Comparison of the mechanisms underlying the positive inotropic actions of dopamine, adrenaline and isoprenaline on the isolated rabbit papillary muscle

Role of Dopamine in the Heart in Health and Disease

Dopamine has effects on the mammalian heart. These effects can include an increase in the force of contraction, and an elevation of the beating rate and the constriction of coronary arteries. Depending on the species studied, positive inotropic effects were strong, very modest, or absent, or even negative inotropic effects occurred. We can discern five dopamine receptors. In addition, the signal transduction by dopamine receptors and the regulation of the expression of cardiac dopamine receptors will be of interest to us, because this might be a tempting area of drug development. Dopamine acts in a species-dependent fashion on these cardiac dopamine receptors, but also on cardiac adrenergic receptors. We will discuss the utility of drugs that are currently available as tools to understand cardiac dopamine receptors. The molecule dopamine itself is present in the mammalian heart. Therefore, cardiac dopamine might act as an autocrine or paracrine compound in the mammalian heart. Dopamine itself might cause cardiac diseases. Moreover, the cardiac function of dopamine and the expression of dopamine receptors in the heart can be altered in diseases such as sepsis. Various drugs for cardiac and non-cardiac diseases are currently in the clinic that are, at least in part, agonists or antagonists at dopamine receptors. We define the research needs in order to understand dopamine receptors in the heart better. All in all, an update on the role of dopamine receptors in the human heart appears to be clinically relevant, and is thus presented here.

Sarcolemmal α2-adrenoceptors control protective cardiomyocyte-delimited sympathoadrenal response

Sustained cardiac adrenergic stimulation has been implicated in the development of heart failure and ventricular dysrhythmia. Conventionally, α2 adrenoceptors (α2-AR) have been assigned to a sympathetic short-loop feedback aimed at attenuating catecholamine release. We have recently revealed the expression of α2-AR in the sarcolemma of cardiomyocytes and identified the ability of α2-AR signaling to suppress spontaneous Ca²⁺ transients through nitric oxide (NO) dependent pathways. Herein, patch-clamp measurements and serine/threonine phosphatase assay revealed that, in isolated rat cardiomyocytes, activation of α2-AR suppressed L-type Ca²⁺ current (I_CaL) via stimulation of NO synthesis and protein kinase G- (PKG) dependent activation of phosphatase reactions, counteracting isoproterenol-induced β-adrenergic activation. Under stimulation with norepinephrine (NE), an agonist of β- and α-adrenoceptors, the α2-AR antagonist yohimbine substantially elevated I_CaL at NE levels >10nM. Concomitantly, yohimbine potentiated triggered intracellular Ca²⁺ dynamics and contractility of cardiac papillary muscles. Therefore, in addition to the α2-AR-mediated feedback suppression of sympathetic and adrenal catecholamine release, α2-AR in cardiomyocytes can govern a previously unrecognized local cardiomyocyte-delimited stress-reactive signaling pathway. We suggest that such aberrant α2-AR signaling may contribute to the development of cardiomyopathy under sustained sympathetic drive. Indeed, in cardiomyocytes of spontaneously hypertensive rats (SHR), an established model of cardiac hypertrophy, α2-AR signaling was dramatically reduced despite increased α2-AR mRNA levels compared to normal cardiomyocytes. Thus, targeting α2-AR signaling mechanisms in cardiomyocytes may find implications in medical strategies against maladaptive cardiac remodeling associated with chronic sympathoadrenal stimulation.

