Endothelin mobilizes Ca2+ from a caffeine- and ryanodine-insensitive intracellular pool in rat atrial cells

Endothelin-1 is a powerful inotropic peptide for the rat atrium. Its action can develop in the absence of L-type Ca2+ channel activity provided that the external Ca2(+)-concentration has been raised to supraphysiological concentrations. Endothelin stimulates phosphatidylinositol hydrolysis in new born rat atrial cells via a mechanism that is insensitive to pertussis toxin. The diacylglycerol/protein kinase C signaling pathway cannot account for the contractile action of endothelin but its activation by phorbol esters induces a partial desensitization of phospholipase C activity. Endothelin-1 and the related peptides, endothelin-2, endothelin-3, and sarafotoxin S6b, raise intracellular Ca2+ levels in rat atrial cells. The actions of endothelin-1, endothelin-2, and sarafotoxin on [Ca2+]i are mutually exclusive, suggesting that they act at the same receptor site. The rise in [Ca2+]i induced by endothelins results both from the mobilization of intracellular stores and from Ca2+ entry through the sarcolemma via a pathway that is not voltage-dependent L-type Ca2+ channels. The Ca2+ store that is mobilized in response to endothelin retains its Ca2+ content when cells were incubated for long periods of time in a 50 nM Ca2+ solution. It is insensitive to caffeine and ryanodine. These two properties distinguish it from the sarcoplasmic reticulum. Contraction experiments in which the pacing rate has been altered to favor Ca2+ accumulation into terminal cisternae of the sarcoplasmic reticulum also suggest that the Ca2+ load of the sarcoplasmic reticulum is increased in endothelin treated rat atria.     

Direct actions of prostaglandins E1, A1, and F2α on myocardial contraction

The inotropic and chronotropic actions of prostaglandin (PG) types PGE₁, PGA₁, and PGF_2α were studied in isolated guinea pig right and left atria, and papillary muscles; rabbit atria; and toad ventricular strips in order to more completely define the cardiac contractile properties of PG. All three prostaglandins, in muscle bath concentrations of 10μg/ml, exerted positive inotropic and chronotropic actions on guinea pig atrium. These contractile effects were persistent after removal of PG from the muscle bath and appeared to limit the relative response to a subsequent dose of PG. The inotropic action of PGE₁ was present over a wide range of bath calcium concentrations (1.1 to 4.4 mM/L). Beta adrenergic receptor blockade, histamine blockade, and pretreatment with reserpine failed to significantly affect the inotropic actions of PG. Norepinephrine and histamine produced more potent inotropic and chronotropic effects on guinea pig atria than did PG and these contractile effects did not exhibit persistence or tachyphylaxis. The prostaglandins did not significantly affect dose response curves for norepinephrine inotropic and chronotropic actions. The prostaglandins had no effect on the force or frequency of contraction in rabbit atria. PGE₁ exerted a positive inotropic effect on toad ventricular myocardium whereas PGA₁ had no effect and PGF_2α had a negative inotropic action.     

Dual effects of mesenteric lymph isolated from rats with burn injury on contractile function in rat ventricular myocytes

Gut-derived factors in intestinal lymph have been shown to trigger myocardial contractile dysfunction. However, the underlying cellular mechanisms remain unclear. We examined the effects of physiologically relevant concentrations of mesenteric lymph collected from rats with 40% burn injury (burn lymph) on excitation-contraction coupling in rat ventricular myocytes. Burn lymph (0.1-5%), but not control mesenteric lymph from sham-burn animals, induced dual positive and negative inotropic effects depending on the concentrations used. At lower concentrations (<0.5%), burn lymph increased the amplitude of myocyte contraction (1.6 +/- 0.3-fold; n = 12). At higher concentrations (>0.5%), burn lymph initially enhanced myocyte contraction, which was followed by a block of contraction. These effects were partially reversible on washout. The initial positive inotropic effect was associated with a prolongation of action potential duration (measured at 90% repolarization, 2.5 +/- 0.6-fold; n = 10), leading to significant increases in the net Ca2+ influx (1.7 +/- 0.1-fold; n = 8). There were no significant changes in the resting membrane potential. The negative inotropic effect was accompanied by a decrease in the action potential plateau (overshoot decrease by 69 +/- 10%; n = 4) and membrane depolarization. Voltage-clamp experiments revealed that the positive inotropic effects of burn lymph were due to an inhibition of the transient outward K+ currents that prolong action potential duration, and the inhibitory effects were due to a concentration-dependent inhibition of Ca2+ currents that lead to a reduction of action potential plateau. These burn lymph-induced changes in cardiac myocyte Ca2+ handling can contribute to burn-induced contractile dysfunction and ultimately to heart failure.     

