The effect of oximes on the dihydropyridine-sensitive Ca current of isolated guinea-pig ventricular myocytes.
Exposure of isolated guinea-pig ventricular myocytes to the uncharged oximes 2,3 butanedionemonoxine (BDM) and norPAM (but not by the charged PAM) results in a dose-dependent reduction of the duration of the action potential. The nifedipine-sensitive Ca current is fully inhibited by BDM (IC(50)5.8 +/- 0.4 mM) and nor PAM but is little affected by PAM. This inhibition is unaltered by the presence of BAY K 8644 but is antagonized by isoprenaline. The effect of isoprenaline is more pronounced when the solution in the patch pipette contains the non-hydrolysable analogue of adenosine 5'-triphosphate, ATP gamma S (the IC50 is increased to 44.0 +/- 5.2 mM). A hastening of the inactivation of the L-type Ca current persists when either 10 mM 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N-tetraacetic acid (BAPTA) or 3 mM ATP gamma S is present in the pipette solution or when BAY K 8644 or isoprenaline are present in the bathing fluid. These results suggest that the inhibition of the Ca current is due to the phosphatase-like activity of the oximes but differs in some respects from previous work where a reduced level of phosphorylation is achieved by the introduction of protein kinase inhibitors or protein phosphatases into the sarcoplasm in guinea-pig myocytes. These differences could be explained if Ca channel availability is regulated by at least two sites of cAMP-dependent phosphorylation with oximes able to rapidly dephosphorylate both sites, while one of these sites is not readily dephosphorylated by the endogenous phosphatases.
Chapman
R A
RA
Department of Physiology, School of Veterinary Science, Bristol, UK.
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D002801:Cholinesterase Reactivators / Q000494:pharmacology*; D003931:Diacetyl / Q000031:analogs & derivatives* / Q000494:pharmacology; D006168:Guinea Pigs; D006352:Heart Ventricles / Q000166:cytology; D007545:Isoproterenol / Q000494:pharmacology*; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D010091:Oximes / Q000494:pharmacology*; D016276:Ventricular Function
D015220:Calcium Channels; D002801:Cholinesterase Reactivators; D010091:Oximes; C004717:diacetylmonoxime; C060977:pyridine-2-aldoxime; D003931:Diacetyl; D007545:Isoproterenol