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

Initial characterization of a transgenic mouse with overexpression of the human D1-dopamine receptor in the heart

Dopamine can exert effects in the mammalian heart via five different dopamine receptors. There is controversy whether dopamine receptors increase contractility in the human heart. Therefore, we have generated mice that overexpress the human D1-dopamine receptor in the heart (D1-TG) and hypothesized that dopamine increases force of contraction and beating rate compared to wild-type mice (WT). In D1-TG hearts, we ascertained the presence of D1-dopamine receptors by autoradiography using [3H]SKF 38393. The mRNA for human D1-dopamine receptors was present in D1-TG hearts and absent in WT. We detected by in-situ-hybridization mRNA for D1-dopamine receptors in atrial and ventricular D1-TG cardiomyocytes compared to WT but also in human atrial preparations. We noted that in the presence of 10 µM propranolol (to antagonize β-adrenoceptors), dopamine alone and the D1- and D5-dopamine receptor agonist SKF 38393 (0.1-10 µM cumulatively applied) exerted concentration- and time-dependent positive inotropic effects and positive chronotropic effects in left or right atrial preparations from D1-TG. The positive inotropic effects of SKF 38393 in left atrial preparations from D1-TG led to an increased rate of relaxation and accompanied by and probably caused by an augmented phosphorylation state of the inhibitory subunit of troponin. In the presence of 0.4 µM propranolol, 1 µM dopamine could increase left ventricular force of contraction in isolated perfused hearts from D1-TG. In this model, we have demonstrated a positive inotropic and chronotropic effect of dopamine. Thus, in principle, the human D1-dopamine receptor can couple to contractility in the mammalian heart.

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.

Molecular and electrophysiological evaluation of human cardiomyocyte subtypes to facilitate generation of composite cardiac models

Paucity of physiologically relevant cardiac models has limited the widespread application of human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes in drug development. Here, we performed comprehensive characterization of hiPSC-derived cardiomyocyte subtypes from 2D and 3D cultures and established a novel 3D model to study impulse initiation and propagation. Directed differentiation approaches were used to generate sinoatrial nodal (SANCM), atrial (ACM) and ventricular cardiomyocytes (VCM). Single cell RNA sequencing established that the protocols yield distinct cell populations in line with expected identities, which was also confirmed by electrophysiological characterization. In 3D EHT cultures of all subtypes, we observed prominent expression of stretch-responsive genes such as NPPA. Response to rate modulating drugs noradrenaline, carbachol and ivabradine were comparable in single cells and EHTs. Differences in the speed of impulse propagation between the subtypes were more pronounced in EHTs compared with 2D monolayers owing to a progressive increase in conduction velocities in atrial and ventricular cardiomyocytes, in line with a more mature phenotype. In a novel binary EHT model of pacemaker-atrial interface, the SANCM end of the tissue consistently paced the EHTs under baseline conditions, which was inhibited by ivabradine. Taken together, our data provide comprehensive insights into molecular and electrophysiological properties of hiPSC-derived cardiomyocyte subtypes, facilitating the creation of next generation composite cardiac models for drug discovery, disease modeling and cell-based regenerative therapies.

GPR55 regulates the responsiveness to, but does not dimerise with, α1A-adrenoceptors

Emerging evidence suggests that G protein coupled receptor 55 (GPR55) may influence adrenoceptor function/activity in the cardiovascular system. Whether this reflects direct interaction (dimerization) between receptors or signalling crosstalk has not been investigated. This study explored the interaction between GPR55 and the alpha 1A-adrenoceptor (α_1A-AR) in the cardiovascular system and the potential to influence function/signalling activities. GPR55 and α_1A-AR mediated changes in both cardiac and vascular function was assessed in male wild-type (WT) and GPR55 homozygous knockout (GPR55^-/-) mice by pressure volume loop analysis and isolated vessel myography, respectively. Dimerization of GPR55 with the α_1A-AR was examined in transfected Chinese hamster ovary-K1 (CHO-K1) cells via Bioluminescence Resonance Energy Transfer (BRET). GPR55 and α_1A-AR mediated signalling (extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation) was investigated in neonatal rat ventricular cardiomyocytes using AlphaScreen proximity assays. GPR55^-/- mice exhibited both enhanced pressor and inotropic responses to A61603 (α_1A-AR agonist), while in isolated vessels, A61603 induced vasoconstriction was attenuated by a GPR55-dependent mechanism. Conversely, GPR55-mediated vasorelaxation was not altered by pharmacological blockade of α_1A-ARs with tamsulosin. While cellular studies demonstrated that GPR55 and α_1A-AR failed to dimerize, pharmacological blockade of GPR55 altered α_1A-AR mediated signalling and reduced ERK1/2 phosphorylation. Taken together, this study provides evidence that GPR55 and α_1A-AR do not dimerize to form heteromers, but do interact at the signalling level to modulate the function of α_1A-AR in the cardiovascular system.

Effect of Phenoxybenzamine on Myocardial Cell Necroses and Blood Levels of Catecholamines in Pigs Subjected to Stress

The degree of myocardial cell necroses after stress was investigated in 6 pigs, weighing 70 to 90 kg. The stress was induced by a myorelaxant, succinylcholine, for about 12 min. The alpha-adrenoceptor blocking agent phenoxybenzamine (PBZ) was given orally 3 times a day in a total dose of 50 to 60 mg per day for 6 to 7 days before the stress. The plasma levels of noradrenaline and adrenaline were assayed in blood samples drawn before, during, and immediately after the period of stress. Heart cell necroses were found in all the cases. The activity of the sympathetic-adrenomedullary system, indicated by the plasma levels of catecholamines, was intense during the stress. When the present results were compared with those of earlier studies the degree of heart cell necroses was significantly smaller after PBZ in control pigs but significantly higher than after the beta-adrenoceptor blocking agent propranolol given for about a week. PBZ exerts a number of different actions, as briefly discussed, some of them appearing to be harmful for the heart during stress while others appear to be protective for the heart.

Inotropic, chronotropic, and arrhythmogenic effects of dopamine on the isolated working heart of rabbit

Some improvements of Kodama's method for perfusing the isolated rabbit heart in its working mode were made. Increases in the right and left atrium pressure, together with an increase in the pulmonary artery pressure, were observed to occur immediately after the start of venous return, and then all of the increased pressures were found to remain at each constant level. In these stable states, the administration of dopamine (DA) into the perfusate was found to produce dose-related increases in contractile activities. In the preparations denervated with reserpine or 6-hydroxydopamine, in which tyramine (Ty) produced no response, the inotropic effectiveness of DA did not differ from that in the normal ones. On the other hand, responses to noradrenaline (NA) were found to increase significantly after the denervation. DA produced a dose-related increase in heart rates in the normal preparation, and this effect was greatly suppressed in the denervated preparations, suggesting that the primary chronotropic effect of DA is an indirect one via the release of NA from the sympathetic nerve terminals. Arrhythmogenic effects of NA, Ty or DA were also observed in these preparations. At all the doses tested, the incidence rates by NA were as high as 50% or more, the type of arrhythmia being recognized as atrial or ventricular extrasystole from the ECG analysis. On the other hand, the rates by DA were relatively low, less than 34%. From a comparison of the incidence rates between the normal and denervated preparations, this effect of DA was considered to be primarily an indirect one.

