Rapid flow chemical quench studies of calcium release from isolated sarcoplasmic reticulum

Abstract

45Ca2+ release from a heavy fraction of rabbit skeletal muscle microsomes was induced by chemical depolarization (replacement of 0.15 M K gluconate with 0.15 M choline Cl), or addition of Ca2+ plus caffeine, or both. The time courses of Ca2+ release were investigated with a multimixing chemical quench apparatus by quenching the Ca2+ release reaction using 10 mM EGTA and 5 microM ruthenium red. At low ATP (e.g. 0.2 mM) and low extravesicular [Ca2+] (e.g. 0.1 microM), the time course of depolarization-induced Ca2+ release was similar to that determined by a spectrophotometric method (Ikemoto, N., Antoniu, B., and Kim, D.H. (1984) J. Biol. Chem. 259, 13151-13158). An increase of the extravesicular [Ca2+] up to 5 microM, or addition of high concentrations of ATP (e.g. 7.5 mM), shortened the lag phase that precedes depolarization-induced Ca2+ release and increased the amount of Ca2+ released. On the other hand, upon addition of several millimolars ATP the rate of (Ca2+ plus caffeine)-induced Ca2+ release was increased, resulting in the same time course as that of depolarization-induced Ca2+ release. Induction of Ca2+ release by combined application of chemical depolarization and Ca2+ plus caffeine resulted in the same time course as that induced by either method alone, suggesting that both types of Ca2+ release are mediated by a common channel rather than separate channels.