Visual transduction in vertebrate photoreceptors. Light activation of guanylate cyclase.

Light activation of guanylate cyclase at different calcium concentrations was studied in the rod outer segments of the toad retina. The enzyme becomes sensitive to calcium ions after a flash of light, showing an enhancement of its activity when Ca2+ concentration is lowered from 10(-4) M to 10(-8) M. A possible pathway of guanylate cyclase activation by light was also investigated by means of the antibody 4A to transducin. When added in excess to transducin, the antibody inhibits light activation of phosphodiesterase as well as of cyclase, suggesting a possible coupling of the two enzymes.