In vivo demonstration of alpha-adrenoceptor-mediated positive inotropy in pithed rats: evidence that noradrenaline does not stimulate myocardial alpha-adrenoceptors

1. This study examines whether positive inotropy via alpha-adrenoceptors could be observed in vivo in pithed rats. Cardiac contractility was measured as the maximum rate of rise of left ventricular pressure (dP/dt(max)). Heart rate and aortic blood pressure were also recorded. 2. The selective alpha1-adrenoceptor agonists, methoxamine, cirazoline, amidephrine and phenylephrine caused dose-related increases in dP/dt(max). This response was progressively reduced by increasing doses of the alpha1-adrenoceptor antagonist prazosin. However, since the concomitant increase in diastolic blood pressure (DBP) was also blocked, the changes in dP/dt(max) may have been a consequence of increased after load. 3. Adrenaline and noradrenaline also increased dP/dt(max), accompanied by pressor responses. Propranolol (1 mg kg(-1)) antagonized the increase in dP/dt(max) in response to noradrenaline, suggesting beta-adrenoceptor involvement, but not that to adrenaline. The additional presence of prazosin (1 mg kg(-1)) further shifted the dose-response curves for both noradrenaline and adrenaline to the right. 4. Analysis of the increases in dP/dt(max) at predetermined increases in DBP by each agonist revealed three groups of regression lines. Adrenaline in the presence of propranolol and the four selective alpha1-adrenoceptor agonists occupied a common central position. Above this group were adrenaline and noradrenaline in the absence of antagonists; their additional effects on contractility were beta-adrenoceptor-mediated since the regression lines were lowered by propranolol. Clearly below the main group of agonists was noradrenaline in the presence of propranolol. 5. Thus, for a given increase in DBP, adrenaline (in the presence of beta-blockade) and the alpha1-adrenoceptor agonists exert an additional inotropic effect to noradrenaline (also in the presence of beta-blockade). This is concluded to be an alpha-adrenoceptor-mediated increase in cardiac contractility which is not shared by noradrenaline.

Effects of dopamine on L-type Ca2+ current in single atrial and ventricular myocytes of the rat

1. The effects of dopamine on the L-type Ca2+ current (ICa,L) of both atrial and ventricular single myocytes and on the force of contraction of atrial trabeculae in rat heart were investigated. 2. Dopamine increased atrial ICa,L at concentrations higher than 1 microM, but had little or no effect on ICa,L at lower concentrations. The increase in ICa,L at high concentrations was reversed by propranolol and acetylcholine, but not by phentolamine. Activation and inactivation kinetics of ICa,L were not altered by dopamine. 3. In rat ventricular myocytes in which the D4 receptor mRNA does not express, dopamine (20-100 microM) also increased the ICa,L amplitude and propranolol reversed this effect. 4. Clozapine, a potent D4 receptor antagonist, blocked the augmenting effect of dopamine on ICa,L. However, this effect could be explained by beta-antagonism, since clozapine also inhibited the isoprenaline effect. 5. In the atrial trabeculae, the increase in contraction by dopamine (1 to 30 microM) was reversed by 1 microM propranolol, but not by 2 microM phentolamine. Low doses of dopamine (0.01 to 0.3 microM) did not affect the contraction in the controls or during a modest stimulation of the beta-adrenoceptor with 0.01 microM isoprenaline. 6. These results indicate that the positive inotropic action of dopamine is mediated through direct stimulation of the beta-adrenoceptor in both atrial and ventricular myocytes. Involvement of D4 receptor appears unlikely in the regulation of the atrial contraction.

Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.

Effects of beta-blockers and Ca(2+)-antagonists on the response of the isolated working rat heart to adrenergic stimulants after cardioplegic arrest

During coronary artery bypass graft (CABG) surgery, patients pretreated with the combination of beta-blocking drugs and Ca2+ antagonists for control of myocardial ischemia often respond inadequately to adrenergic stimulants administered after cardioplegic arrest. In this study, the effects of the combination of a beta-blocker (propranolol) and a Ca2+ antagonist (nifedipine) on the spontaneous recovery, as well as the adrenergic response of the isolated, perfused, working rat heart after a period of cardioplegic arrest were evaluated. After pretreatment of the animals with propranolol and/or nifedipine, hearts were removed, perfused in the presence of pretreatment drugs, subjected to 45 minutes of normothermic cardioplegic arrest, reperfused, and finally stimulated with exponentially increasing concentrations of a sympathomimetic drug. Propranolol, and to a lesser extent nifedipine, protected the hearts during cardioplegic arrest, as indicated by the improved recovery and maximum response to adrenergic stimulation after cardioplegia. Isoprenaline, a beta-stimulant, (at a 100 x higher than conventional concentration), elicited an adequate inotropic and chronotropic response. Stimulation by the alpha, beta-stimulant adrenaline or dobutamine improved only the inotropic response of propranolol and combination treated hearts. Cautious extrapolation of the results to human may suggest continuation of drug therapy of patients before CABG surgery.

Induction by endogenous noradrenaline of an alpha 1-adrenoceptor-mediated positive inotropic effect in rabbit papillary muscles

1. The possible involvement of alpha 1-adrenoceptors in the inotropic and electrophysiological responses to endogenous noradrenaline released by tyramine was examined in rabbit papillary muscles. 2. A concentration-dependent positive inotropic effect was produced by tyramine. This effect of tyramine was not observed in muscles from rabbits pretreated with reserpine. 3. The positive inotropic effect of tyramine was greatly inhibited by propranolol, but not altered by prazosin. However, when beta-adrenoceptors were blocked by pretreatment with propranolol, tyramine still produced a positive inotropic effect, an effect which was antagonized by prazosin. 4. Tyramine caused a decrease in action potential duration (APD) and an increase in action potential amplitude in a concentration-dependent manner. Isoprenaline also produced the same electrophysiological effects. These electrophysiological effects of both agents were inhibited by propranolol. 5. When beta-adrenoceptors were blocked by propranolol, the observed prazosin-sensitive positive inotropic effect of tyramine was not accompanied by any change in APD. In contrast, APD was markedly prolonged by alpha 1-adrenoceptor stimulation with phenylephrine in the presence of propranolol, in association with the positive inotropic effect. 6. It is concluded that in rabbit papillary muscles, endogenous noradrenaline causes a positive inotropic effect predominantly mediated by beta-adrenoceptors, but can still evoke a positive inotropic effect through alpha 1-adrenoceptors when beta-adrenoceptor stimulation is eliminated. This suggests that the alpha 1-adrenoceptor-mediated positive intropic mechanism(s) may be masked by simultaneous activation of beta-adrenoceptors. In addition, this study indicates that APD prolongation is not involved in the alpha 1-adrenoceptor-mediated inotropic responses to endogenous noradrenaline.

