title: Activators of protein kinase C decrease Ca2+ spark frequency in smooth muscle cells from cerebral arteries.
abstract: Local Ca2+ transients ("Ca2+ sparks") caused by the opening of one or the coordinated opening of a number of tightly clustered ryanodine-sensitive Ca(2+)-release (RyR) channels in the sarcoplasmic reticulum (SR) activate nearby Ca(2+)-dependent K+ (KCa) channels to cause an outward current [referred to as a "spontaneous transient outward current" (STOC)]. These KCa currents cause membrane potential hyperpolarization of arterial myocytes, which would lead to vasodilation through decreasing Ca2+ entry through voltage-dependent Ca2+ channels. Therefore, modulation of Ca2+ spark frequency should be a means to regulation of KCa channel currents and hence membrane potential. We examined the frequency modulation of Ca2+ sparks and STOCs by activation of protein kinase C (PKC). The PKC activators, phorbol 12-myristate 13-acetate (PMA; 10 nM) and 1,2-dioctanoyl-sn-glycerol (1 microM), decreased Ca2+ spark frequency by 72% and 60%, respectively, and PMA reduced STOC frequency by 83%. PMA also decreased STOC amplitude by 22%, which could be explained by an observed reduction (29%) in KCa channel open probability in the absence of Ca2+ sparks. The reduction in STOC frequency occurred in the presence of an inorganic blocker (Cd2+) of voltage-dependent Ca2+ channels. The reduction in Ca2+ spark frequency did not result from SR Ca2+ depletion, since caffeine-induced Ca2+ transients did not decrease in the presence of PMA. These results suggest that activators of PKC can modulate the frequency of Ca2+ sparks, through an effect on the RyR channel, which would decrease STOC frequency (i.e., KCa channel activity).
authors.affiliation: Department of Pharmacology, College of Medicine, University of Vermont, Colchester 05446, USA.
authors.forename: A D
authors.initials: AD
authors.lastname: Bonev
authors.forename: J H
authors.initials: JH
authors.lastname: Jaggar
authors.forename: M
authors.initials: M
authors.lastname: Rubart
authors.forename: M T
authors.initials: MT
authors.lastname: Nelson
mesh_terms: D000818:Animals; D001488:Basilar Artery / Q000187:drug effects / Q000502:physiology*; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D004075:Diglycerides / Q000494:pharmacology*; D004789:Enzyme Activation; D066298:In Vitro Techniques; D007700:Kinetics; D008954:Models, Biological; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000502:physiology*; D011494:Protein Kinases / Q000378:metabolism*; D051381:Rats; D012433:Ryanodine / Q000494:pharmacology; D013755:Tetradecanoylphorbol Acetate / Q000031:analogs & derivatives / Q000494:pharmacology*
chemical_list: D015220:Calcium Channels; D004075:Diglycerides; C044295:1,2-dioctanoylglycerol; D012433:Ryanodine; C020922:4-O-methyl-12-O-tetradecanoylphorbol 13-acetate; D011494:Protein Kinases; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium