Influence of acetylcholine on the positive inotropic effect evoked by alpha- or beta-adrenoceptor stimulation in the rabbit heart

Adenosine A1 receptors heterodimerize with β1- and β2-adrenergic receptors creating novel receptor complexes with altered G protein coupling and signaling

G protein coupled receptors play crucial roles in mediating cellular responses to external stimuli, and increasing evidence suggests that they function as multiple units comprising homo/heterodimers and hetero-oligomers. Adenosine and β-adrenergic receptors are co-expressed in numerous tissues and mediate important cellular responses to the autocoid adenosine and sympathetic stimulation, respectively. The present study was undertaken to examine whether adenosine A1ARs heterodimerize with β1- and/or β2-adrenergic receptors (β1R and β2R), and whether such interactions lead to functional consequences. Co-immunoprecipitation and co-localization studies with differentially epitope-tagged A1, β1, and β2 receptors transiently co-expressed in HEK-293 cells indicate that A1AR forms constitutive heterodimers with both β1R and β2R. This heterodimerization significantly influenced orthosteric ligand binding affinity of both β1R and β2R without altering ligand binding properties of A1AR. Receptor-mediated ERK1/2 phosphorylation significantly increased in cells expressing A1AR/β1R and A1AR/β2R heteromers. β-Receptor-mediated cAMP production was not altered in A1AR/β1R expressing cells, but was significantly reduced in the A1AR/β2R cells. The inhibitory effect of the A1AR on cAMP production was abrogated in both A1AR/β1R and A1AR/β2R expressing cells in response to the A1AR agonist CCPA. Co-immunoprecipitation studies conducted with human heart tissue lysates indicate that endogenous A1AR, β1R, and β2R also form heterodimers. Taken together, our data suggest that heterodimerization between A1 and β receptors leads to altered receptor pharmacology, functional coupling, and intracellular signaling pathways. Unique and differential receptor cross-talk between these two important receptor families may offer the opportunity to fine-tune crucial signaling responses and development of more specific therapeutic interventions.

Positive inotropic, negative chronotropic, and coronary vasoconstrictor effects of acetylcholine in isolated rat hearts: role of muscarinic receptors, prostaglandins, protein kinase C, influx of extracellular ca2+, intracellular Ca2+ release, and endothelium

The involvement of nitric oxide (NO), muscarinic receptors, prostaglandins, calcium influx via slow calcium channels, Ca2+ release from intracellular stores, protein kinase C, and endothelium in the positive inotropic, negative chronotropic, and coronary vasoconstrictor effects of acetylcholine (ACh) has been investigated in isolated rat hearts. The perfusion of hearts with ACh (10(-7), 5 x 10(-7), and 10(-6) M) produced marked decreases in heart rate and coronary flow and a marked increase in contractile force. Similar effects have been observed during the perfusion of hearts with ACh in the presence of Nomega-nitro-L-arginine methyl ester (L-NAME), which is an inhibitor of NO synthesis. The positive inotropic, negative chronotropic, and coronary vasoconstrictor effects of ACh were abolished by muscarinic receptor blocker atropine. In hearts pretreated with cyclooxygenase inhibitor indomethacin, ACh significantly decreased heart rate but did not significantly affect coronary flow and contractile force. In the presence of calcium channel antagonist verapamil or protein kinase C inhibitor staurosporine, ACh produced a significant drop in heart rate but did not significantly affect coronary perfusion pressure and force of contraction. In the presence of the inhibitor of the release of Ca2+ from intracellular stores dantrolene sodium, ACh produced a significant increase in coronary perfusion pressure and a marked decline in heart rate, but did not significantly affect force of contraction. Furthermore, the disruption of endothelium by perfusing the hearts with saponin abolished the vasoconstrictor effect of ACh but did not alter negative chronotropic and positive inotropic effect. Our results suggest that ACh causes vasoconstrictor, negative chronotropic, and positive inotropic effects in isolated rat hearts. Cardiac effects of ACh are related to muscarinic receptor activation, and prostaglandins modulate ACh-induced vasoconstriction and positive inotropy. Our data also suggest that protein kinase C and calcium influx from extracellular source may be responsible for the vasoconstrictor and positive inotropic effect of ACh. The calcium release from intracellular stores may mediate the positive inotropic effect, and the vasoconstrictor effect of ACh depends on an intact endothelium.

