Chloride-induced Ca2+ release from the sarcoplasmic reticulum of chemically skinned rabbit psoas fibers and isolated vesicles of terminal cisternae

Abstract

There is increasing evidence that Ca2+ release from sarcoplasmic reticulum (SR) of mammalian skeletal muscle is regulated or modified by several factors including ionic composition of the myoplasm. We have studied the effect of Cl- on the release of Ca2+ from the SR of rabbit skeletal muscle in both skinned psoas fibers and in isolated terminal cisternae vesicles. Ca2+ release from the SR in skinned fibers was inferred from increases in isometric tension and the amount of release was assessed by integrating the area under each tension transient. Ca2+ release from isolated SR was measured by rapid filtration of vesicles passively loaded with 45Ca2+. Ca2+ release from SR was stimulated in both preparations by exposure to a solution containing 191 mm choline-Cl, following pre-equilibration in Ca2+-loading solution that had propionate as the major anion. Controls using saponin (50 microg/ml), indicated that the release of Ca2+ was due to direct action of Cl- on the SR rather than via depolarization of T-tubules. Procaine (10 mM) totally blocked Cl-- and caffeine-elicited tension transients recorded using loading and release solutions having ([Na+] + [K+]) x [Cl-] product of 6487.69 mm2 and 12361.52 mm2, respectively, and blocked 60% of Ca2+ release in isolated SR vesicles. Surprisingly, procaine had only a minor effect on tension transients elicited by Cl- and caffeine together. The data from both preparations suggests that Cl- induces a relatively small amount of Ca2+ release from the SR by activating receptors other than RYR-1. In addition, Cl- may increase the Ca2+ sensitivity of RYR-1, which would then allow the small initial release of Ca2+ to facilitate further release of Ca2+ from the SR by Ca2+-induced Ca2+ release.