title: Effects of phorbol esters on excitation-contraction coupling and protein kinase C activity of frog twitch muscle fibers.
abstract: By recording the calcium transients evoked by voltage-clamp depolarizing pulse with arsenazo III as a calcium indicator, it has been shown that 1 micromol/l phorbol 12,13-dibutyrate (PDBu), a protein kinase C (PKC) agonist, causes a transient potentiation and then a depression of the calcium transients of twitch muscle fibers in frogs. PDBu also produces an initial translocation and activation of PKC, which is followed by a down-regulation. To find out whether the effect of PDBu on the calcium transients depends on PKC, a correlated study of the effect of phorbol esters on calcium transients and PKC activity was performed. The calcium transients and PKC activity were similarly affected by PDBu in ordinary and cold-accommodated frogs, but the effects occurred more quickly in the latter. However, they still changed in parallel as in ordinary frogs. 1 or 10 micromol/l, 4-alpha-phorbol, a PKC-inactive analogue of phorbol ester, caused a partial depression of the calcium transients in cold-accommodated frogs, while PKC activity was not affected. Moreover, the transient potentiation of the calcium transients induced by 1 micromol/l PDBu could be antagonized by the PKC inhibitors 10 micromol/l chelerythrine chloride or 10 micromol/l polymyxin B (PMB). All these results suggest that: (1) the transient potentiation of calcium transients induced by PDBu is caused by activation of PKC; (2) phorbol ester can depress the calcium transients by a mechanism that is independent of PKC.
authors.affiliation: Unit of Cell Signal Transduction, Laboratory of Neurobiology, Shanghai Institute of Physiology, Chinese Academy of Sciences, Shanghai 200031, China.
authors.forename: X H
authors.initials: XH
authors.lastname: Zhang
authors.forename: J H
authors.initials: JH
authors.lastname: Sun
authors.forename: P H
authors.initials: PH
authors.lastname: Zhu
mesh_terms: D000470:Alkaloids; D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D053119:Benzophenanthridines; D020013:Calcium Signaling / Q000187:drug effects; D003080:Cold Temperature; D015536:Down-Regulation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000187:drug effects; D018485:Muscle Fibers, Skeletal / Q000187:drug effects* / Q000201:enzymology*; D019319:Okadaic Acid / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D010617:Phenanthridines / Q000494:pharmacology; D015240:Phorbol 12,13-Dibutyrate / Q000494:pharmacology*; D011112:Polymyxin B / Q000494:pharmacology; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000502:physiology*; D011898:Ranidae
chemical_list: D000470:Alkaloids; D000900:Anti-Bacterial Agents; D053119:Benzophenanthridines; D004791:Enzyme Inhibitors; D010617:Phenanthridines; D019319:Okadaic Acid; D015240:Phorbol 12,13-Dibutyrate; C016299:chelerythrine; D011493:Protein Kinase C; D011112:Polymyxin B