Alpha1-adrenenoceptor stimulation inhibits cardiac excitation-contraction coupling through tyrosine phosphorylation of beta1-adrenoceptor

Adrenoceptor stimulation is a key determinant of cardiac excitation-contraction coupling mainly through the activation of serine/threonine kinases. However, little is known about the role of protein tyrosine kinases (PTKs) activated by adrenergic signaling on cardiac excitation-contraction coupling. A cytoplasmic tyrosine residue in β1-adrenoceptor is estimated to regulate Gs-protein binding affinity from crystal structure studies, but the signaling pathway leading to the phosphorylation of these residues is unknown. Here we show α1-adrenergic signaling inhibits β-adrenergically activated Ca(2+) current, Ca(2+) transients and contractile force through phosphorylation of tyrosine residues in β1-adrenoceptor by PTK. Our results indicate that inhibition of β-adrenoceptor-mediated Ca(2+) elevation by α1-adrenoceptor-PTK signaling serves as an important regulatory feedback mechanism when the catecholamine level increases to protect cardiomyocytes from cytosolic Ca(2+) overload.

Signal transduction and Ca2+ signaling in intact myocardium

The experimental procedures to simultaneously detect contractile activity and Ca(2+) transients by means of the Ca(2+) sensitive bioluminescent protein aequorin in multicellular preparations, and the fluorescent dye indo-1 in single myocytes, provide powerful tools to differentiate the regulatory mechanisms of intrinsic and external inotropic interventions in intact cardiac muscle. The regulatory process of cardiac excitation-contraction coupling is classified into three categories; upstream (Ca(2+) mobilization), central (Ca(2+) binding to troponin C), and/or downstream (thin filament regulation of troponin C property or crossbridge cycling and crossbridge cycling activity itself) mechanisms. While a marked increase in contractile activity by the Frank-Starling mechanism is associated with only a small alteration in Ca(2+) transients (downstream mechanism), the force-frequency relationship is primarily due to a frequency-dependent increase of Ca(2+) transients (upstream mechanism) in mammalian ventricular myocardium. The characteristics of regulation induced by beta- and alpha-adrenoceptor stimulation are very different between the two mechanisms: the former is associated with a pronounced facilitation of an upstream mechanism, whereas the latter is primarily due to modulation of central and/or downstream mechanisms. alpha-Adrenoceptor-mediated contractile regulation is mimicked by endothelin ET(A)- and angiotensin II AT(1)-receptor stimulation. Acidosis markedly suppresses the regulation induced by Ca(2+) mobilizers, but certain Ca(2+) sensitizers are able to induce the positive inotropic effect with central and/or downstream mechanisms even under pathophysiological conditions.

Muscarinic regulation of Ca2+ signaling in mammalian atrial and ventricular myocardium

The differential regulation of the contractility of mammalian atrial and ventricular myocardium upon activation of muscarinic receptors can be ascribed, for the most part, to alterations in intracellular Ca2+ transients. However, alterations in myofibrillar sensitivity to Ca2+ ions also contribute to such regulation. In atrial muscle, the following actions are all associated with the corresponding alterations in the amplitude of Ca2+ transients in the same direction as those in the strength of the contractile force: (1) the direct inhibitory action on the basal force of contraction; (2) the increase (recovery) in force that is induced during the prolonged stimulation of muscarinic receptors; and (3) the rebound increase in force induced by washout of muscarinic receptor agonists. In addition, for a given decrease in force induced by muscarinic receptor stimulation in atrial muscle, the amplitude of Ca2+ transients is decreased to a smaller extent than the decrease in amplitude induced by reduction of extracellular Ca2+ concentration ([Ca2+]o), an indication that muscarinic receptor stimulation might increase myofibrillar sensitivity to Ca2+ ions simultaneously with the reduction in the amplitude of Ca2+ transients during induction of the direct inhibitory action. In mammalian ventricular myocardium, the direct inhibitory action of muscarinic receptor stimulation exhibits a wide range of species-dependent variation. A pronounced direct inhibitory action is induced in ferret papillary muscle, which is also associated with a definite increase in myofibrillar sensitivity to Ca2+ ions. By contrast, in the ventricular myocardium of other species including the rabbit and the dog, muscarinic receptor stimulation scarcely affects the baseline Ca2+ transients and the force, but it results in a pronounced decrease in Ca2+ transients and force when applied in the presence of beta-adrenoceptor stimulation, a phenomenon known as 'accentuated antagonism' or the 'indirect inhibitory action' of muscarinic receptor stimulation in mammalian ventricular myocardium. During induction of the indirect inhibitory action in mammalian ventricular myocardium, muscarinic receptor stimulation reverses all the effects induced by beta-adrenoceptor stimulation, including the increase in Ca2+ transients, the positive inotropic and lusitropic effects, and the decrease in myofibrillar sensitivity to Ca2+ ions. The relationship between the amplitude of Ca2+ transients and force is unaffected during induction of the indirect inhibitory action in rabbit and dog ventricular myocardium. The direct and indirect inhibitory actions of muscarinic receptor stimulation on Ca2+ transients have clearly different dependences on frequency: the former is more pronounced at a higher rate of stimulation, while the latter is more pronounced at a lower rate. The more complex interaction of muscarinic receptor and beta-adrenoceptor stimulation in mammalian atrial muscle and ferret ventricular muscle might be explained by the contribution of both the direct and the indirect regulatory mechanisms to the interaction.