Adrenergic-cholinergic interactions in left atria. II. Comparison of the antagonism of inotropic responses to alpha- and beta-adrenoceptor stimulation and BAY K 8644 by carbachol, D-600, and nifedipine

The muscarinic agonist carbachol has previously been shown to reverse positive inotropic responses of rabbit left atrial strips to equiactive doses of the beta-adrenoceptor agonist isoproterenol and to the alpha-adrenoceptor agonist phenylephrine. Responses to phenylephrine were measured in the presence of the beta-blocker timolol. However, carbachol was not able to reverse the increase in tension produced by elevating the extracellular Ca2+ concentration. To gain more information about the nature of the functional interaction of carbachol with alpha- and beta-receptor stimulants in left atria, the interaction of carbachol with these agonists, as well as with elevated Ca2+ and the Ca2+ activator compound BAY K 8644, was compared with that of the Ca2+ antagonists D-600 and nifedipine. The results demonstrate that the Ca2+ antagonists exhibit a selectivity similar to that of carbachol, in that responses to both isoproterenol and phenylephrine plus timolol were blocked by low concentrations of D-600 and nifedipine, which had no effect on positive inotropic responses to elevated Ca2+. Higher concentrations of these antagonists shifted the Ca2+ dose-response curve to the right. In addition, although phenylephrine and BAY K 8644 increased tension to a similar extent, responses to phenylephrine were more sensitive than responses to BAY K 8644 to inhibition by both carbachol and D-600. These similarities between the effects of low concentrations of D-600 and nifedipine and those of carbachol are consistent with the hypothesis that carbachol antagonizes responses to alpha- and beta-receptor stimulation in left atria primarily by blocking increases in Ca2+ influx produced by these agonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of inotropic agents on isolated guinea pig heart under conditions that modify calcium pools involved in contractile activation

Positive inotropic effects of strophanthidin were compared with those of isoproterenol, BAY K 8644, grayanotoxin, veratridine, and monensin in electrically stimulated left atrial muscle preparations of guinea pig heart under conditions in which the calcium pool, playing a primary role in contractile activation, was altered. In concentrations that caused similar degrees of increase in developed tension under 1 Hz stimulation, grayanotoxin and strophanthidin caused a relatively large increase in potentiated postrest contraction compared with that caused by isoproterenol, whereas the effect of BAY K 8644 on the postrest contraction was the smallest. The effect of high concentrations of grayanotoxin or strophanthidin, however, resembled that of isoproterenol. The sensitivity of the isolated heart muscle to these agents was compared under conditions in which utilization of various calcium pools contributing to contractile activation was suppressed. Mn2+, which reduces contribution of very superficial Ca2+, reduced sensitivity of heart muscle to the positive inotropic effect of isoproterenol and enhanced the inotropic effect of monensin or veratridine. Verapamil, nifedipine, diltiazem, or ryanodine did not have marked effects on the positive inotropic action of Ca2+, monensin, veratridine, or strophanthidin. These results suggest that the positive inotropic actions of veratridine, grayanotoxin, and strophanthidin share a common mechanism and that low concentrations of strophanthidin may increase loading of Ca2+ pool, which plays an important role in potentiated postrest contraction.     

Endothelin action on vascular smooth muscle involves inositol trisphosphate and calcium mobilization

Cultured endothelial cells release a potent vasoconstrictor peptide, endothelin. Cumulative addition of synthetic endothelin to isolated rabbit aortic rings elicited a concentration-dependent increase in contractile tension which was endothelium-independent. In cultured rabbit vascular smooth muscle cells loaded with the fluorescent dye fura 2, endothelin induced a concentration-dependent increase in [Ca2+]i over the range of 0.01 to 100 nM. Moreover, in the absence of extracellular Ca2+, endothelin could still induce an increase in [Ca2+]i. In addition, endothelin stimulated 45Ca2+ efflux from preloaded vascular smooth muscle cells in the presence and absence of extracellular Ca2+, as well as stimulating 45Ca2+ influx in a concentration-dependent manner. Measurement of inositol phosphates in [3H]-myoinositol-labelled vascular vascular trisphosphate. Unlabelled endothelin inhibited (125I)-endothelin binding to cultured rabbit vascular smooth muscle cells in a concentration-dependent manner. Binding was not inhibited by other vasoactive hormones or calcium channel ligands, suggesting cell surface receptors specific for endothelin. We conclude that one of the initial membrane events in the action of endothelin is to induce phospholipase C-stimulated PIP2 hydrolysis and that this signalling mechanism is initiated by endothelin/receptor interaction at the plasma membrane.     

