Stimulation of protein phosphatases as a mechanism of the muscarinic-receptor-mediated inhibition of cardiac L-type Ca2+ channels.
Acetylcholine decreases currents through cardiac L-type Ca2+ channels after stimulation with agents which elevate levels of cyclic adenosine monophosphate, such as isoproterenol, but there is still a controversy over the mechanisms of this muscarinic effect. We tested the hypothesis of whether, after isoproterenol stimulation, protein phosphatases are activated by acetylcholine. Whole-cell currents were recorded from guinea-pig ventricular myocytes. The effect of 10(-5) M acetylcholine on currents induced by 10(-8) M isoproterenol was studied in the absence or presence of protein phosphatase inhibitors. Three agents reduced the acetylcholine response: okadaic acid (3 or 9 x 10(-6) M) and cantharidin (3 x 10(-6) M) added to the pipette solution, and bath-applied fluoride (3 mM). In contrast, pipette application of other phosphatase inhibitors, namely the inhibitor PPI2 (1000 U/ml), ciclosporin (10(-5) M), or calyculin A (10(-6) M) did not significantly diminish the acetylcholine effect. Interestingly, there was no correlation between the effects of the compounds on basal Ca2+ current and their interference with the muscarinic response. An activation of type 2A phosphatases by acetylcholine would explain these findings. Indeed, okadaic acid is 3 orders of magnitude more potent in vitro in its inhibition of this isoform (purified from cardiac myocytes) than is calyculin A, while type-1 phosphatases are inhibited equally. The data support the attractive possibility that stimulation of protein phosphatases is part of the signal transduction cascade of Ca2+ channel inhibition by acetylcholine.
Herzig
S
S
Institut für Pharmakologie, Universität Kiel, Germany.
Meier
A
A
Pfeiffer
M
M
Neumann
J
J
D000109:Acetylcholine / Q000494:pharmacology*; D000818:Animals; D015220:Calcium Channels / Q000187:drug effects*; D002642:Chick Embryo; D004305:Dose-Response Relationship, Drug; D004988:Ethers, Cyclic / Q000494:pharmacology; D005260:Female; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D008387:Marine Toxins; D019319:Okadaic Acid; D010080:Oxazoles / Q000494:pharmacology; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors / Q000187:drug effects*; D011976:Receptors, Muscarinic / Q000187:drug effects*; D013997:Time Factors
D015220:Calcium Channels; D004988:Ethers, Cyclic; D008387:Marine Toxins; D010080:Oxazoles; D011976:Receptors, Muscarinic; D019319:Okadaic Acid; C059041:calyculin A; D010749:Phosphoprotein Phosphatases; D007545:Isoproterenol; D000109:Acetylcholine