Electrophysiologic and inotropic effects of alpha-adrenoceptor stimulation in human isolated atrial heart muscle

The effects of alpha-adrenoceptor stimulation on force of contraction were investigated in human atrial heart muscle and compared with those of beta-adrenoceptor stimulation. The maximal positive inotropic effect produced by stimulation of alpha-adrenoceptors with phenylephrine (in the presence of atenolol 10 mumol/l) was significantly smaller than that seen in response to beta-adrenoceptor stimulation with isoprenaline. The maximal effect of phenylephrine (25% of the maximal effect of isoprenaline) required far higher concentrations (1 mmol/l) than isoprenaline (100 nmol/l); the EC50 values amounted to 33.1 mumol/l and 3.3 nmol/l, respectively. In the presence of the alpha-adrenoceptor blocking agent phentolamine (1 mumol/l), the concentration-response curve of phenylephrine was displaced to higher concentrations of the agonist; under these conditions, the EC50 value amounted to 52.5 mumol/l. The effects of the catecholamines noradrenaline and adrenaline on force of contraction remained unchanged in the presence of phentolamine (1 mumol/l) or prazosin (1 mumol/l). The positive inotropic effect of phenylephrine (1 mmol/l) was associated with a slight decrease in action potential duration; the effects on action potential were completely blocked in the presence of phentolamine (1 mumol/l). These findings support the view that selective stimulation of alpha-adrenoceptors may mediate a small but detectable positive inotropic effect in human atrial tissue under in vitro conditions. The requirement of high concentrations of alpha-adrenoceptor agonists and the lack of effects of the endogenous catecholamines adrenaline and noradrenaline on alpha-adrenoceptors (in concentrations which fully elicit the beta-adrenoceptors-mediated response) do not provide a basis for a functional role of alpha-adrenoceptor-mediated effects under in vivo conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of inotropic and chronotropic responses in rat isolated atria and ventricles

1. The positive inotropic and chronotropic responses to adrenoceptor agonists (noradrenaline, phenylephrine), to compounds which increase cAMP by post-adrenoceptor mechanisms (forskolin, theophylline and dibutyryl cAMP) and to calcium chloride were measured in isolated rat atria and papillary muscles from both ventricles. 2. Noradrenaline produced similar maximal inotropic responses to calcium chloride in all tissues. Forskolin gave similar responses to calcium chloride in atrial but not ventricular tissues; the reverse was observed with dibutyryl cAMP. Phenylephrine and theophylline produced significantly smaller inotropic responses than calcium chloride in all tissues, especially in ventricular tissues. 3. Maximal chronotropic responses to noradrenaline, theophylline and dibutyryl cAMP were similar. Forskolin produced significantly greater responses while calcium chloride and phenylephrine produced significantly smaller responses than noradrenaline. 4. These results show that the maximal positive inotropic response of some agonists is markedly dependent on the tissue chosen. Further, chronotropic responses in right atria do not mimic inotropic responses in left atria.

Myocardial alpha 1-adrenoceptors mediate positive inotropic effect and changes in phosphatidylinositol metabolism. Species differences in receptor distribution and the intracellular coupling process in mammalian ventricular myocardium

Species-dependent variations of myocardial alpha 1-adrenoceptor-mediated positive inotropic effects of epinephrine were assessed in relation to characteristics of alpha 1-receptor bindings and acceleration of phosphatidylinositol metabolism in the isolated rat, rabbit, and dog ventricular myocardium. Epinephrine in the presence of the beta-adrenoceptor antagonist bupranolol (10(-6) M) elicited a positive inotropic effect through activation of alpha 1-adrenoceptors in rat and rabbit, whereas in dog ventricular myocardium, bupranolol abolished the positive inotropic effect of epinephrine. [3H]Prazosin bound to membrane fractions derived from rat, rabbit, and dog ventricular muscle with high affinities in a saturable and reversible manner. In dog, Bmax and Kd values of alpha 1-adrenoceptor binding sites were identical to those in rabbit ventricular muscle. The Bmax value of alpha 1-adrenoceptors in rat ventricle was the highest, amounting to two to four times those in rabbit and dog. Epinephrine displacement curves for the specific binding of [3H]prazosin in the membrane fraction of these species showed high and low affinity sites with slope factors significantly less than unity, which were shifted to single low affinity sites with slope factors close to unity by addition of 5'-guanylylimidodiphosphate. Accumulation of [3H]inositol 1-phosphate [( 3H]IP1) in ventricular slices prelabeled with [3H]myo-inositol was increased by epinephrine in a time- and concentration-dependent manner in rat ventricular slices. [3H]IP1 accumulation likewise was facilitated by alpha 1-adrenoceptor stimulation in rabbit ventricular slices, whereas the extent of [3H]IP1 accumulation was much less than that in rat. In dog ventricular slices, [3H]IP1 was not accumulated by epinephrine. In rabbit papillary muscle, the time course of increase in contractile force induced by alpha-adrenoceptors coincided with the prolongation of the action potential duration with a similar time course, which is in strong contrast to previous findings in rat that the contractile response was dissociated from the electrophysiological response to alpha-adrenoceptor stimulation. The present results indicate that a wide range of variation of alpha 1-adrenoceptor-mediated regulation of myocardial contractility may be ascribed to different contributions of facilitatory as well as inhibitory regulatory processes that lead to intracellular Ca2+ mobilization subsequent to myocardial alpha 1-adrenoceptor activation among mammalian species.

Alpha-adrenoceptors do not contribute to the chronotropic or inotropic responses of the avian heart to noradrenaline

1. The chronotropic and inotropic responses of the young chick heart to noradrenaline have been investigated in isolated right atria, left atria and ventricular strips from 14-day-old chicks. 2. In the presence of desipramine and metanephrine to inhibit amine uptake, concentration-response curves to noradrenaline in all three tissues were shifted to the right by propranolol (1 microM) but were not altered by the presence of either phentolamine (5 microM) or prazosin (10 microM). 3. Similar results were obtained in the presence of cocaine (10 microM) and corticosterone (10 microM) to inhibit amine uptake. Propranolol (0.3-3.0 microM) produced rightward shifts of noradrenaline concentration-response curves which gave pA2 values of 8.1-8.4. Phentolamine (5 microM), in contrast, did not affect responses to noradrenaline in any tissue, either in the absence or presence of propranolol (1 microM). 4. Isoprenaline produced positive chronotropic responses in right atria and positive inotropic responses in left atria and ventricular strips. Methoxamine elicited positive inotropic responses in left atria but only negative chronotropic responses in right atria and negative inotropic responses in ventricular strips. 5. These results demonstrate that cardiac responses to noradrenaline in the chick heart are mediated via beta-adrenoceptors only, and that cardiac alpha-adrenoceptors are not involved in these responses.