Cyclic GMP-dependent protein kinase activation in the absence of negative inotropic effects in the rat ventricle

1. It has been suggested that activation of cyclic GMP-dependent protein kinase (PKG) is a necessary step in the chain of events leading to the production of negative inotropy by muscarinic receptor agonists in mammalian ventricles, and that some cyclic GMP-elevating agents, such as sodium nitroprusside (SNP), fail to exert a negative inotropic effect because they elevate cyclic GMP levels in a pool that does not activate the kinase. This hypothesis was tested in the present study by monitoring the effects of carbachol, SNP and atrial natriuretic peptide (ANP) on contractility, cyclic GMP content and PKG activity in rat intact ventricular preparations and freshly isolated ventricular cardiomyocytes. 2. The presence of PKG in both the intact vehicle and in isolated ventricular cardiomyocytes was confirmed by MonoQ anion exchange chromatography and Western blotting. The elution profile indicated that the conditions of the PKG assay were selective for measuring PKG activity. 3. Carbachol induced a marked negative inotropic effect in intact, perfused hearts and ventricular strips in the presence of isoproterenol. The negative inotropic effect of carbachol was not associated with significant changes in cyclic GMP content or PKG activity in intact ventricular tissue, or in PKG activity in isolated cardiomyocytes. 4. SNP and ANP significantly increased cyclic GMP levels and activated PKG in intact ventricular preparations. Both drugs also activated PKG in isolated cardiomyocytes. However, neither drug had any negative inotropic effect in isoprenaline-stimulated perfused hearts and ANP did not change the contractility of isoprenaline-stimulated isolated cardiomyocytes. 5. The results of this study demonstrate that the negative inotropic effects of muscarinic receptor agonists can occur in the absence of significant activation of PKG. Conversely, marked increases in ventricular cyclic GMP content and PKG activity caused by SNP or ANP were not accompanied by a negative inotropic effect. 6. These results suggest that increases in cyclic GMP levels and activation of PKG do not play important roles in the regulation of rat ventricular contractility by muscarinic receptor agonists.

The effects of various drugs on the myocardial inotropic response

1. The signal transduction process mediated by cyclic AMP that leads to the characteristic positive inotropic effect (PIE) in association with a positive lusitropic effect (acceleration of rate of twitch relaxation) has been well established. Relationships between accumulation of cyclic AMP, changes in intracellular Ca2+ transients and the PIE differ, however, depending on the mechanism of particular drugs that affect different steps in the metabolism of cyclic AMP. Selective partial agonists of beta 1-adrenoceptors and inhibitors of phosphodiesterase (PDE) III cause the accumulation of less cyclic AMP for a given PIE than does isoproterenol. In addition, in aequorin-microinjected canine ventricular muscle, selective inhibitors of PDE III, OPC 18790 and Org 9731, produced smaller decreases in the responsiveness of myofilaments to Ca2+ ions than isoproterenol, while a partial agonist of beta 1-adrenoceptors, denopamine, elicits a decrease in Ca2+ responsiveness of the same extent as does isoproterenol. 2. Activation of myocardial alpha 1-adrenoceptors, as well as stimulation of receptors for endothelin and angiotensin II, which accelerates hydrolysis of phosphoinositide (PI) to result in production of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) are associated with very similar inotropic regulation: (1) the dependence on the species of animals of induction of the PIE; (2) an excellent correlation between the extent of acceleration of hydrolysis of PI and the PIE; (3) isometric contraction curves associated with a negative lusitropic effect; (4) the PIE associated with increases in myofibrillar responsiveness to Ca2+ ions; and (5) the selective inhibition of the PIE by an activator of protein kinase C (PKC), phorbol 12,13-dibutyrate (PDBu), with little effect on the PIE of isoproterenol and Bay k 8644. 3. A novel class of cardiotonic agents, namely, Ca2+ sensitizers such as EMD 53998 and Org 30029, act on the Ca(2+)-binding site of troponin C, increasing the affinity of these sites for Ca2+ ions, or at the actin-myosin interface to facilitate the cycling of cross-bridges. These agents produce a PIE with little change or decrease in Ca2+ transients and may bring about a significant breakthrough in the development of drugs for reversal of myocardial failure in the treatment of congestive heart failure.

Muscarinic receptor stimulation and cyclic AMP-dependent effects in guinea-pig ventricular myocardium

1. The effect of carbachol on force of contraction, contraction duration, intracellular Na+ activity and cyclic AMP content was studied in papillary muscles of the guinea-pig exposed to isoprenaline or the phosphodiesterase inhibitor 3-isobutyl, 1-methyl xanthine (IBMX). The preparations were obtained from reserpine-pretreated animals and were electrically driven at a frequency of 0.2 Hz. 2. Isoprenaline (10 nM) and IBMX (100 microM) produced comparable positive inotropic effects of 9.8 and 9.7 mN, respectively. Carbachol (3 microM) attenuated the inotropic effects by 82% (isoprenaline) and by 79% (IBMX). The shortening of contraction duration which accompanied the positive inotropic effect of isoprenaline (by 14.9%) and of IBMX (by 22.4%) was not significantly affected by 3 microM carbachol. 3. The positive inotropic effect of 10 nM isoprenaline and of 100 microM IBMX was accompanied by an increase in cellular cyclic AMP content of 58 and 114%, respectively. Carbachol (3 microM) failed to reduce significantly the elevated cyclic AMP content of muscles exposed to either isoprenaline or IBMX. 4. In the quiescent papillary muscle, isoprenaline (10 nM) and IBMX (100 microM) reduced the intracellular Na+ activity by 28 and 17%, respectively. This decline was not influenced by the additional application of 3 microM carbachol. 5. The results demonstrate that muscarinic antagonism in guinea-pig ventricular myocardium exposed to cyclic AMP-elevating drugs is restricted to force of contraction. The underlying mechanism does not apparently involve the cytosolic signal molecule cyclic AMP.

