CHLORIDE-DEPENDENT SARCOPLASMIC-RETICULUM CA2+ RELEASE CORRELATES WITH INCREASED CA2+ ACTIVATION OF RYANODINE RECEPTORS

The mechanism by which chloride increases sarcoplasmic reticulum (SR) Ca2+ permeability was investigated. In the presence of 3 mu M Ca2+, Ca 2+ release from Ca-45(2+)-loaded SR vesicles prepared from porcine ske letal muscle was increased approximately 4-fold when the media contain ed 150 mM chloride versus 150 mM propionate, whereas in the presence o f 30 nM Ca2+, Ca2+ release was similar in the chloride- and the propio nate-containing media. Ca2+-activated [H-3]ryanodine binding to skelet al muscle SR was also increased (2- to 10-fold) in media in which prop ionate or other organic anions were replaced with chloride; however, c hloride had little or no effect on cardiac muscle SR Ca-45(2+) release or [H-3]ryanpdine binding. Ca2+-activated [H-3]ryanodine binding was increased similar to 4.5-fold after reconstitution of skeletal muscle RYR protein into liposomes, and [H-3]ryanodine binding to reconstitute d RYR protein was similar in chloride- and propionate- containing medi a, suggesting that the sensitivity of the RYR protein to changes in th e anionic composition of the media may be diminished upon reconstituti on. Together, our results demonstrate a close correlation between chlo ride-dependent increases in SR Ca2+ permeability and increased Ca2+ ac tivation of skeletal muscle RYR channels. We postulate that media cont aining supraphysiological concentrations of chloride or other inorgani c anions may enhance skeletal muscle RYR activity by favoring a confor mational state of the channel that exhibits increased activation by Ca 2+ in comparison to the Ca2+ activation exhibited by this channel in n ative membranes in the presence of physiological chloride (less than o r equal to 10 mM). Transitions to this putative Ca2+-activatable state may thus provide a mechanism for controlling the activation of RYR ch annels in skeletal muscle.