Adrenergic-cholinergic interactions in the heart

• Main Author: Ray, Abhijit
• Language: English
• Published: 2008
• Online Access: http://hdl.handle.net/2429/2194

In the mammalian atrial myocardium the muscarinic receptor agonist carbacholcan inhibit the positive inotropic responses to isoproterenol, a f3-adrenoceptor agonist, forskolin, a direct activator of adenylate cyclase, IBMX, a phosphodiesterase inhibitor, and phenylephrine, an a-adrenoceptor agonist. While the inhibitory effect of carbacholon the isoproterenol-stimulated cAMP generation is believed to contribute to the negative inotropic effect of carbachol in the presence of isoproterenol, it is not known how carbachol inhibits the positive inotropic responses to phenylephrine, forskolin and IBMX in the atrial myocardium. One of the objectives of the present study was to investigate whether the reversal by carbachol of the positive inotropic responses of left atria tophenylephrine, forskolin and IBMX is related to the ability of carbachol to open potassium channels. In rabbit left atria, carbachol exerted a direct negative inotropic response and inhibited the positive inotropic response to phenylephrine. Carbachol also promoted the efflux of 86 Rb in the presence and absence of phenylephrine from left atria. The ability of carbachol to increase the rate-constant of 86 Rb-efflux was attenuated by atropine (100nM), 4-aminopyridine (50 and 500 gM), a potassium channel blocker, and pre-treatment of rabbits with pertussis toxin (0.5 and 1 lig/kg), an uncoupler of muscarinic receptors from potassium channels. Although both 4-aminopyridine and pertussis toxin attenuated the negative inotropic response to carbachol, both agents had a greater attenuating effecton the carbachol-stimulated 86 Rb-efflux than on the carbachol-induced negative inotropy. The abilities of carbachol to promote the 86 Rb-efflux and to exert a negative inotropic response in the presence of phenylephrine were attenuated by pertussis toxin pre-treatment of rabbits. Although 4-aminopyridine was able to attenuate the inhibitory effect of carbachol on the phenylephrine-induced positive inotropy, only 500 1.1M 4-aminopyridine slightly reduced the carbachol-stimulated increase in the rate constant of86 Rb efflux in the presence of phenylephrine. 4-Aminopyridine did not have any effect on the carbachol-induced inhibition of the isoproterenol-stimulated cAMP generation suggesting that 4-aminopyridine was notacting as a muscarinic receptor antagonist. 4-Aminopyridine inhibited only modestly the inhibitory effect of carbachol on the isoproterenol-induced positive inotropy, a cAMP-dependent response. The potassium channel openers, pinacidil and cromakalim, did not have any inhibitory effect on the isoproterenol-induced positive inotropy but inhibited ina concentration-dependent manner the positive inotropic responses to phenylephrine. Uncoupling of muscarinic receptors from adenylate cyclase using pertussis toxin(2.2 μg/kg) attenuated only partially the negative inotropic responses of left atria tocarbachol in the presence of forskolin and IBMX suggesting that at least part of the reversal by carbachol of positive inotropic responses to forskolin and IBMX occurs by acAMP-independent mechanism. 4-Aminopyridine attenuated in a concentration-dependent manner the negative inotropic responses to carbachol in the presence of forskolin and IBMX in left atria from both saline and pertussis toxin pre-treated rabbits. These results suggest that the ability of carbachol to open potassium channels may not explain completely the direct negative inotropic response to carbachol but may contribute to the negative inotropic responses to carbachol in the presence of phenylephrine, forskolin and IBMX. It is established that muscarinic receptors are linked to potassium channels andadenylate cyclase in the atrium and only to adenylate cyclase in the ventricle by means of pertussis toxin sensitive G-protein(s). The second objective of this study was to investigate if inhibition of the isoproterenol-stimulated adenylate cyclase by carbachol in the rabbit atrium is more sensitive to pertussis toxin than either the same response to carbachol in the ventricle or the ability of carbachol to open potassium channels in the atrium. Injection of rabbits with 0.5 μg/kg pertussis toxin, a dose which ADP-ribosylated60 % of atrial and ventricular G-proteins, resulted in complete uncoupling of muscarinicreceptors from the atrial adenylate cyclase. On the other hand, in rabbits injected with 1μg/kg pertussis toxin, the ability of carbachol to inhibit the ventricular adenylate cyclasewas not altered and to open the atrial potassium channels was only partially attenuated. These data suggest that the muscarinic receptor-mediated inhibition of adenylate cyclasein the atrium is more sensitive to pertussis toxin than either the muscarinic receptor-mediated activation of potassium efflux in the atrium or inhibition of adenylate cyclase inthe ventricle. This suggests that there are differences in the coupling of muscarinic receptors to adenylate cyclase and potassium channels in the atrium and ventricle.