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 μM Ca²⁺, Ca²⁺ release from ⁴⁵Ca²⁺-loaded SR vesicles prepared from porcine skeletal muscle was increased approximately 4-fold when the media contained 150 mM chloride versus 150 mM propionate, whereas in the presence of 30 nM Ca²⁺, Ca²⁺ release was similar in the chloride- and the propionate-containing media. Ca²⁺-activated [³H]ryanodine binding to skeletal muscle SR was also increased (2- to 10-fold) in media in which propionate or other organic anions were replaced with chloride; however, chloride had little or no effect on cardiac muscle SR ⁴⁵Ca²⁺ release or [³H]ryanodine binding. Ca²⁺- activated [³H]ryanodine binding was increased ~-4.5-fold after reconstitution of skeletal muscle RYR protein into liposomes, and [³H]ryanodine binding to reconstituted RYR protein was similar in chloride- and propionate-containing media, suggesting that the sensitivity of the RYR protein to changes in the anionic composition of the media may be diminished upon reconstitution. Together, our results demonstrate a close correlation between chloride-dependent increases in SR Ca²⁺ permeability and increased Ca²⁺ activation of skeletal muscle RYR channels. We postulate that media containing supraphysiological concentrations of chloride or other inorganic anions may enhance skeletal muscle RYR activity by favoring a conformational state of the channel that exhibits increased activation by Ca²⁺ in comparison to the Ca²⁺ activation exhibited by this channel in native membranes in the presence of physiological chloride (≤10 mM). Transitions to this putative Ca²⁺-activatable state may thus provide a mechanism for controlling the activation of RYR channels in skeletal muscle.