Mechanism of muscarinic control of the high-threshold calcium current in rabbit sino-atrial node myocytes
Résumé
The mechanism of the action of acetylcholine (ACh) on the L-type calcium current (Ic,,D was examined using a whole-cell voltage-clamp technique in single sino-atrial myocytes from the rabbit heart. ACh depressed basal Ica,L at concentrations in the range 0.05-10 I-tM, without previous fl-adrenergic stimulation. The ACh-induced reduction of IC,,L was reversed by addition of atropine, indicating that muscarinic receptors mediate it. Incubation of cells with a solution containing pertussis toxin led to abolition of the ACh effect, suggesting that this effect is mediated by G proteins activated by muscarinic receptors. Dialysis of cells with protein kinase inhibitor or 5'-adenylyl imidodiphosphate, inhibitors of the cAMP-dependent protein kinase, decreased basal Ica,L by about 85 % and suppressed the effect of ACh. The ACh effect was also absent in ceils dialysed with a non-hydrolysable analogue of cAME 8-bromo-cAMR. The results suggest that, in basal conditions, a large part of the L-type calcium channels should be phosphorylated by protein kinase A stimulated by a high cAMP level correlated with a high adenylate cyclase activity. The depressing effect of ACh on ICa,L may occur via inhibition of the high basal adenylate cyclase activity leading to a decrease of cAMP-dependent protein kinase stimulation and thus to a dephosphorylation of calcium channels.