Effect of UK-52,046, an alpha 1-adrenoceptor antagonist, on baroreflex function in man

1. In a placebo controlled study (six healthy male subjects), the effects of UK-52,046 (0.4 microgram kg-1 i.v.) and prazosin (0.25 mg i.v.) on baroreflex function were compared, at doses which produced antagonism to phenylephrine, but which had no effect on supine blood pressure. 2. Baroreflex function [delta R-R interval ms mm Hg-1 change in SBP] was assessed following increases in systolic blood pressure (SBP) with phenylephrine and during the Valsalva manoeuvre. 3. At these doses neither UK-52,046 nor prazosin had an effect on supine SBP or heart rate; however following prazosin, standing SBPs at 5 s (69.7 +/- 7.6 mm Hg) and at 3 min (65.5 +/- 11.7 mm Hg) were less than the respective pre-treatment (P less than 0.05) values (96.0 +/- 2.9, 110.3 +/- 6.2 mm Hg) and placebo (82.7 +/- 5.6, 98.7 +/- 11.1 mm Hg). UK-52,046 had no significant effects on standing SBP at 5 s or 3 min. At 5 s, pre- and post-treatment R-R intervals (584 +/- 26, 541 +/- 27 ms respectively) were not significantly different with prazosin, but at 3 min the post-treatment R-R interval following prazosin (519 +/- 17 ms) was less (P less than 0.05) than the pre-treatment value (658 +/- 36 ms). 4. UK-52,046 had no effect on baroreflex sensitivity (12.7 +/- 1.3 ms mm Hg-1) compared with placebo (17.9 +/- 2.7 ms mm Hg-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenoceptors in myocardial regulation: concomitant contribution from both alpha- and beta-adrenoceptor stimulation to the inotropic response

The studies presented deal with the alpha 1-adrenoceptor mediated inotropic effects of noradrenaline obtained by exclusive ("pure") alpha 1-adrenoceptor stimulation or by concomitant stimulation of alpha 1- and beta-adrenoceptors in myocardium. The pure beta-adrenergic effects of noradrenaline were also quantified. Interactions between the two receptor systems were studied. The pure alpha 1- and beta-adrenergic effects of noradrenaline, respectively, were achieved separately in the presence of high concentrations of appropriate receptor blockers. The experiments were performed on isolated ventricular myocardium from rat, rabbit, and man. The pure alpha 1-adrenergic inotropic effects were about 35-50% of control (basal) and half the pure beta-adrenergic effects both in rat and rabbit myocardium. Ventricular myocardium from man exhibited an alpha 1-adrenergic inotropic effect of the same magnitude (50% of control [basal]) as did rabbit papillary muscle. Determination of the alpha 1-adrenergic inotropic component during concomitant beta-adrenoceptor stimulation was associated with difficulties. Several experimental approaches on rat and rabbit myocardium are presented and discussed. Some types of experimental approaches obviously underestimate the alpha 1-adrenergic component. The methods regarded as reliable revealed an alpha 1-adrenergic inotropic effect of about 20-30% during combined adrenoceptor stimulation by noradrenaline. Concomitant beta-stimulation reduced the alpha 1-adrenergic effect by about 50%, while alpha 1-stimulation attenuated the beta-effect to a lesser degree (about 20-25%). A model is presented on a mutual attenuation of the functional expression of the two receptor systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Do both adrenaline and noradrenaline stimulate cardiac alpha-adrenoceptors to induce positive inotropy of rat atria?

1. The positive inotropic responses of rat paced left atria to adrenaline and noradrenaline were recorded. Desmethylimipramine (DMI, 1 microM) and metanephrine (10 microM) were initially present throughout. 2. The positive chronotropic responses of spontaneously beating right atria to adrenaline were used as a reference. In these, pindolol, in increasing concentrations, caused progressive shift of the concentration-response curves to the right, which yielded a pA2 value (8.15) compatible with antagonism of beta-adrenoceptors. 3. The left atrial tension responses to adrenaline showed an initial progressive displacement by pindolol (up to 3 microM) which gave an unexpectedly low pA2 value (6.48). However, with further increases in pindolol concentration there was no additional shift of the curve. In the presence of pindolol (3 microM), prazosin (0.1 microM) displaced the curve to the right but the pA2 value derived from this shift (7.75) was less than expected for alpha 1-adrenoceptor antagonism. 4. When the experiments in the presence of pindolol (3 microM) were repeated in the absence of DMI, prazosin displaced the concentration-response curves for adrenaline-induced left atrial tension to a greater extent and the pA2 value (8.76) was now compatible with adrenaline stimulating typical alpha 1-adrenoceptors. 5. The concentration-response curves for noradrenaline-induced left atrial tension were also progressively displaced to the right by pindolol (0.1, 0.3 and 1.0 microM). These concentrations yielded a Schild plot of unity slope and a pA2 value of 7.94 +/- 0.04. This was not significantly different from the pA2 value of 8.02 +/- 0.07 determined for pindolol against isoprenaline in the left atria, which indicates a normal interaction of noradrenaline with beta-adrenoceptors in the absence and presence of low concentrations of pindolol. 6. A further increase in the concentration of pindolol to 3 microM failed to induce an additional shift of the noradrenaline curves, whether a 'before and after' antagonist or a 'naïve tissue' design was adopted. Similarly, the rightwards shift of the concentration-response curves by timolol reached a limit as the concentration was increased. In all cases the limit of shift occurred at a noradrenaline EC50 value of 5-10 microM. 7. At the limit of beta-adrenoceptor antagonism, prazosin and dibenamine did not displace the noradrenaline curves further. The residual inotropic response to noradrenaline therefore appeared to be mediated via neither alpha- nor beta-adrenoceptors. 8. DMI, in the absence of beta-blockade, produced the potentiation of adrenaline and noradrenaline expected of a neuronal uptake inhibitor. However, in the presence of pindolol, there was no potentiation of the right atrial rate response to adrenaline while its left atrial tension responses were antagonized. This suggested that DMI was acting as an alpha-adrenoceptor antagonist. It also explained the less-than-expected shift by prazosin of the adrenaline responses in the presence of both pindolol and DMI, the latter drug already exerting some alpha-blocking activity. In contrast, the left atrial tension responses to noradrenaline in the presence of pindolol (1 microM) were neither potentiated nor antagonized by DMI. 9. When the effects of prazosin upon left atrial tension responses to noradrenaline in the presence of pindolol (10 microM) were examined in the presence of a lower concentration of DMI (O.1 microM) or cocaine (1O microM), again there was no further shift of the curve. However, when the effect of prazosin) The Macmillan Press Ltd 1989 598 K.L. WILLIAMSON & K.J. BROADLEY was examined in the absence of DMI, but in the presence of pindolol (1 and 1O microM) or timolol (3 microM), there was a small shift of the curves by prazosin (0.1 microM). This yielded pA2 values of 7.19, 7.34 +/- 0.1 and 7.66 +/- 0.09, which were at least one order of magnitude less than literature values and that obtained with adrenaline (8.76 +/- 0.18), and are not consistent with noradrenaline stimulating an alpha 1-adrenoreceptor in the presence of beta-adrenoceptor blockade, the increase in left atrial tension by noradrenaline does not appear to be mediated by beta l- or typical alpha-adrenoceptors. This is in contrast to adrenaline which in these conditions stimulates typical alpha 1-adrenoceptors.

Relations between beta-adrenoceptor occupancy and increases of contractile force and adenylate cyclase activity induced by catecholamines in human ventricular myocardium. Acute desensitization and comparison with feline ventricle

The function of beta-adrenoceptors was investigated in ventricular myocardium obtained from patients undergoing open heart surgery. 1. Dopamine increased contractile force up to 1/2 and 1/4 of the maximum increase caused by (-)-noradrenaline or (-)-adrenaline in right and left ventricular preparations, respectively. 2. beta-Adrenoceptors were labelled with 3H-(-)-bupranolol. For 3/4 of the receptors (beta 1) the affinity of (-)-noradrenaline was 20 times higher than for the remaining 1/4 (beta 2). (-)-Adrenaline and dopamine appeared to be non-selective for beta 1 and beta 2. 3. Dopamine was able to stimulate the adenylate cyclase only up to 1/3 of the maximum stimulation caused by (-)-noradrenaline and (-)-adrenaline. 4. Increases in contractile force by (-)-noradrenaline were closely associated with small increases of cyclase activity through beta 1-adrenoceptors, consistent with a common link. 5. The experiments on human myocardium were compared with similar experiments on feline myocardium. Feline ventricle exhibited a 20- to 30-fold higher sensitivity to catecholamines as activators of contractile force than did human ventricle. However, the binding affinities for catecholamines were similar in cat and man. 6. A 3 h exposure of human and feline ventricular myocardium to (-)-isoprenaline caused desensitization by uncoupling beta-adrenoceptors from the adenylate cyclase. Desensitization reduced the maximum contractile response to (-)-isoprenaline in human but not in feline ventricle. 7. The more efficient activation of contractile force by (-)-noradrenaline in cat, compared to man, appears to be related to a 2-fold higher density of beta 1-adrenoceptors, a 6-fold higher production of cyclic AMP per beta 1-adrenoceptor and possibly to a more effective use of cyclic AMP for contraction.

