Effect of light and calcium on cyclic GMP synthesis in rod outer segments of toad retina.
The rod outer segments of toad retina contain a guanylate cyclase activity of about 3 +/- 1 nmol of cGMP formed/min per mg protein. In darkness this value is largely independent of the Ca2+ concentration, although it is enhanced by light upon lowering the Ca2+ concentration from 10(-5) to 10(-8) M. The activating effect of light on cyclase at low Ca2+ concentrations is enlarged upon increasing the light intensity. With a flash of light bleaching 7 X 10(-2) percent of rhodopsin, cyclase activity increased by a factor of 30 when Ca2+ levels dropped from 10(-5) to 10(-8) M. In view of recent observations that shortly after a flash of light the calcium activity inside the photoreceptor cell decreases, it seems likely that Ca2+ plays a regulatory role on cGMP metabolism in visual excitation.
D000818:Animals; D002023:Bufo bufo; D002118:Calcium / Q000494:pharmacology*; D006152:Cyclic GMP / Q000096:biosynthesis*; D004307:Dose-Response Relationship, Radiation; D006162:Guanylate Cyclase / Q000528:radiation effects; D010786:Photoreceptor Cells / Q000378:metabolism*; D012243:Rhodopsin / Q000528:radiation effects; D012374:Rod Cell Outer Segment / Q000187:drug effects / Q000378:metabolism* / Q000528:radiation effects
D012243:Rhodopsin; D006162:Guanylate Cyclase; D006152:Cyclic GMP; D002118:Calcium