Cyclic GMP stimulation of a light-activated ATPase in rod outer segments

Rod outer segments (ROSs) of vertebrate photoreceptor cells have been reported to contain several enzyme systems including a dark, Ca2+-stimulated ATPase, a rhodopsin kinase, a phosphodiesterase and a GTPase, all of which are light-stimulated. Recently, Thacher has found a light-stimulated Mg2+-ATPase in frog ROSs while our own laboratory has identified a dark, Ca2+-inhibited Mg2+-ATPase in bovine ROSs. Here we extend our observations on the Mg2+-ATPase and demonstrate that flash illumination following the dark ATPase process stimulated ATPase activity at a rate considerably faster than the dark process. In addition, we find that both the dark and light stimulated ATPase activities are markedly enhanced by cyclic GMP and inhibited by Ca2+.