title: Guanosine 3',5'-cyclic monophosphate stimulates release of actively accumulated calcium in purified disks from rod outer segments of bovine retina.
abstract: Parallel lines of evidence have suggested that light initiates changes in both cGMP metabolism and calcium levels in rod outer segments (ROS). We report that cGMP stimulates release of a pool of Ca2+ actively accumulated within purified ROS disks. Disks were purified and actively loaded with 45Ca2+ by an associated ATP-dependent calcium uptake activity as previously described [Puckett, K.L., Aronson, E.T., & Goldin, S.M. (1985) Biochemistry 24, 390-400]. Spikes of 45Ca2+ released from disks were observed in a rapid superfusion system. The Ca2+ release was specifically stimulated by physiological levels of cGMP (Kapp approximately 20 microM; Hill coefficient = 1.7). 8-Bromo-cGMP could also activate the release mechanism, but cAMP was ineffective. At cGMP levels of greater than or equal to 100 microM, approximately 20% of the loaded Ca2+ was released. The Ca2+ release rate at saturating cGMP levels reached a maximum within the 10-s time resolution of the assay system. In contrast to other recent reports of cGMP activation of ROS ion conductances, the majority of the release activity terminated in a spontaneous manner, suggestive of an intrinsic inactivation process. The amount of Ca2+ released and the release kinetics were similar to the presence or absence of an unbleached pool of rhodopsin. Cyclic nucleotides did not stimulate release from disks passively equilibrated with 45Ca2+, i.e., in the absence of ATP but otherwise under identical conditions. Preincubation of the disks with cGMP also reduced the level of ATP-dependent Ca2+ uptake (approximately 30%); this apparent inhibition may be due to activation of the release mechanism, rather than direct modulation of the uptake activity.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D002132:Calcium Radioisotopes; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002417:Cattle; D006152:Cyclic GMP / Q000494:pharmacology*; D006157:Guanosine Monophosphate / Q000494:pharmacology; D006160:Guanosine Triphosphate / Q000494:pharmacology; D007700:Kinetics; D010786:Photoreceptor Cells / Q000378:metabolism*; D012374:Rod Cell Outer Segment / Q000187:drug effects / Q000378:metabolism*
chemical_list: D002132:Calcium Radioisotopes; D006157:Guanosine Monophosphate; D006160:Guanosine Triphosphate; D000252:Calcium-Transporting ATPases; D006152:Cyclic GMP; D002118:Calcium