Light triggers the cyclic nucleotide cascade in photoreceptor disk membranes. We report here that light-induced changes in the apparent viscosity of disk membrane suspensions can also be observed using either native disk membranes or washed membranes reconstituted with G protein and PDE. The viscosity changes are light- and GTP-dependent and require the presence of G protein and PDE. The magnitude of the viscosity change increases with increasing membrane concentration. Under the same conditions in which light elicits a change in viscosity, we observe a large increase in light scattering by the disk membrane suspension.
