Influence of calcium on guanosine 3',5'-cyclic monophosphate levels in frog rod outer segments

Abstract

In the presence of 10-9 M calcium, rod outer segments freshly detached from dark-adapted frog retinas contain between 0.01 and 0.02 moles of guanosine 3',5'-cyclic monophosphate (cyclic GMP) per mole of rhodopsin. The dark level of cyclic GMP is reduced ~50 by illumination that bleaches 5 × 105 rhodopsin molecules/outer segment-s. The dark levels of cyclic GMP also can be suppressed to ~0.007 mol/mol of rhodopsin by increasing the concentration of calcium from 10-9 M to 2 × 10-9 M, and they remain at this level as calcium concentration is raised to 10-3 M. The final level to which illumination reduces cyclic GMP is unaffected by the calcium concentration between 10-9 and 10-3 M. The maximal light-induced decrease in cyclic GMP occurs within 1s from the onset of illumination at all calcium concentrations. The magnitude and time-course of the light-induced decrease in cyclic GMP measured in these experiments are comparable to values obtained previously (Woodruff et al. 1977. J. Gen. Physiol. 69:677-679; Woodruff and Bownds. 1979. J. Gen. Physiol. 73:629-653). The data are consistent with a role for cyclic GMP in visual transduction irrespective of the calcium concentration. © 1981, Rockefeller University Press., All rights reserved.