Relaxant effect of dopamine on the isolated rat uterus

The effect of dopamine was studied on the isolated uterus of diethylstilboestrol-treated rats. Dopamine, at concentrations (10(7)-10(-4) M) produced a concentration-dependent relaxation in the K+-depolarized rat uterus. On a molar basis, dopamine was about 500 times less potent than adrenaline in relaxing the uterus, the maximum degree of relaxation obtained with both drugs was the same. Pretreatment of the rats with reserpine (5 mg/kg) did not produce any modification of the dose-response curve to dopamine. Similarly, cocaine (3 x 10(-6) M) failed to modify the relaxant effect of dopamine. The dopamine induced relaxation was inhibited by propranolol (10(-9)-10(-7) M) in a dose-dependent manner. Prazosin (10(-7) M), SCH 23390 (10(-7) M) and sulpiride (10(-7) M) did not affect the dopamine dose-response curve. In the isolated rat uterus which was not preconstricted by KCl neither dopamine nor adrenaline produced any effect when added to the organ bath. This lack of response to both catecholamines was present even in tissues pretreated with propranolol or sulpiride. It is concluded that dopamine produced a concentration-dependent relaxation of the uterus from diethylstilboestrol-treated rats by direct activation of beta-adrenoceptors. There was no evidence for indirect action (catecholamine release and neuronal uptake mechanisms) and specific dopamine receptor mediated relaxation and alpha-adrenoceptor mediated contractions have not been found in this preparation.

Adrenoceptor-mediated changes of action potential and force of contraction in human isolated ventricular heart muscle

1. The effects of alpha-adrenoceptor stimulation on the action potential and force of contraction were investigated in human isolated ventricular heart muscle and compared with those of beta-adrenoceptor stimulation. 2. The maximal stimulation by isoprenaline of beta-adrenoceptors produced large changes in the force of contraction, which were accompanied by moderate increases in the height of the action potential. The maximal inotropic effect produced by stimulation of alpha-adrenoceptors with phenylephrine, in the presence of propranolol (1 mumol 1(-1)) was much smaller (about 10% of that seen in response to beta-adrenoceptor stimulation), and no significant changes of the action potential configuration were observed. 3. The effects of noradrenaline and adrenaline on the force of contraction were not affected by prazosin. 4. It is concluded that the adrenoceptor-mediated changes of the force of contraction (in the presence of either noradrenaline or adrenaline) in the human ventricle are due virtually exclusively to the stimulation of beta-adrenoceptors.

Intracoronary infusion of dobutamine to patients with and without severe congestive heart failure. Dose-response relationships, correlation with circulating catecholamines, and effect of phosphodiesterase inhibition

We infused dobutamine into the left main coronary artery of 24 patients with severe congestive heart failure (CHF) and 8 normal subjects without hemodynamic dysfunction. The maximal +dP/dt response to intracoronary (IC) dobutamine in CHF patients was only 37% of that in normals. This decrease in maximal response was not associated with a rightshift in the EC50 for dobutamine's effect on +dP/dt, or a decrease in the affinity of myocardial beta adrenergic receptors for dobutamine determined in vitro. In nine of the CHF patients, IC dobutamine infusion was followed by IC infusion of the phosphodiesterase inhibitor milrinone, and subsequently, by a second IC infusion of dobutamine. After IC milrinone, the increase in +dP/dt caused by IC dobutamine (74 +/- 10%) was significantly greater than that caused by the first infusion of dobutamine (52 +/- 11%; P less than 0.003) or milrinone (42 +/- 6%; P less than 0.001). Resting plasma norepinephrine was markedly elevated in CHF patients (837 +/- 208 ng/liter), but not in normal subjects (142 +/- 32 ng/liter); and the increase in +dP/dt caused by IC dobutamine was inversely related to resting plasma norepinephrine levels (r = -0.653; P less than 0.001). IC dobutamine caused a dose-related decrease in plasma norepinephrine (maximal effect, -160 +/- 31 ng/liter; P less than 0.001). Thus, (a) the maximal inotropic response to dobutamine is markedly depressed in patients with severe CHF, and is significantly greater after pretreatment with the phosphodiesterase inhibitor milrinone; (b) the impairment in inotropic response to dobutamine is inversely related to circulating norepinephrine levels; and (c) myocardial stimulation by dobutamine results in withdrawal of sympathetic tone.

Actions of sympathomimetic amines on the Ca2+ transients and contractions of rabbit myocardium: reciprocal changes in myofibrillar responsiveness to Ca2+ mediated through alpha- and beta-adrenoceptors

The effects of sympathomimetic amines on Ca2+ transients and isometric contractions were assessed in isolated rabbit papillary muscles in which multiple superficial cells had been microinjected with the calcium-sensitive bioluminescent protein aequorin. In the presence of beta-adrenoceptor blockade, the alpha-receptor agonist phenylephrine increased both the amplitude of the aequorin signals and the force of contraction in a concentration-dependent manner. However, the maximum increase in the aequorin signals was less than 10% of that produced by the beta-receptor agonist isoproterenol, while the maximum increase in force of contraction produced by alpha-stimulation was about 50% of that elicited via beta-adrenoceptors. For a given increase in the force of contraction, stimulation of alpha-adrenoceptors produced much less change in the amplitude of the aequorin signals than did elevation of the extracellular Ca2+ concentration; we interpret this to mean that the positive inotropic effect of alpha-adrenoceptor stimulation is in large part the result of an increase in myofibrillar sensitivity to Ca2+. Stimulation of alpha-adrenoceptors produced little change or a slight decrease in the duration of the aequorin signal and an increase in the duration of contraction, while stimulation of beta-adrenoceptors significantly decreased the time to peak and duration of both the aequorin signals and the contractions. For a given level of inotropic effect, high concentrations of isoproterenol often increased the aequorin signals more than did elevations of Ca2+, which is consistent with other evidence that the cyclic AMP-dependent phosphorylation of troponin I leads to a decrease in myofibrillar Ca2+ sensitivity. However, concentrations of isoproterenol that did not produce evidence of this sort of desensitization also abbreviated the contractions much more than they did the aequorin signals. This suggests that the traditionally accepted mechanisms--a decrease in the Ca2+ affinity of troponin C and an acceleration of Ca2+ uptake by the sarcoplasmic reticulum--may not be sufficient to account for the actions of beta-receptor stimulation on the time course of contraction. In the absence of blocking agents, the naturally occurring catecholamines norepinephrine, epinephrine, and dopamine appear to influence the function of the rabbit papillary muscle through both alpha- and beta-adrenoceptors. Dopamine has a relatively greater effect on alpha-adrenoceptors than the other catecholamines.

