Ion-induced Release of Calcium from Isolated Sarcoplasmic Reticulum

Choline Cl addition to either longitudinal reticulum or terminal cisternae of skeletal muscle sarcoplasmic reticulum caused release of Ca2+ which had previously accumulated in the presence of ATP. However the extent of release was considerably greater in terminal cisternae. Ca2+ accumulation and release by terminal cisternae were also observed using chlorotetracycline as a probe for membrane-associated Ca2+. Among a number of salts and ions tested for effectiveness in causing Ca2+ release the order was gluconate- less than cacodylate- less than isethionate- = methane sulfonate- less than methylsulfate- less than SCN- for anions, and K+ = Na+ = Li+ less than choline+ = tetramethylammonium+ for cations. Valinomycin enhanced Ca2+ accumulation in the presence of ATP both in the absence and presence of the releasing agent, choline Cl. The concentration of sucrose in the medium exerted no discernible effect on the rate or extent of Ca2+ release from terminal cisternae. The rate of release was estimated using a stopped-flow mixing apparatus. The rapid phase of release was complete in 6 sec when choline Cl or KSCN were employed to initiate release. Ca2+ efflux was slower when release was initiated by EGTA addition. The estimated rate of release was 4-6 nmol/mg protein/sec. The fluorescent probe, 1-anilino-8-naphthalene sulfonate was employed to estimate the influence of ions on the surface potential of terminal cisternae. A broad inverse correlation was observed between the fluorescence of the probe in the presence of various salts and their ability to induce Ca2+ release.