Regulation of force and intracellular calcium transients by cyclic AMP generated by forskolin, MDL 17,043 and isoprenaline, and its modulation by muscarinic receptor agents: a novel mechanism for accentuated antagonism

The relation of changes in intracellular calcium transients and force of isometric contractions in response to an elevation or reduction of cyclic AMP levels was investigated in isolated dog ventricular trabeculae and rabbit papillary muscles, in which multiple superficial cells have been microinjected with the calcium sensitive bioluminescent protein aequorin. Forskolin, MDL 17,043 and isoprenaline elevated the tissue cyclic AMP level, increased consistently the peak aequorin signals and force, and abbreviated the duration of both signals in a concentration-dependent manner. When the effect of isoprenaline was compared with that of alteration of extracellular calcium concentration [( Ca2+]0), the increase in force by isoprenaline was associated with higher peak aequorin signals than that by alteration of [Ca2+]0 for a given increase in force, indicating the decrease in calcium sensitivity of myofibrils by cyclic AMP generated by beta-adrenoceptor stimulation. Carbachol, which did not affect significantly the basal force and cyclic AMP levels, lowered the cyclic AMP levels elevated previously by forskolin, MDL 17,043 or isoprenaline in the isolated dog ventricular trabeculae. It antagonized the increase in peak aequorin signals and force caused by these agents in a concentration-dependent manner. When carbachol had been administered prior to isoprenaline and the concentration-response curve for isoprenaline was determined in the presence of carbachol, the relation of force peak aequorin signals was not modified by carbachol in the rabbit papillary muscle. Carbachol, when administered during induction of the positive inotropic action by forskolin, MDL 17,043 and isoprenaline, decreased the force more than peak aequorin signals in a concentration-dependent manner in the dog ventricular trabeculae. Therefore, the relation of force to peak aequorin signals was shifted downwards during the carbachol-induced inhibition, indicating a further decrease of calcium sensitivity of myofibrils by carbachol. This effect of carbachol appears to be specific to the cyclic AMP-mediated positive inotropic action, since the alpha-adrenoceptor-mediated (cyclic AMP-independent) action was unaffected by carbachol. This mechanism may play an important role for "accentuated antagonism" in the mammalian ventricular myocardium.

Diminution by benzodiazepines of the chronotropic responses to noradrenaline in rat isolated atria

The effects of various benzodiazepines on chronotropic responses were assayed in spontaneously beating rat isolated atria. The increases in atrial rate obtained from concentration-response curves to noradrenaline were reduced dose dependently by both the peripheral agonist, Ro 5-4864 5 and 10 microM, and the mixed agonist, diazepam 5, 10 and 50 microM, but not by the central benzodiazepine agonist, clonazepam 10 and 30 microM. The inhibitory effects of the benzodiazepines on the atrial responses to noradrenaline were not counteracted by either the peripheral benzodiazepine antagonist, PK 11195 10 microM, or the central benzodiazepine antagonist, Ro 15-1788 10 and 100 microM. Both 10 microM Ro 5-4864 and 10 microM diazepam also reduced the increases in atrial rate produced by either the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, or the adenylate cyclase activator, forskolin. On the contrary, diazepam and Ro 5-4864 did not modify the chronotropic responses of the atria either to direct exposure to CaCl2 or to the calcium agonist, BAY K 8644. The increases in the intracellular levels of cAMP induced by noradrenaline were not modified by Ro 5-4864 and were even increased by 50% in the presence of diazepam. It is concluded that benzodiazepines probably reduce the chronotropic responses to noradrenaline in rat isolated atria through the interaction with the cAMP-linked chain of events that follows the activation of beta-adrenoceptors.

h-ANF does not play a role in the regulation of myocardial force of contraction

In the present study, h-ANF failed to reduce myocardial force of contraction alone and in the presence of isoprenaline. This lack of effect was observed in electrically driven, isolated human papillary muscle strips and rat papillary muscles. In contrast, adenosine and carbachol exerted antiadrenergic actions in rat and human myocardium. It is concluded that h-ANF obviously does not play a role in the regulation of human and rat myocardial force of contraction, whereas adenosine and carbachol are capable to produce antiadrenergic effects in the human and rat myocardium.