Endothelin is a positive inotropic agent in human and rat heart in vitro

We have investigated the response to endothelin of isolated atrial and ventricular trabeculae from failing human hearts obtained at transplant. Results indicate that endothelin exerts a significant positive inotropic effect on human atrial and ventricular tissue, with increases in developed tension of 74.6 +/- 14.1% (+/- SEM) and 9.9 +/- 4.0%, respectively. Further studies on rat cardiac muscle demonstrate that the greater inotropic effect on atrial than ventricular muscle is also exhibited by the rat heart in vitro, with 39.9 +/- 10.7% and 17.1 +/- 5.9% increases in developed tension for atria and papillary muscle, respectively. Studies in rat atria also provide no evidence for an effect of endothelin on the frequency of spontaneous contractions. These results suggest that the potential exists for regulation of cardiac function in humans and rats by endothelial-derived factors such as endothelin, possibly via augmentation of atrial systole.     

Alterations in alpha 1-adrenoceptor stimulation of isolated atria from experimental diabetic rats

This purpose of this investigation was to determine the influence of experimental diabetes (3 months) on the responsiveness of rat isolated atria to alpha 1-adrenoceptor stimulation by phenylephrine. Diabetes was chemically induced with streptozotocin (65 mg/kg i.v.) in 42- to 43-day-old, nonfasted male Sprague-Dawley derived rats. Chronotropic (right atria) and inotropic (left atria) indices were recorded in response to alpha 1-adrenoceptor stimulation by phenylephrine. These experiments were performed in the presence of beta-adrenoceptor antagonism (timolol). Isolated right atria from diabetic rats demonstrated a greater increase in heart rate in response to phenylephrine than did corresponding control atria. Left atria were supersensitive (decrease in EC50 values) and hyperresponsive to alpha 1-adrenoceptor stimulation by phenylephrine when compared with stimulation of control left atria. Diabetic left atria in response to phenylephrine were observed to exchange more radioactive calcium (45Ca2+) than control left atria, whereas both diabetic and control left atria exchanged the same amount of 45Ca2+ during basal contractile conditions. Phenylephrine had no effect on 45Ca2+ efflux from either diabetic or control atria. These results indicate that 3 months of uncontrolled experimental diabetes in the rat produces an enhancement of alpha 1-adrenoceptor activation of isolated atria, and that there is an alteration in Ca2+ mobilization which may contribute to the enhanced receptor activation.     

Possible mechanisms of positive inotropic action of synthetic human calcitonin gene-related peptide in isolated rat atrium

The effect of synthetic human calcitonin gene-related peptide (hCGRP) on the isolated and electrically driven left atria of rats were investigated. The peptide at concentrations of 3 X 10(-9)-3 X 10(-7) M produced positive inotropic effects on the left atria in a dose-dependent manner. Verapamil (10(-5) M) and adenosine (10(-4) M) reduced the positive inotropic effect of hCGRP at concentrations of 3 X 10(-9) and 3 X 10(-8) M, but not at that of 3 X 10(-7) M. Ouabain (5 X 10(-5) M) inhibited the effect of hCGRP in concentrations of 3 X 10(-7) and 3 X 10(-8) M, but not in that of 3 X 10(-9) M. Simultaneous pretreatment with verapamil (10(-5) M) and ouabain (5 X 10(-5) M) suppressed the positive inotropy by hCGRP at all concentrations tested. On the other hand, tetrodotoxin (10(-6) M) potentiated only the positive inotropic effect of 3 X 10(-7) M hCGRP. Metoprolol (10(-7) M) and theophilline (10(-3) M) did not affect the inotropic effect of hCGRP. These results suggest that the positive inotropic effect of hCGRP is not mediated by beta-adrenoceptors but by two distinct mechanisms of action, which was inhibited by verapamil but not by ouabain (facilitation of Ca++ influx in lower concentrations of hCGRP) and which was blocked by ouabain but not by verapamil and potentiated by tetrodotoxin (inhibition of Na+/Ca++ exchange mechanism at higher concentrations of hCGRP).     

