beta-adrenergic blocking agent, Pindolol (4 X 10(-7) mol/l) nor H2-receptor antagonist, cimetidine by (2 X 10(-6) mol/l) but was antagonized by H1-receptor blocker, mepyramine (10(-5) mol/l) and by the selective Ca/2-channel blocker D-600 (2 X 10(-6) mol/l). The results suggest that in this species histamine may stimulate the slow inward Ca2+ current mediated by H1-receptors.

alpha-Sympathomimetic amines and calcium-mediated action potentials in guinea-pig ventricular muscle

1 The ability of amines, having alpha- or alpha- and beta-adrenoceptor stimulating activity, to restore excitability and contractility in heart preparations partially depolarized by potassium, was investigated in guinea-pig ventricular muscle in order to elucidate the mechanism of the positive inotropic effect mediated via alpha-adrenoceptors. 2 In preparations in which fast sodium channels were inactivated by K+-rich medium (22 mM) slow electrical responses as well as contractions were consistently induced by high concentrations of phenylephrine (10(-4) to 3 X 10(-4) M) and synephrine (3 X 10(-4) M). 3 The restorative effective effects of both phenylephrine and synephrine were unaffected by phentolamine (10(-5) M) but were readily abolished by practolol (10(-5) M) or sotalol (10(-5) M). 4 Methoxamine induced a dose-dependent positive inotropic effect in ventricular strips paced at 0.5 Hz in normal Tyrode solution; the maximum increase in contractile tension was obtained with methoxamine 10(-4) M. However, at the same concentration, the amine did not induce slow electrical responses in potassium-depolarized preparations. 5 It is concluded that the induction of slow responses by phenylephrine and synephrine is due to beta-adrenoceptor stimulation, and that the increase in cardiac contractility caused by alpha-adrenoceptor stimulation does not involve an increase in slow inward calcium current.

Chronotropic effect of histamine on cultured neonatal rat heart cells

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

Studies on the positive inotropic effect of dopamine in the guinea-pig heart

The positive inotropic effect of dopamine has been studied in isolated ventricular strips of guinea-pig heart. The concentration-inotropic response curve for dopamine was significantly shifted to the right by pretreatment with reserpine. In preparations obtained from animals pretreated with reserpine (2.5 mg/kg, 24 h prior to the experiment) the dose-response curve was not significantly affected by haloperidol, a dopamine vascular receptor antagonist (10(-6)-3X10(-6) M). The inotropic effect of dopamine was antagonized by practolol (3X10(-7)-10(-6) M), but not by phentolamine (3X10(-6)-10(-5) M); moreover the alpha-adrenoceptor blocking drug (10(-5) M) did not affect the curve for dopamine in the presence of practolol (3X10(-7) M). In preparations in which fast sodium channels were blocked by K+ -rich medium, slow electrical responses (calcium-mediated action potentials) as well as contractions were induced by high concentrations of dopamine (10(-4)-3X10(-4) M); again these responses were unaffected by phentolamine or haloperidol, but were blocked by practolol. It was concluded that in the guinea-pig ventricular muscle dopamine induced a positive inotropic effect through both indirect and direct action, and that the latter is due to the activation of beta-adrenoceptors.