Biochemical correlates of adaptation processes in isolated frog photoreceptor membranes.

Abstract

Frog rod outer segments isolated in suspension can maintain much of their in vivo activity. This provided a simpler system than the intact retina for correlating biochemical and physiological changes. The relevant physiological process, a decrease of Na+ permeability by illumination, was assayed as light suppression of outer segment swelling in a modified Ringer''s solution. This decrease was observed over approximately 4 log units of input light intensity and varied with the logarithm of intensity at light levels which bleach between 5.cntdot.102 and 5.cntdot.104 rhodopsin molecules/outer segment. In this illumination range responsiveness to light decreased as intensity increases. This sensitivity control sytem may be linked to light-activated rhodopsin phosphorylation, for inhibitors of this reaction increase light sensitivity. The presence of a 2nd system, which controls the maximum amplitude of in vitro response to light, was revealed in experiments with cyclic nucleotide phosphodiesterase inhibitors. Papaverine addition raised intracellular cyclic GMP levels and increased the magnitude of the dark permeability but did not have a large influence on the amount of illumination required for suppresion of this permeability. Sensitivity and amplitude, as they are expressed in this in vitro system, are apparently regulated by pharmacologically distinct pathways which use 2 different light-sensitive enzyme systems.