Local ATP regeneration is important for sarcoplasmic reticulum Ca2+ pump function.

Abstract

Ca2+ pump function of skeletal muscle sarcoplasmic reticulum (SR) vesicles was measured by monitoring Ca2+ uptake and efflux with a Ca(2+)-sensitive minielectrode and adenosinetriphosphatase (ATPase) activity of the same preparation under the same conditions. The efficiency of Ca2+ transport into SR vesicles, defined by the amount of Ca2+ transported per ATP hydrolyzed (coupling ratio), varied significantly depending on assay conditions. Coupling ratio increased in parallel with increase in precipitating anion concentration, which is supposed to decrease accumulation of free Ca2+ inside vesicles and its subsequent efflux. Membrane-bound creatine kinase-creatine phosphate (CK-CP) system, acting as a ADP sensor and local ATP regenerator, significantly improved Ca2+ pump function when the pump worked with low efficiency (coupling ratio < 1). The effect of CK-CP system on Ca2+ pump function was also dependent on extravesicular Ca2+ concentration ([Ca2+]o), the effect being most significant at high initial [Ca2+]o. Under conditions in which SR vesicles were allowed to decrease [Ca2+]o, as occurs also during muscle relaxation, plateau values of Ca(2+)-ATPase activity were reached at significantly higher [Ca2+]o (54 +/- 5.7, n = 6), compared with leaky vesicles or the condition in which [Ca2+]o was maintained. By preventing local accumulation of ADP, generated in ATPase reactions, CK-CP system also inhibited Ca2+ efflux under conditions in which this efflux was stimulated by the increase of free Ca2+ inside vesicles. This effect was at least partially responsible for the CK-CP-supported increase in Ca2+ uptake and coupling ratios that were more expressed at low precipitating anion concentration. We hypothesize that local ATP regeneration by CK-CP system is one mechanism the cell can use to improve Ca2+ uptake by SR in emergency conditions, where excessive increase in cytoplasmic [Ca2+] may have deleterious effects.