POTENTIATION OF THE TWITCH RESPONSES BY INHIBITORS OF SARCOPLASMIC-RETICULUM CA2-ATPASE IN FROG ATRIAL FIBERS()

In frog atrial fibres, cyclopiazonic acid as well as thapsigargin, whi ch are inhibitors of sarcoplasmic reticulum Ca2+-ATPase, induced a sig nificant increase in the twitch amplitude without detectable changes i n its kinetics. The measurements performed on chemically skinned fibre s show that cyclopiazonic acid has no effect on the properties of cont ractile proteins. In the presence of a T-type Ca2+ channel blocker or L-type Ca2+ channel blocker, cyclopiazonic acid still induced a potent iation of the twitch while no effect was found in the presence of a Na +-Ca2+ exchange blocker. The effect of cyclopiazonic acid was not rela ted to any modification in myofibrillar Ca2+ sensitivity or in Ca2+ in flux through Ca2+ channels. It is proposed that the inhibition of the sarcoplasmic reticulum Ca2+-ATPase resulted in a potentiation of the e ffect of the Ca2+ influx and that the major role of the sarcoplasmic r eticulum was to limit the intracellular Ca2+ concentration.