Cardiac alpha- and beta-adrenoceptor sensitivity and binding characteristics after chronic reserpine pretreatment

Cardiac alpha- and beta-adrenoceptor sensitivities were examined after chronic pretreatment of rats with reserpine. Increases in sensitivity would indicate that the receptor is under the influence of the sympathetic innervation, removal by catecholamine depletion with reserpine of the tonic effect of neurotransmitter release would permit receptor upregulation. The positive inotropic responses of paced left atria and papillary muscles and the positive chronotropic responses of spontaneously beating right atria were recorded. A concentration-response curve to isoprenaline (beta-adrenoceptor-mediated) was followed, in the presence of beta-blockade, by one to methoxamine (alpha-adrenoceptor-mediated). Methoxamine exerted positive inotropy of left atria and papillary muscles, the maxima being 43.2 +/- 2.7 and 26.8 +/- 4.4% of the isoprenaline maxima. A small positive chronotropy (16.5 +/- 5.6% maximum) of right atria occurred. After pretreatment with reserpine (1.0 mg kg-1 i.p. daily) for 7 days, the three preparations displayed supersensitivity to isoprenaline, revealed as a significant displacement (P less than 0.05) of the concentration-response curves to the left of those for control rats. Reserpine pretreatment, however, had no effect on the sensitivity to methoxamine. The increase in beta-adrenoceptor sensitivity to isoprenaline after reserpine pretreatment was accompanied by a significant 41.3% increase (P less than 0.05) in the number of [3H]-dihydroalprenolol [( 3H]-DHA) binding sites (Bmax) in ventricular membranes, although the dissociation constant (KD) was unaffected. There were more alpha-adrenoceptor [3H]-prazosin binding sites in ventricular than atrial membranes. However, there was no difference in KD or Bmax between reserpine-pretreated and control tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac alpha-1 adrenoceptors are not involved in heart rate control of the anaesthetized dog

To study the possible role of cardiac postsynaptic alpha-1 adrenoceptors in heart rate control of the anaesthetized open-chest dog we injected a specific alpha-1 agonist (amidephrine) into the right coronary artery or stimulated electrically the right stellate ganglion. Reflex influences were minimized by bilateral cervical vagotomy and de-afferentiation of both stellate ganglia. Activation of alpha-2, beta- and muscarinic receptors was prevented by intravenous administration of yohimbine, propranolol and atropine, respectively. Since alpha-1 receptor stimulation could affect heart rate indirectly via coronary constriction, a continuous intracoronary infusion of adenosine (0.25 mg/kg/h) was given. Amidephrine did not affect heart rate at the lower dose (1-10 microgram). After the highest dose (100 micrograms) the maximum variation in heart rate was an increase of 2.2 +/- 1.1 bpm at 3 min after injection (mean +/- SEM; P less than 0.05). This slight cardioacceleration was simultaneous with an aortic pressure rise of 13.8 +/- 3.4 mm Hg and it was abolished by alpha-1 blockade with prazosin (1 mg/kg i.v.). After propranolol (1 mg/kg +0.5 mg/kg/h) the residual positive chronotropic effect of sympathetic stimulation (12.2 +/- 4.0 bpm) was not significantly altered (13.8 +/- 5.7 bpm) by prazosin administration. Similar results were recorded without adenosine infusion. We conclude that in the anaesthetized dog chronotropic effects directly mediated by alpha-1 adrenoceptors either do not exist or lack physiological significance.

Functional alpha-adrenoceptors in human atrial preparations in the presence of beta-receptor blockade

Inotropic effects via cardiac alpha-adrenoceptors were studied in electrically driven auricular strips (1 Hz, 37 degrees C) from patients treated with beta-blockers for months prior to open heart surgery. Marked alpha-mediated positive inotropic effects were demonstrated with adrenaline (A), noradrenaline (NA) and phenylephrine (PHE) in the presence of beta-blocker and with blockers of the muscarinic receptor and of the neuronal and extraneuronal uptake mechanisms for the catecholamines. In the presence of approximately 10(-6) M propranolol the maximal effects as well as the potencies (pD2-values) for A and NA were not significantly different while higher than for PHE. The alpha 1-blocker, prazosin (10(-6) M), markedly reduced the pD2-values but not the intrinsic activities (alpha-values) for A, NA and PHE in the beta-blocked preparations. Methoxamine, however, induced negative inotropic responses at normal and low frequencies (1, 0.5 and 0.1 Hz) of stimulation, suggestive of non-specific, cardiodepressant effects. Other agonists with alpha-effects in other types of tissue (oxymethazoline, xylomethazoline and clonidine) were without effects on the force and velocity of contraction in the auricular strips under the present experimental conditions. The results show alpha 1-type of adrenoceptor-induced inotropic effects for A, NA and PHE during beta-blockade in human auricular strips, indicating that cardiac alpha 1-receptors may have clinical importance by increasing the inotropy of the human myocardium treated with beta-blocking agents.

The myocardial contractile responses to protamine sulfate and heparin

The administration of protamine sulfate for the reversal of heparin anticoagulation has been associated with adverse hemodynamic changes including hypotension and decreased cardiac output. The possible direct toxic effect of protamine on human right atrial trabeculae contracting isometrically in vitro was studied. Muscles were stimulated to contract at 1 Hz in Tyrode's solution (maintained at 34 degrees C, pH 7.4) into which protamine was continuously added. Following a polynomial regression analysis, a parabolic dose-response curve resulted. The equation was: y = 95.13 + 38.76x - 278.71x2 where y = relative developed force and x = concentration of protamine (milligrams per milliliters) (r = 0.82). The estimated concentration of protamine resulting in 50% developed force was 0.48 mg/ml. In a second series of experiments, protamine was added to the bath along with a neutralizing amount of heparin. This resulted in a limited reduction in the fall of relative developed force. Thus, protamine in high concentrations alone or in complex with heparin has a direct toxic effect on human myocardial muscle mechanics, and care is warranted in its clinical use.

Dobutamine: positive inotropy by nonselective adrenoceptor agonism in isolated guinea pig and human myocardium

Positive inotropic responses to dobutamine have been examined using isolated myocardium from guinea pigs und humans. The potency (EC50) of dobutamine was 1.5 X 10(-6) mol/l on guinea pig papillary muscles, 1.8 X 10(-6) mol/l on guinea pig left atria and 2.5 X 10(-6) mol/l on human papillary muscle strips. In guinea pig cardiac muscles, Schild plots for the beta 1-selective antagonist, 1-practolol, using dobutamine as agonist, had slopes of less than unity. This suggested the involvement of other receptors in the inotropic response to dobutamine. The beta 2-selective antagonist, ICI 118,551, but not the alpha 1-selective antagonist, prazosin, attenuated the dobutamine response in guinea pig papillary muscles. Both ICI 118,551 and prazosin shifted the dobutamine concentration-response curve in guinea pig left atria. The positive inotropic response to dobutamine in human papillary muscles was antagonised by l-practolol and ICI 118,551 but not by prazosin. The maximal inotropic response to dobutamine was 90% that of calcium measured in the same guinea pig papillary muscles but only 37% that of calcium in human papillary muscle strips. This reduced maximal effect of dobutamine in human myocardium is probably a disease-induced change but species variations cannot be excluded.