Effect of alpha 1-adrenoceptor blockade on ventricular ectopic beats inacute myocardial infarction

Experimental studies have shown that alpha1-adrenoceptor blockade can reduce ventricular arrhythmia associated with myocardial ischaemia. To examine the efficacy of prazosin in clinical acute infarction 38 patients were randomized, on presentation, to prazosin or placebo. Oral therapy was commenced at 0.5 mg, incremented and continued for seven days, Holter recordings being obtained for the first 48 hours and on day 7. The final dose of prazosin was 2.5 +/- 1.7 (SD) mg and placebo, 3.1 +/- 2.0 mg. During dose titration in the first 24 hours, and on day 7, there was no difference in ventricular ectopic beats. In the second 24 hours, ventricular ectopic beats averaged two per hour in the prazosin group (n = 9) and 60 per hour in placebo (n = 15) (P = 0.05, Mann-Whitney rank testing). The results indicate that alpha1-adrenoceptor blockade may reduce ventricular arrhythmia in clinical acute myocardial infarction. While early and adequate therapy is currently limited by vasodilation, this small study suggests that more extensive clinical trials will be warranted as relatively cardio-selective alpha1-adrenoceptor blocking drugs are developed.

Temperature sensitivity of cardiac muscarinic receptors in bat atria and ventricle

In contrast to other mammals, muscarinic receptors in the bat ventricle can mediate significant decrease in basal contractile force (greater than 50%), not only at 37 degrees C but also at hibernation temperature (12 degrees C). At frequencies of contraction that approximate in vivo values for 37-12 degrees C, no significant shift in receptor affinity or maximum response to applied acetylcholine was found for either ventricular or atrial muscle. Low temperature does not appear to compromise receptor function in hibernators. The atypical cholinergic innervation of the ventricle may maintain a regulative role during hibernation.

Myocardial pharmacodynamics of dopamine, dobutamine, amrinone and isoprenaline compared in the isolated rabbit heart

The isolated spontaneously beating rabbit heart was used for comparing the myocardial effects of isoprenaline, dobutamine, dopamine and amrinone. Both isoprenaline and dobutamine produced a progressive concentration-dependent increase in contractility from 100% to a maximum of about 200% (pD2 7.81 and 7.01, respectively) as measured by the increase in isotonic contraction rate. The simultaneous augmentations in contraction amplitude reached maxima of about 127 and 143% (pD2 7.83 and 7.05) for each of the drugs and the heart frequency rose to 202 and 162% (pD2 7.80 and 6.63), respectively. The accompanying oxygen consumption increased from 100 to 194% (pD2 7.70) for isoprenaline and to only 177% (pD2 6.36) for dobutamine. Coronary flow rate rose to 153 and 134%, respectively. Dopamine increased the contraction rate to 181% (pD2 6.26), contraction amplitude to about 122% (pD2 6.25) and heart rate to 162% (pD2, 5.85), while oxygen consumption rose to a maximum of 202% (pD2 5.69). Coronary flow rate rose to 156%. In contrast amrinone produced an unexpected slowly progressing decrease in contraction rate and contraction amplitude to about 66% (pD2 4.45 and 4.01, respectively). Oxygen consumption increased to 159% (pD2 4.10) and coronary flow rate to 210%. The positive inotropic effect of dobutamine thus equalled that of isoprenaline but with a distinct lower concomitant increase in heart frequency and oxygen consumption which may reflect a better myocardial efficiency during the action of dobutamine.

