The transduction of light-energy into a receptor potential in the rod photoreceptor of the vertebrate retina commences with absorption of the light by the membrane bound visual pigment rhodopsin. Upon absorption of light, rhodopsin undergoes a sequence of conformational changes and these changes lead, through an unknown mechanism, to the change in plasma membrane ionic permeability which underlies the rod photoresponse. The object of the project proposed herein is to understand the mechanisms which couple the conformational change of rhodopsin to the change in plasma membrane permeability, in particular, the possibility that rhodopsin acts as a light-dependent ion translocator. This object will be pursued through an investigation of the ionic permeabilities of the rhodopsin-containing disc membranes with a novel method which permits complete control of the ionic electrochemical gradients across the membrane. The quantitative characteristics of the light-dependent ion transport function of rhodopsin will be measured in model lipid membranes into which rhodopsin is incorporated as the only protein. To investigate in molecular detail the mechanism of this transport function of rhodopsin, planar films containing structurally intact rhodopsin and separating two aqueous phases will be studied with electrophysiological methods. To understand the energy sources for the transport of ions in rod outer segments, the metabolic state of functionally intact outer segments will be investigated through the measurements of levels of phosphate nucleotides, in the dark and the light, using NMR techniques.