The inotropic effects of dopamine and its precursor levodopa on isolated human ventricular myocardium

The direct positive inotropic effects of dopamine and its precursor, levodopa, were measured using isolated, contracting human papillary muscle strips taken from patients during mitral valve replacement. Levodopa did not produce any positive inotropic effect at concentrations up to 3 X 10(-3) M. The positive inotropic effects of dopamine were observed at concentrations above 1 X 10(-5) M with the maximal effect at 3 X 10(-3) M - concentrations higher than those observed in therapy. This inotropic effect was reduced by the beta 1-selective antagonist, 1-practolol (1 X 10(-6) M); the beta 2-selective antagonist, ICI 118,551 HCl (1 X 10(-6) M); the dopamine antagonist, haloperidol (3 X 10(-6) M); the neuronal uptake inhibitor, cocaine (3 X 10(-5) M), but not by the alpha 1-antagonist, prazosin (1 X 10(-7) M). This indicates that dopamine exerts its positive inotropic effects on human heart muscle mainly through release of noradrenaline, together with possible interactions at beta- and dopamine-receptors. The maximal inotropic effect of dopamine was about 50% that of calcium (15 mM, 6.2 +/- 0.7 mN) or ouabain (1 X 10(-7) M, 5.0 +/- 0.8 mN) when measured in the same muscle strips, possibly due to the reduced cardiac noradrenaline content together with the reduced beta-receptor number in congestive heart failure. This concentration of ouabain (1 X 10(-7) M) gave almost maximal inotropy without marked toxicity; when dopamine was then added, only toxicity developed without any further increases in force of contraction. Any haemodynamic benefits of dopamine therapy in optimally digitalis-treated patients are probably due to other cardiovascular effects such as vasodilatation.

[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.

Interactions of the enantiomers of 3-O-methyldobutamine with alpha- and beta-adrenoceptors in vitro

The enantiomers of 3-O-methyldobutamine, a metabolite of dobutamine, were evaluated for their alpha- and beta-adrenoceptor mediated effects in vitro in a variety of isolated organs and in radioligand binding studies. Neither enantiomer of 3-O-methyldobutamine possessed alpha 1-adrenoceptor agonist activity in isolated guinea pig aorta. However, both enantiomers of 3-O-methyldobutamine were competitive alpha 1-adrenoceptor antagonists, with the (+)-enantiomer being approximately 10-fold more potent than the (-)-enantiomer as assessed either in guinea pig aorta or by displacement of 3H-prazosin binding from alpha 1-adrenoceptors in rat cerebral cortex. The alpha 1-adrenoceptor blocking activity of (+)-3-O-methyldobutamine was relatively potent and corresponded to a pA2 of 7.33 in guinea pig aorta and a -log Ki of 7.72 in radioligand binding studies. Neither enantiomer of 3-O-methyldobutamine possessed alpha 2-adrenoceptor agonist activity in field-stimulated guinea pig ileum. Although (+)-3-O-methyldobutamine weakly inhibited the twitch response in field-stimulated guinea pig ileum, the response was not blocked by the selective alpha 2-adrenoceptor antagonist, yohimbine, and was found to result from weak anticholinergic activity (pA2 = 5.06). Neither enantiomer of 3-O-methyldobutamine possessed beta 1-adrenoceptor agonist activity in guinea pig atria, however the (+)-enantiomer was a weak noncompetitive antagonist at beta 1-adrenoceptors. In contrast, both enantiomers of 3-O-methyldobutamine were weak beta 2-adrenoceptor agonists in rat uterus, however these weak effects were not highly stereoselective, which was also confirmed in radioligand binding studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between the stereoselective negative inotropic effects of verapamil enantiomers and their binding to putative calcium channels in human heart

Ventricular preparations from patients with mitral disease and hypertrophic obstructive cardiomyopathy (HOCM) were set up to contract isometrically. Ventricular membrane particles were also prepared and putative calcium channels were labelled with [3H]-nimodipine. Positive staircase was induced by varying the rate of stimulation of isolated strips from 6 min-1 to 120 min-1 in the presence of 6-60 microM (-)-adrenaline or (-)-noradrenaline. (-)-Verapamil 3-5 microM or (+)-verapamil 20-30 microM reversed the force-frequency relationship (i.e. caused negative staircase) in preparations from patients with mitral disease or HOCM. In subendocardial strips of ventricular septum from 5 patients with HOCM paced at 60 min-1, both (-)-verapamil and (+)-verapamil caused cardiodepression. Half-maximal cardiodepression was observed with 0.4 microM (-)-verapamil and with 3 microM (+)-verapamil. [3H]-nimodipine bound to ventricular membrane particles in a saturable, reversible fashion to a high affinity site with an equilibrium dissociation constant of 0.23 nM. The density of these sites was 95 fmol mg-1 of membrane protein. Binding of the tritiated 1,4-dihydropyridine was stereoselectively inhibited by 1,4-dihydropyridine enantiomers and nifedipine. (-)-Verapamil and (+)-verapamil inhibited high affinity [3H]-nimodipine binding in a negative heterotropic allosteric manner with (-)-verapamil being 5 times more potent than (+)-verapamil on an IC50 basis. At a given [3H]-nimodipine concentration, (+)-verapamil inhibited a greater fraction of specific [3H]-nimodipine binding. The allosteric mode of (+)-verapamil inhibition of [3H]-nimodipine binding was confirmed by kinetic studies. (-)-Verapamil shifted (+)-verapamil-binding inhibition curves to the right in an apparently competitive fashion. The inversion of staircase caused by both verapamil enantiomers suggests that they cause a use-dependent channel blockade. The similar potency ratios for binding and for cardiodepression are indicative of a common locus of action for both verapamil enantiomers within the calcium channel.

Systemic hemodynamic effects of dopamine, (+/-)-dobutamine and the (+)-and (-)-enantiomers of dobutamine in anesthetized normotensive rats

The hemodynamic effects of dopamine, (+/-)-dobutamine (racemic mixture) and the (+)- and (-)-enantiomers of dobutamine were evaluated in anesthetized normotensive rats. Dopamine and (+/-)-dobutamine produced hemodynamic effects in anesthetized rat that were qualitatively similar to those reported for these compounds in man. The increase in cardiac output produced by dopamine and (+/-)-dobutamine was due mainly to an increase in stroke volume, with heart rate being only minimally affected. Dopamine produced a large increase in mean arterial pressure and slightly increased total peripheral vascular resistance, whereas (+/-)-dobutamine only modestly increased blood pressure and significantly reduced total peripheral resistance. The (-)-enantiomer of dobutamine, which possesses mainly alpha 1-adrenoceptor agonist activity, produced marked increases in cardiac output, stroke volume, total peripheral resistance and mean arterial pressure, but did not significantly increase heart rate. In contrast, (+)-dobutamine, which possesses predominantly beta 1-and beta 2-adrenoceptor agonist activity, elicited only a modest increase in cardiac output which was due entirely to increased heart rate since stroke volume was not increased. Total peripheral vascular resistance and mean arterial blood pressure were both reduced by (+)-dobutamine, characteristic of a beta-adrenoceptor agonist. The increase in cardiac output and blood pressure produced by (+/-)-dobutamine, but not the positive chronotropic effect, were significantly inhibited by alpha 1-adrenoceptor blockade with prazosin.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions of three inotropic agents, ASL-7022, dobutamine and dopamine, with alpha- and beta-adrenoceptors in vitro