[Alpha-adrenoceptors in the myocardium: incidence and functional significance]

Alpha-adrenoceptors mediating positive inotropic effects are well established in the heart of various species including human heart. The mechanism by which alpha-adrenoceptor stimulation increases force of contraction is not known. cAMP is unlikely to be involved as a mediator. Evidence has been presented that an increase in magnitude and duration of the slow Ca++ inward current may be partly responsible for the positive inotropic effect. In addition, stimulation of alpha-adrenoceptors may increase Ca++ sensitivity of the contractile proteins. Stimulation of alpha-adrenoceptors by endogenous catecholamines may serve as a reserve mechanism under various conditions of impaired beta-adrenergic influence, e.g. hypothyroidism, bradycardia or ischemia. Furthermore, alpha-adrenoceptors may be involved in the genesis of reperfusion arrhythmias in ischemic heart.

beta 2-Adrenoceptors regulate induction of myocardial ornithine decarboxylase in mice in vivo

The pharmacological characteristics of the myocardial adrenoceptor of the mouse have been examined during embryogenesis by measuring ornithine decarboxylase (ODC, EC 4.1.1.17) induction. 2 A four fold elevation of ODC activity was observed after isoprenaline (10 mg/kg, s.c.), and enzyme activity was increased two to three fold following adrenaline (1 mg/kg, s.c.) or terbutaline given by direct injection to the foetus (10 microgram/500 mg). 3 Pretreatment with the beta-adrenoceptor antagonist, propranolol (10 mg/kg), totally blocked the increase in ODC activity. 4 Elevation of myocardial ODC activity was not inhibited by metoprolol, a relatively specific beta-adrenoceptor antagonist, at a dose of 10 mg/kg. 5 Since the increase in ODC activity was blocked by a beta-adrenoceptor antagonist (propranolol) and enzyme activity was stimulated by terbutaline, a beta 2-agonist, we conclude that beta 2-adrenoceptors are selectively coupled to the regulation of murine cardiac ODC activity following catecholamine stimulation.

Differences between alpha-adrenergic and beta-adrenergic inotropic effects in rat heart papillary muscles

alpha-And beta-adrenergic inotropic effects have been shown to be qualitatively different. In order to further characterize these difference we compared the mechanical response to alpha- and beta-adrenoceptor stimulation, respectively, in electrically driven left ventricular papillary muscles from rat heart. The muscles were stimulated by either isoprenaline (Beta-adrenoceptor stimulation), phenylephrine in the presence of propranolol (alpha-adrenoceptor stimulation) or phenylephrine alone (combined alpha-and Beta-adrenoceptor stimulation). Isometric tension (T), rate of rise and decline of tension (first derivate=T') and rate of transition from tension rise to tension decline (negative part of second derivative=T') were recorded. These recordings disclosed qualitative differences between the alpha-and Beta-inotropic response both in dose-response and time course experiments. Maximal Beta-adrenoceptor stimulation caused a small increase in Tmax (18%), intermediate increases in T'max (45%) and T'min (68%) and considerable increase in T'min (145%) ("Beta-type" effect). Maximal alpha adrenoceptor stimulation increased all qualities by about the same degree (23-24% ("a-type" effect). While Beta-adrenoceptor stimulation gave a dose-dependent and pronounced increase in the ratio T"min/T'max (relaxation-onset index), alpha adrenoceptor stimulation decreased it to subcontrol values and phenylephrine alone gave a small dose-dependent increase at higher dose. The time course of the alpha-adrenoceptor stimulation was characterized by a transient decrease in all qualities followed by an increase which reached maximum at 4-5 min. Beta-Adrenoceptor stimulation gave a monophasic response which reached maximum after 1-2 min. Phenylephrine alone gave mainly an "a-type" effect although T"min increased significantly more in the absence than in the presence of propranolol and T"min/T'max showed a small increase which developed slowly. Thus Beta-adrenoceptor stimulation activated relaxation compared to contraction by a higher degree than did alpha-adrenoceptor stimulation. This probably reflects different mechanisms of action. While the alpha-effect may rely primarily on an increased calcium influx, the Beta-effect probably is the final result of several subcellular effects of cyclic AMP.

