Synthesis of adenosine triphosphate during release of intravesicular and membrane-bound calcium ions from passively loaded sarcoplasmic reticulum

Abstract

Sarcoplasmic reticulum isolated from rabbit skeletal muscle and incubated in a medium containing Ca2+ in the absence of ATP retains intravesicular and/or membrane-bound Ca2+. The synthesis of ATP coupled with the release of intravesicular Ca2+ is totally inhibited by the ionophore X-537A. Release of the membrane-bound Ca2+, retained after short periods of incubation (10min) or after release of the intravesicular Ca2+ by ionophore X-537A, still supports some synthesis of ATP. The ratios of Ca2+ released to ATP synthesized are 2.5-3.2, when bound and intravesicular Ca2+ are released simultaneously, and 3.1-4.0, when only bound Ca2+ is released. The results show that the synthesis of ATP by sarcoplasmic reticulum during release of passively accumulated Ca2+ by EGTA [ethanedioxybis(ethylamine)tetra-acetic acid] is accompanied by a loss of membrane-bound Ca2+.

Mechanism of Ca2+Transport by Sarcoplasmic Reticulum

“…The Ca 2+ -ATPase can also be phosphorylated by Pi in the absence of a transmembrane Ca 2+ gradient with the formation of a phosphoenzyme intermediate (267,268,270,298,398,611). Under optimum conditions (pH 6.0, 2-4 mM Pi, 10 mM Mg, 1 mM EGTA) the steady-state concentration of phosphoenzyme (2-3 nmol/mg protein) is similar to that obtained by gradient-dependent phosphorylation (115).…”

“…Light microsomes of rabbit skeletal muscle, which contain relatively small amounts of calsequestrin, accumulate Ca 2+ as well or better than the heavy microsomes with relatively high calsequestrin content (405). These observations suggest that calsequestrin even in rabbit SR may not be the only major Ca 2+-binding component (609,611) and raise the possibility that the principal function of calsequestrin is not the binding of accumulated Ca 2+. Varsanyi and Heilmeyer (622) observed a Ca 2+-insensitive protein kinase activity in purified calsequestrin preparations that is distinct from phosphorylase kinase.…”

“…Vale et al (611) observed that on release of membrane-bound Ca 2+ by EGTA in the presence of X-537A, which is presumed to abolish Ca 2+ gradients, synthesis of ATP occurs.…”