Ca2+ induces an increase in cGMP-phosphodiesterase activity in squid retinal photoreceptors

In invertebrate photoreceptors, light elicits the opening of cationic channels to produce a depolarizing receptor potential. One hypothesis is that cGMP is the agent that gates the channels. It has been previously proposed that the light-induced rise in intracellular Ca2+ down-regulates phosphodiesterase activity, thereby eliciting an increase in intracellular cGMP concentration. cGMP-phosphodiesterase activity from squid photoreceptors was monitored both by a continuous fluorescence assay using 2'-o-(N-methylanthraniloyl)-cGMP and by hydrolysis of [3H]cGMP. The activity of cGMP-phosphodiesterase was found to be increased by Ca2+ in the physiological range of concentrations. These findings suggest that the previously known light-induced increase in Ca2+ in invertebrate photoreceptors cannot directly account for light-elicited down-regulation of cGMP-phosphodiesterase.