Molecular mechanism of action of the vasoconstrictor peptide endothelin

Endothelin, one of the most potent vasoconstrictor known, has been suggested to act as an endogenous agonist of L-type Ca2+ channels. In this paper we show that endothelin stimulates the metabolism of inositol phosphates and induces the mobilization of intracellular Ca2+ stores. The transient activation of Ca2+-sensitive K+ channel provokes an hyperpolarization of the membrane. It is followed by a sustained depolarization which is due to the opening of a non-specific cation channel which is permeable to Ca2+ and Mg2+. The depolarization then activates L-type Ca2+ channels. This mechanism of action explains why part of the endothelin-induced vasocontriction is eliminated by L-type Ca2+ channel blockers.     

Mechanisms of cardiodepression by an Na+-H+ exchange inhibitor methyl-N-isobutyl amiloride (MIA) on the heart: lack of beneficial effects in ischemia-reperfusion injury

Although Na+-H+ exchange (NHE) inhibitors such as methyl-N-isobutyl amiloride (MIA) are known to depress the cardiac function, the mechanisms of their negative inotropic effect are not completely understood. In this study, isolated rat hearts were perfused with MIA to study its action on cardiac performance, whereas isolated subcellular organelles such as sarcolemma, myofibrils, sarcoplasmic reticulum, and mitochondria were treated with MIA to determine its effect on their function. The effect of MIA on intracellular Ca2+ mobilization was examined in fura-2-AM-loaded cardiomyocytes. MIA was observed to depress cardiac function in a concentration-dependent manner in HCO3- -free buffer. On the other hand, MIA had an initial positive inotropic effect followed by a negative inotropic effect in HCO3-containing buffer. MIA increased the basal concentration of intracellular Ca2+ ([Ca2+]i) and augmented the KCl-mediated increase in [Ca2+]i. MIA did not show any direct effect on myofibrils, sarcolemma, and sarcoplasmic reticulum ATPase activities; however, this agent was found to decrease the intracellular pH, which reduced the myofibrils Ca2+-stimulated ATPase activity. MIA also increased Ca2+ uptake by mitochondria without having any direct effect on sarcoplasmic reticulum Ca2+ uptake. In addition, MIA did not protect the hearts subjected to mild Ca2+ paradox as well as ischemia-reperfusion-mediated injury. These results suggest that the increase in [Ca2+]i in cardiomyocytes may be responsible for the initial positive inotropic effect of MIA, but its negative inotropic action may be due to mitochondrial Ca2+ overloading as well as indirect depression of myofibrillar Ca2+ ATPase activity. Thus the accumulation of [H+]i as well as occurrence of intracellular and mitochondrial Ca2+ overload may explain the lack of beneficial effects of MIA in preventing the ischemia-reperfusion-induced myocardial injury.     

Callipeltin A, a cyclic depsipeptide inhibitor of the cardiac sodium-calcium exchanger and positive inotropic agent

Callipeltin A, a cyclic depsipeptide from the New Caledonian Lithistida sponge Callipelta sp., is a macrocyclic lactone containing four amino acids in the L configuration, Ala, Leu, Thr (2 residues); one (Arg) in the D configuration; two N-methyl amino acids, N-MeAla and N-MeGln; a methoxy tyrosine, a 3, 4-dimethyl-l-glutamine; and a 4-amino-7-guanidino-2,3 dihydroxypentanoic acid (AGDHE), formally derived from L-Arg. In cardiac sarcolemmal vesicles Callipeltin A induces a powerful (IC(50) = 0.85 microM) and selective inhibition of the Na(+)/Ca(2+) exchanger. In electrically driven guinea-pig atria, at concentrations ranging between 0.7 and 2.5 microM, Callipeltin A induces a positive inotropic effect, which at the highest concentrations is accompanied by a rise in resting tension. It is suggested that the positive inotropic effect is linked to the inhibition of the Na(+)/Ca(2+) exchanger and that Callipeltin A may be an useful tool to study the role of the cardiac Na(+)/Ca(2+) exchanger in physiological and pathological conditions.     