Stimulation by adrenaline and dopamine but not by noradrenaline of myocardial alpha-adrenoceptors mediating positive inotropic effects in human atrial preparations

In isolated electrically driven right auricular strips of human hearts (1 Hz at 37 degrees C) the ability of the endogenous catecholamines noradrenaline, adrenaline, and dopamine to mediate positive inotropic effects by stimulation of myocardial alpha-adrenoceptors was investigated. 1. The concentration-response curve for the positive inotropic effect of noradrenaline was shifted to the right by pindolol, 10(-8)M, whereas the additional blockade by phentolamine, 3 x 10(-6)M, was without further effect; therefore, the positive inotropic effect of noradrenaline is mediated by beta-adrenoceptors only. 2. In contrast, the positive inotropic effects of adrenaline as well as of dopamine are caused by stimulation of both, beta- and alpha-adrenoceptors. 3. These results are discussed with respect to the possible physiological and pathophysiological function of myocardial alpha-adrenoceptors.

Studies on the mechanism of the positive inotropic action evoked by epinine on the rabbit isolated papillary muscle at different rates of beating

1. Effects of epinine on cyclic AMP and contractility were investigated in rabbit papillary muscles driven at a rate of 0.5 or 2.0 Hz. 2. When the frequency of stimulation was increased from 0.5 to 2.0 Hz, the log dose-response curve for the positive inotropic effect of epinine was displaced to the left, whereas the maximum of the developed tension was not changed. 3. At both frequencies phentolamine (1 mumol/l) shifted the lower part of the log dose-response curve for epinine to the right, whereas pindolol (30 nmol/l) affected mainly the upper part. In the presence of both alpha- and beta-adrenoceptor antagonists, the whole curve was shifted to the right in a parallel manner. However, cocaine (30 mumol/l) did not significantly influence the log dose-response curve of epinine. 4. At 0.5 Hz a submaximal effective concentration of epinine (100 mumol/l) led to an approximately 100% increase of the cyclic AMP level after 60s; the same increase of the cyclic AMP level was induced at 2.0 Hz by one-third the concentration of epinine (30 mumol/l). 5. Phentolamine (1 mumol/l) did not affect the increase of the cyclic AMP level evoked by epinine, whereas pindolol (30 nmol/l) completely depressed it. 6. The present results indicate that epinine produces its positive inotropic effect through direct stimulation of myocardial alpha-adrenoceptors as well as beta-adrenoceptors, depending upon the concentration: in lower concentrations it acts mainly on alpha-adrenoceptors, whereas in higher concentrations it acts predominantly on beta-adrenoceptors. The positive inotropic effect through beta-adrenoceptor stimulation is mediated by cyclic AMP, while that through alpha-adrenoceptors is not.