Enhancement of the alpha-adrenergic inotropic component of noradrenaline by simultaneous stimulation of muscarinic acetylcholine receptors in rat myocardium

The contribution of an alpha-adrenoceptor-mediated component to the final inotropic response to noradrenaline in the absence and presence of muscarinic acetylcholine receptor stimulation (which exerts a 'functional' antagonism of effects mediated through beta-adrenoceptors but not through alpha-adrenoceptors) was evaluated by recording contraction and relaxation in isolated, paced rat papillary muscles. In the absence of muscarinic acetylcholine receptor stimulation, the alpha 1-selective adrenoceptor blocker prazosin (0.12 microM) did not significantly influence the dose-dependent response to noradrenaline with respect to either contractility or to relaxation. In the presence of concomitant muscarinic acetylcholine receptor stimulation by 10 microM carbachol, prazosin reduced by 32% (alpha = 0.028) the maximal increase in contractility (expressed as (dT/dt)max) evoked by noradrenaline compared to the absence of prazosin. Prazosin also did not influence the effect of noradrenaline upon relaxation under these conditions. Carbachol itself did not significantly reduce the maximal contractile response to noradrenaline. Thus cholinergic stimulation increases both relatively and absolutely the alpha-adrenergic inotropic component of noradrenaline. These observations indicate a ternary regulatory system of myocardial contractility through the autonomic receptors.

Increased responsiveness to stimulation of alpha- but not beta-adrenoceptors in the hereditary cardiomyopathy of the Syrian hamster. Intact adenosine- and cholinoceptor-mediated isoprenaline antagonistic effect

The effects of phenylephrine (in the presence of propranolol) or isoprenaline and of adenosine or carbachol (alone and in the presence of isoprenaline) were studied on the force of contraction in electrically driven papillary muscles isolated from the hearts of cardiomyopathic (strain BIO 8262) and age-matched, healthy Syrian hamsters. All experiments were performed in the so-called prenecrotic stage of the disorder within the first 30 days of life. Phenylephrine exerted a positive inotropic effect in all preparations from the cardiomyopathic hamsters. In contrast, in thirteen preparations from healthy Syrian hamsters, phenylephrine increased force of contraction in only four preparations. The positive inotropic effect of isoprenaline was similar in both cardiomyopathic and healthy Syrian hamsters. Adenosine and carbachol apparently reduced the isoprenaline-induced increase in force of contraction in both cardiomyopathic and healthy Syrian hamsters. We conclude that an increased responsiveness to alpha-adrenergic stimulation occurs in the hearts of cardiomyopathic Syrian hamsters and may be related to the myocardial injury occurring in this syndrome. An increased responsiveness to beta-adrenoceptor stimulation or an impaired adenosine-mediated or cholinoceptor-mediated feedback inhibition is unlikely to play a role in the aetiology of this syndrome.

Increased sensitivity to alpha-adrenoceptor stimulation but intact purinergic and muscarinergic effects in prehypertensive cardiac hypertrophy of spontaneously hypertensive rats

The effects of phenylephrine, isoprenaline and adenosine, (-)-N6-phenylisopropyladenosine (PIA) or carbachol alone and in the presence of isoprenaline on force of contraction were studied in isolated electrically driven papillary muscles of spontaneously hypertensive rats (SHR) and age-matched Wistar control rats. In SHR an increased heart to body weight ratio was observed when blood pressure was not yet elevated. During this stage of the syndrome (i.e. between the 27th and 35th day of life) phenylephrine was about 3.4 times more potent to increase force of contraction in SHR (mean EC50: 2.8 mumol l-1) than in control rats (mean EC50: 9.4 mumol l-1). The positive inotropic effect of isoprenaline was similar in SHR and control rats. Also no difference could be detected in the isoprenaline-antagonistic effect of adenosine, the adenosine receptor agonist PIA or carbachol. We conclude that an increased sensitivity to cardiac alpha-adrenoceptor stimulation might be related to prehypertensive cardiac hypertrophy in SHR.

[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.

The interaction of carbachol and forskolin in rabbit papillary muscles

The ability of the muscarinic agonist, carbachol, to overcome increases in tension and in cAMP produced in response to the adenylate cyclase activator, forskolin, in rabbit papillary muscles was measured. Forskolin alone increased cAMP levels to a much greater extent than did a concentration of isoproterenol that produced an equivalent increase in tension. Carbachol completely overcame the increase in tension, while having no effect on the increase in cAMP, produced by forskolin. The response to carbachol was associated with a significant increase in cGMP levels. It appears that carbachol may block the inotropic response to forskolin by antagonizing the effects of forskolin-induced increases in cAMP.