Calcium release from rod outer segments: evidence for a cGMP-sensitive calcium binding protein.
Numerous studies investigating the cGMP-gated cation conductance in rod disk membranes have purported to measure efflux of Ca2+ entrapped in rod disk membrane vesicles. We have utilized sonication and osmotic shock as additional tests for sensitivity of cGMP- and A23187-induced Ca2+ release to elimination of the transvesicular Ca2+ gradient. We find that 1) Treatment with sonication or osmotic shock in low Ca2+ medium does not release Ca2+ from either native cGMP/Ca2(+)-loaded vesicles or solubilized, reconstituted "Ca2(+)-loaded" vesicles, 2) 70-100% of the cGMP-induced "flux" and 90-100% of the A23187-induced Ca2+ "flux" is insensitive to elimination of the Ca2+ gradient by sonication or osmotic shock in low Ca2+ medium, and 3) total amount of releasable Ca2+ is related to membrane surface area rather than vesicle entrapment volume. We conclude that 1) A23187 disrupts binding of Ca2+ to proteins and phospholipids as well as releasing entrapped Ca2+ and 2) a large fraction of the cGMP-induced release observed in rod disk vesicles is due to release of bound Ca2+.
Devenny
J J
JJ
Department of Ophthalmology and Visual Science, Yale University School of Medicine, New Haven, CT 06510.
Clack
J W
JW
D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D002417:Cattle; D006152:Cyclic GMP / Q000494:pharmacology*; D007444:Inulin; D007700:Kinetics; D012374:Rod Cell Outer Segment / Q000378:metabolism*; D014465:Ultrasonics
D002135:Calcium-Binding Proteins; D000001:Calcimycin; D007444:Inulin; D006152:Cyclic GMP; D002118:Calcium