Calcium release from rod outer segments : evidence for a cGMP-sensitive calcium binding protein

Abstract Numerous studies investigating the cGMP-gated cation conductance in rod disk membranes have purported to measure efflux of Ca 2+ entrapped in rod disk membrane vesicles. We have utilized sonication and osmotic shock as additional tests for sensitivity of cGMP- and A23187-induced Ca 2+ release to elimination of the transvesicular Ca 2+ gradient. We find that 1) Treatment with sonication or osmotic shock in low Ca 2+ medium does not release Ca 2+ from either native cGMP Ca 2+ -loaded vesicles or solubilized, reconstituted “Ca 2+ -loaded” vesicles, 2) 70–100% of the cGMP-induced “flux” and 90–100% of the A23187-induced Ca 2+ “flux” is insensitive to elimination of the Ca 2+ gradient by sonication or osmotic shock in low Ca 2+ medium, and 3) total amount of releasable Ca 2+ is related to membrane surface area rather than vesicle entrapment volume. We conclude that 1) A23187 disrupts binding of Ca 2+ to proteins and phospholipids as well as releasing entrapped Ca 2+ and 2) a large fraction of the cGMP-induced release observed in rod disk vesicles is due to release of bound Ca 2+ .