EFFECT OF ACTIVATION OF PROTEIN-KINASE-C ON EXCITATION-CONTRACTION COUPLING IN FROG TWITCH MUSCLE-FIBERS

Intracellular Ca2+ transients were recorded from frog twitch muscle fi bres in response to voltage- clamp depolarizing pulses, using arsenate III as an intracellular Ca2+ indicator. The effect of the activation of protein kinase C (PKC) on the Ca2+ transients was studied. With 1 m u M phorbol 12,13-dibutyrate (PDBu), a PKC activator, the peak of the Ca2+ transients increased to about 120% of control during the first 0. 5 h, and then decreased gradually to a plateau of 44% of control withi n the following 2 h. This effect of PDBu could be alleviated significa ntly by PKC inhibitors, 10 mu M polymyxin B (PMB) or 30 mu M 1-(5-isoq uinolinylsulphonyl)-2-methyl-piperazine (H-7). Moreover, PDBu caused a n upward shift of the strength/duration curve. In Li+-loaded muscle fi bres the Ca2+ transients could not fully recover after 80 mM K+ exposu re for 15 min, while the post-K+ Ca2+ transients could be completely r estored in the fibres not loaded with Li+. In the presence of 10 mu M PMB or 30 mu M H-7, a full restoration of the post-K+ Ca2+ transients was seen in Li+-loaded fibres. PMB supplemented after high-K+ exposure also could result in a complete recovery of the post-K+ Ca2+ transien ts in Li+-loaded fibres. The role of PKC in modulating excitation-cont raction coupling in frog twitch muscle fibres is c clearly indicated, but the mechanism(s) and physiological significance remain to be estab lished.