The effects of hepes, bicarbonate and calcium on the cGMP content of vertebrate rod photoreceptors and the isolated electrophysiological effects of cGMP and calcium

To evaluate the role of cGMP in vertebrate rod photoreceptors, two extracellular effectors of cGMP were used (calcium and buffer), and both cGMP content and electrophysiological responses were measured. In the dark when the cGMP content was reduced by 80%, electrophysiological effects mimicking a weak background light were observed. Continuous illumination, sufficient to suppress all electrophysiological responses, had only minor effects on cGMP. The data seem inconsistent with suggestions that steady-state levels of cGMP alone control the light responses of rod photoreceptors. The electrophysiological effects of extracellular calcium (under conditions of unaltered cGMP) were distinguishable from those of cGMP.