The positive inotropic effect of the aqueous extract of Convallaria keiskei in beating rabbit atria

The positive inotropic effect of the aqueous extract of Convallaria keiskei (ACK) and the possible mechanisms responsible for this effect were investigated in beating rabbit atria. ACK significantly increased atrial stroke volume, pulse pressure, and cAMP efflux in beating rabbit atria. The effects were not altered by pre-treatment with staurosporine and diltiazem, a non-selective protein kinase inhibitor and an L-type Ca2+ channel blocker, respectively. In addition, ACK markedly increased the K+ concentration in the beating atria-derived perfusate. Convallatoxin, a well-known digitalis-like cardiac glycosidic constituent of ACK, also increased atrial stroke volume and pulse pressure but did not alter the cAMP efflux level. The increases in atrial stroke volume and pulse pressure induced by convallatoxin were not also altered by pre-treatment with diltiazem. These results suggest that the ACK-induced positive inotropic effect in beating rabbit atria may, at least in part, be due to the digitalis-like activity of convallatoxin.     

Rat atrial muscle responses with caffeine: dose-response, force frequency, and postrest contractions.

Caffeine has been reported to have a positive and (or) a negative inotropic effect on cardiac muscle. In this study, the force-frequency and postrest characteristics of rat atrium were studied in the presence of caffeine (1.0-10 mM) to see if the interval between beats affected the response of cardiac muscle to caffeine. When stimulation frequency was 0.5 or 2.0 Hz, there was a positive followed by a negative inotropic response with 1, 5, or 10 mM caffeine. Incomplete relaxation occurred under these circumstances, giving rise to contracture. At low frequency of stimulation (0.1 Hz) caffeine had only a negative inotropic effect, and this effect was greater with 1 mM caffeine than with 5 mM caffeine. In the absence of caffeine, when stimulation at 0.5 or 3 Hz was interrupted, a pause of 2-20 s resulted in potentiation. When caffeine was present (2.0 mM), postrest potentiation was severely attenuated, but the steady-state contraction amplitude within the range 0.5-3.0 Hz was not different. These results are consistent with the hypothesis that caffeine induces a leak of Ca2+ from the sarcoplasmic reticulum, and this Ca2+ is extruded from the cell, possibly by Na+/Ca2+ exchange. Sarcoplasmic reticular uptake of Ca2+ and the translocation to release sites appear not to be affected by caffeine within 1-5 mM concentrations.     

Positive inotropic effect of the inhibition of cyclic GMP-stimulated 3',5'-cyclic nucleotide phosphodiesterase (PDE2) on guinea pig left atria in eu- and hyperthyroidism

The significance of PDE2 on the atrial inotropy was studied in eu- and hyperthyroidism. The contractile force was measured and negative inotropic capacity of N6-cyclopentyladenosine (CPA) was determined on left atria isolated from 8-day thyroxine- or solvent-treated guinea pigs, in the presence or absence of EHNA (adenosine deaminase and PDE2 inhibitor) or NBTI (nucleoside transporter inhibitor). EHNA was administered to inhibit PDE2, while NBTI was used to model the accumulation of endogenous adenosine. The reduction of the contractile force caused by EHNA was smaller in the thyroxine-treated atria than in the solvent-treated samples. Contrary, NBTI induced a decrease in the contractile force without significant difference between the two groups. In addition, EHNA enhanced the efficiency of CPA in thyroxine-treated atria and did not affect it in solvent-treated samples, while the response to CPA was decreased by NBTI in all atria, especially in hyperthyroidism. On the basis of greater retention of the contractile force and sustained/enhanced responsiveness to CPA in the presence of EHNA we conclude that PDE2's inhibition has a significant positive inotropic effect in guinea pig atria and this effect is proven to be augmented in hyperthyroidism.     

Inhibition by nicardipine of endothelin-mediated inositol phosphate formation and Ca2+ mobilization in smooth muscle cell

