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

Abstract

Rod outer segments (ROSs) of vertebrate photoreceptor cells have been reported to contain several enzyme systems including a dark, Ca²⁺-stimulated ATPase¹, a rhodopsin kinase^2,3, a phosphodiesterase^4,5 and a GTPase^6,7, all of which are light-stimulated. Recently, Thacher has found a light-stimulated Mg²⁺-ATPase in frog ROSs^8,9 while our own laboratory has identified a dark, Ca²⁺-inhibited Mg²⁺-ATPase in bovine ROSs^10–12. 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⁺.