Three inotropic agents, ASL-7022, dobutamine and dopamine, were evaluated for their alpha- and beta-adrenoceptor mediated effects in vitro in a variety of isolated organs and in radioligand binding studies. All compounds were alpha 1-adrenoceptor agonists in rat and guinea pig aortae, but the rank orders of potency were exactly opposite in these two tissues. Only the rank potency order of dobutamine greater than ASL-7022 greater than dopamine obtained in rat aorta was consistent with the results obtained in radioligand binding studies to alpha 1-adrenoceptors in rat cerebral cortex and to previous results obtained in vivo in the pithed rat. The results obtained in guinea pig aorta did not parallel the radioligand binding studies in rat brain or our previous results in pithed rat, and suggests that species differences exist between postsynaptic vascular alpha 1-adrenoceptors in rat and guinea pig aorta, consistent with previous conclusions. ASL-7022 was found to be a potent alpha 2-adrenoceptor agonist in field-stimulated guinea pig ileum, and was approximately 10-fold more potent than dobutamine in this respect, which was also confirmed by radioligand binding studies to alpha 2-adrenoceptors in rat cerebral cortex. The beta 1-adrenoceptor mediated effects of these compounds were evaluated in guinea pig atria, where the rank order of potency was dobutamine greater than ASL-7022 greater than dopamine. An identical rank order of affinity was established for these compounds by displacement of 3H-dihydroalprenolol from beta 1-adrenoceptors in rat cerebral cortex. The beta 1-adrenoceptor mediated effects of dobutamine and ASL-7022 in guinea pig atria were completely direct in nature and not secondary to the release of endogenous catecholamines. In contrast, a major component of the beta 1-adrenoceptor mediated tachycardia produced by dopamine in guinea pig atria was indirect in nature as evidenced by the marked attenuation in potency that occurred following catecholamine depletion with reserpine. All three compounds elicited beta 2-adrenoceptor mediated inhibition of tone in rat uterus, with the rank order of potency being ASL-7022 greater than dobutamine greater than dopamine. Again, this rank order of beta 2-adrenoceptor potency was also reflected in beta 2-adrenoceptor affinity as assessed by displacement of 3H-dihydroalprenolol from beta 2-adrenoceptors in rat cerebellum. Based on these results, it may be concluded that for alpha-adrenoceptors, dobutamine is a selective alpha 1-adrenoceptor agonist, ASL-7022 is a selective alpha 2-adrenoceptor agonist, and dopamine is a nonselective alpha-adrenoceptor agonist.(ABSTRACT TRUNCATED AT 400 WORDS)

Beta-adrenoceptors of the human myocardium: determination of beta 1 and beta 2 subtypes by radioligand binding

beta-Adrenoceptors of the human myocardium were investigated with binding studies using 125iodocyanopindolol (ICYP) as ligand. Inhibition of ICYP-binding by betaxolol (a selective beta 1-antagonist) and ICI 118551 (a selective beta 2-blocking drug) resulted in non-linear Scatchard-plots suggesting that both beta-adrenoceptor subtypes are present in human left atrium and left ventricle. Computer analysis of the data gave a beta 1/beta 2-adrenoceptor ratio of approximately 65:35 both for left atrium and for left ventricle.

Age-dependent differences in the positive inotropic effect of phenylephrine on rat isolated atria

The age-dependent differences in the involvement of alpha-adrenoceptors in the positive inotropic effect of phenylephrine (Phe) were examined in isolated atria of male Wistar rats 6 weeks (6W), 10 weeks and 7 months (7M) of age. The maximal increase in tension development induced by Phe increased with age, whereas the EC50 values for the positive inotropic effect of Phe did not change with age. The inhibitory effect of phentolamine on the response to Phe increased with age. Propranolol caused only slight inhibition of the effect of Phe in both 6W and 7M rats, and the EC50 values for Phe in the presence of propranolol did not change significantly with age. The EC50 values for isoprenaline and 5-hydroxytryptamine in 7M rats were higher than those in 6W rats. In 7M rats, the duration of the tension development was only slightly affected by Phe in the presence or absence of propranolol, but it was markedly decreased by Phe in the presence of phentolamine. The dose-response curve for Phe was markedly shifted to the left by papaverine in 6W rats, but slightly in 7M rats. The dose-response curve for isoprenaline was markedly shifted to the left by papaverine in both groups. These results are consistent with effects of Phe being mediated by both alpha- and beta-adrenoceptors in both 6W and 7M rats, but there is a shift in the balance from rather more beta-receptors in the young animals to more alpha-receptors in the adults.

An initial characterization of human heart beta-adrenoceptors and their mediation of the positive inotropic effects of catecholamines

The positive inotropic effects of catecholamines were studied on samples of ventricular myocardium taken from patients undergoing open heart surgery. The adenylyl cyclase and binding of 3H-(-)-bupranolol were examined in membrane particles prepared from similarly obtained samples. The equilibrium dissociation constant (KD) for (-)-bupranolol was estimated in 4 ways: blockade of the positive inotropic effects of catecholamines, blockade of the stimulation of the adenylyl cyclase by catecholamines, saturation binding of 3H-(-)-bupranolol, inhibition of the binding of 3H-(-)-bupranolol by its unlabeled stereoisomers. The estimates of KD fall in the range 0.5-1.4 nmol/l. The stereo-selectivity ratio (KD (+)-isomer/KD (-)-isomer) is 73. Both values for bupranolol are very similar in cat and man. The inotropic potency of (-)-noradrenaline is nearly 2 orders of magnitude higher in cat heart tissues than in tissues from human hearts. The difference in inotropic potencies between species is only partially accounted for by the five-fold lower potency of (-)-noradrenaline for the human heart adenylyl cyclase as compared to the cat enzyme.

alpha-Adrenoceptors mediating the positive inotropic effect of phenylephrine in the right ventricular muscle of the monkey (Macaca fuscata)

The property of adrenoceptors mediating the positive inotropic effect (PIE) in the ventricular muscle of the Japanese monkey (Macaca fuscata) was investigated by the use of phenylephrine (PE) and adrenoceptor antagonists. The intrinsic activity (0.6) and the pD2-value (5.41) for PE were comparable to those in other mammalian species. The beta-adrenoceptor antagonist, pindolol (3 x 10(-8) mol/l) shifted only the upper part of the concentration-response curve (CRC) for PE to the right; the pD2-value for PE was not significantly affected by pindolol. On the other hand, phentolamine (10(-6) mol/l) shifted the lower part of the CRC for PE more than the upper part. In the presence of both pindolol and phentolamine the curve was shifted to the right in a parallel manner. The time required for twitch relaxation was negatively correlated to the degree of PIE of PE in the presence of phentolamine but not pindolol. These results indicate that both beta- and alpha-adrenoceptors mediate the positive inotropic action in the ventricular muscle of the Japanese monkey and that in contrast to the action via beta-adrenoceptors the action via alpha-adrenoceptors is not accompanied by the "relaxant effect".