We have investigated the effects of endothelin on phosphoinositide metabolism and Ca2+ mobilization in cultured A10 cells. Endothelin stimulated a significant increase in inositol phosphate formation in a time- and dose-dependent manner. IP3 was significantly elevated by 30 sec and reached a 2.0-fold above control at 1 min. The EC50 for endothelin was 0.5 nM. The initiation of inositol phosphate formation was independent of extracellular Ca2+, and the Ca2+ ionophore, A23187, did not stimulate IP3 formation. However, the sustained elevation of inositol phosphates was partially inhibited by incubating cells in buffer lacking Ca2+ or in buffer containing nicardipine. Endothelin mobilized both intracellular and extracellular Ca2+ reaching a peak intracellular concentration of 350 +/- 11 nM by 1 min when cells were bathed with Ca2+-complete buffer. Intracellular Ca2+ remained 2-fold above baseline for at least 15 min. In contrast, when cells were exposed to endothelin in Ca2+-free buffer, the peak value of [Ca2+]i was 195 +/- 20 nM and returned to baseline by 2 min. Nicardipine completely blocked the influx of extracellular Ca2+ but did not interfere with the mobilization of intracellular stores. We conclude that endothelin produces a rapid and sustained elevation in inositol phosphate formation. The rapid production of IP3 is consistent with the time course for mobilization of intracellular Ca2+. Elevated cytosolic Ca2+ levels are maintained by the influx of extracellular Ca2+ through a nicardipine-sensitive Ca2+ channel and are involved in the sustained formation of inositol phosphates. These data provide an explanation for the sustained, nicardipine-inhibitable contraction of coronary artery strips induced by endothelin.     

Effects of prostaglandin E1 (PGE1) on the positive inotropic response of heart muscle to elevation of external Ca++-concentration, to increased driving frequency, and to paired stimulation.

Electrically driven left guinea pig atria were exposed to positive inotropic stimuli which are thought to be related to the turnover of calcium ions. For increasing contractibility, the following procedures were used: a) varying the concentration of CaCl2 in the bath fluid; b) stimulation at frequencies from 1.0 to 3.0 Hz; c) paired stimulation. Positive inotropic responses to the increase of the rate of stimulation and to paired stimulation were not affected by 0.1 microgram/ml tetrodotoxin (TTX). This excludes the adrenergic contribution to the positive inotropic effects observed. Actions of the positive inotropic stimuli were studied both in the absence and in the presence of 0.1--1.0--10.0--1000.0 ng/ml of PGE1-PGE1 in the highest concentration used increased contractile force. The inotropic stimulus-response curves were not affected by PGE1 at any concentration. This finding suggests there is no interaction between Ca ions and PGE1 in the contractile mechanism of the guinea pig heart muscle.     

Comparative studies on the positive inotropic effect of isoproterenol and bromobenzoyl-methyladamantylamine in guinea pig ventricular myocardium

The effects of bromobenzoyl-methyladamantylamine (BMA) on the transmembrane potentials, contractile force, and 42K efflux were investigated and compared to that of isoproterenol (IPR) in guinea pig ventricular myocardium. Both drugs exerted positive inotropic effect. BMA lengthened the action potential duration, depolarized the membrane, and decreased the Vmax. IPR increased the height of the plateau, accelerated repolarization, slightly increased the resting potential. In preparations depolarized partially by 26 mmol/l K+, both BMA (10(-4) mol/l) and IPR (10(-7) mol/l) induced slow response action potentials, but the duration of BMA-induced ones was twice longer than that of IPR-induced ones. BMA markedly reduced the 42K efflux from ventricular myocardium, whereas IPR had no effect on it. Moreover, BMA also decreased the 26 mmol/l K+-induced increment in 42K efflux, while IPR did not. It is concluded that BMA and IPR exert their positive inotropic effects on different ways. IPR increases the slow inward Ca2+ current directly by activating a phosphorylation process, whereas BMA enhances it indirectly by reducing the K+ conductance, lengthening the repolarization and consequently prolonging the time during which the slow inward Ca2+ current can be operative.     

Changes in sensitivity to histamine of guinea pig cardiac muscles during postnatal development

Histamine stimulates the heart by interacting with cardiac histamine receptors. We investigated the postnatal changes in histamine sensitivity with spontaneously beating right atria and electrically driven left atria and right ventricular papillary muscles from 0-, 5-, and 10-day-old and adult guinea pigs. The positive chronotropic response to histamine in right atria was antagonized by cimetidine but not by chlorpheniramine at any age. Chlorpheniramine did not antagonize the positive inotropic effect of histamine and 2-(2-pyridyl)ethylamine in the immature left atria but it blocked the positive inotropic effect in the adult; cimetidine had no effect. The positive inotropic effect of histamine in right ventricular muscles was not affected by chlorpheniramine in immature right ventricular muscles but was antagonized in the adult. These results suggest that, in immature left atria and right ventricular muscles, there is no H1-receptor system mediating the positive inotropic effect of histamine and that, as age advances, this system begins to mediate the positive inotropic effect. In immature left atria, non-H1 and non-H2 receptors exist and mediate the positive inotropic effect of histamine.