Mechanism of muscarinic control of the high-threshold calcium current in rabbit sino-atrial node myocytes.
The mechanism of the action of acetylcholine (ACh) on the L-type calcium current (ICa,L) 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 microM, without previous beta-adrenergic stimulation. The ACh-induced reduction of ICa,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 cells dialysed with a non-hydrolysable analogue of cAMP, 8-bromo-cAMP. 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.
Petit-Jacques
J
J
Laboratoire de Physiologie Générale, CNRS U.R.A. 290, Faculté des Sciences, Université de Poitiers, France.
Bois
P
P
Bescond
J
J
Lenfant
J
J
D000109:Acetylcholine / Q000494:pharmacology*; D037361:Adenylate Cyclase Toxin; D000266:Adenylyl Imidodiphosphate / Q000494:pharmacology; D000818:Animals; D001285:Atropine / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D000242:Cyclic AMP / Q000494:pharmacology / Q000502:physiology; D004553:Electric Conductivity; D019204:GTP-Binding Proteins / Q000502:physiology; D007545:Isoproterenol / Q000494:pharmacology; D037342:Pertussis Toxin; D047428:Protein Kinase Inhibitors; D011817:Rabbits; D011976:Receptors, Muscarinic / Q000502:physiology*; D012849:Sinoatrial Node / Q000187:drug effects / Q000502:physiology*; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D037361:Adenylate Cyclase Toxin; D015220:Calcium Channels; D047428:Protein Kinase Inhibitors; D011976:Receptors, Muscarinic; D010566:Virulence Factors, Bordetella; D000266:Adenylyl Imidodiphosphate; D001285:Atropine; D000242:Cyclic AMP; D037342:Pertussis Toxin; D019204:GTP-Binding Proteins; D007545:Isoproterenol; D000109:Acetylcholine