Demonstration in human atrial preparations of alpha-adrenoceptors mediating positive inotropic effects

In isolated, electrically driven right auricular strips of the human heart the inotropic effect of phenylephrine was studied. 1. First, the influence of the driving rate on the tension developed (i.e., the frequency-force relationship) was determined by stimulation of the preparations at 0.1, 0.5, 1, 2 and 3 HZ. The force of contraction was lowest at a stimulation rate of 0.1 HZ (36.9 g/g dry weight). The maximally developed force of contraction observed at frequencies of 0.5, 1 and 2 HZ amounted to about 200 g/g dry weight. The values did not significantly differ from each other. 2. The negative log of the EC50 (-log EC50) for the positive inotropic effect of phenylephrine determined at a frequency of 0.5 and 1.0 HZ amounted to 5.28 +/- 0.08 and 5.34 +/- 0.11, respectively. The alpha-adrenolytic drug phentolamine (3 x 10(-6) M) diminished significantly the -log EC50 to 5.01 +/- 0.04 and 4.89 +/- 0.10, respectively. 3. At a frequency of 1 HZ a shift of the concentration-response curve to the right was observed after treatment with the beta-adrenolytic drug pindolol (3 x 10(-8) M); the -log EC50 of phenylephrine decreased significantly to 4.08 +/- 0.07. 4. From these results it is concluded that alpha-adrenoceptors are present in human atria; they mediate positive inotropic effects and are stimulated by phenylephrine.

Qualitative differences between beta-adrenergic and alpha-adrenergic inotropic effects in rat heart muscle

If beta- and alpha-adrenergic inotropic effects are cyclic AMP dependent and cyclic AMP independent, respectively, they may be qualitatively different. The inotropic effects of beta-receptor stimulation (isoprenaline) and alpha-receptor stimulation (phenylephrine combined with propranolol) were characterized in isolated perfused rat hearts, rat atria and rat papillary muscles. The beta-effect reached its maximum before the alpha-effect. The alpha-effect followed a three-phasic time-course indicating both stimulatory and inhibitory components. The aortic pressure wave (perfused heart) indicated a shorter contraction phase after beta-stimulation than after alpha-stimulation. The time to peak tension (atrium, papillary muscle) was relatively shorter after isoprenaline than after alpha-stimulation, which tended to prolong it. The contraction-relaxation cycles (atrium, papillary muscle) were examined by recording the isometric tension (T), its first (T') and second (T'') deri derivatives. alpha and beta-stimulation both increased Tmax, T'max (maximal rate of tension rise), T'min (maximal rate of tension decline) and T''min (maximal rate of transition from rise to decline of tension). Isoprenaline increased T'min (papillary muscle) and T''min (atrium, papillary muscle) relatively more than did alpha-stimulation, i.e. the relaxing processes were activated relatively more by beta-stimulation. The results indicate different mechanisms for the two adrenergic inotropic effects. The relatively larger activation of relaxation by beta-stimulation is assumed to be caused by clic AMP.

No evidence for involvement of dopaminergic receptors in the positive inotropic action of dopamine on the isolated rabbit papillary muscle

Experiments were carried out on the isolated rabbit papillary muscle driven at 0.5 Hz in order to further elucidate the mechanism of the positive inotropic effect evoked by dopamine. The dose-response curve for dopamine was not affected by the antagonists pimozide (10(-6) M), yohimbine (10(-5) M) pindolol (3 x 10(-8) M) and phentolamine (10(-6) M) when these agents were given separately. Only the simultaneous administration of yohimbine plus pindolol and phentolamine plus pindolol, respectively, shifted the entire curve to the right. This shift was not further influenced by pimozide. Dopamine (10(-4) M) increased the cyclic AMP content of the papillary muscle by about 50%; this increase was not affected by pimozide, but was markedly elevated by yohimbine and completely depressed by pindolol. From the present results it is concluded, that dopamine produces its positive inotropic effect through stimulation of myocardial alpha-as well as beta-adrenoceptors to about the same degree; stimulation of specific dopaminergic receptors, however, is not involved. The stimulation of beta-adrenoceptors is accompanied by an increase of the cyclic AMP level, while that of alpha-adrenoceptors is not.