CHARACTERIZATION OF GUANYLATE-CYCLASE ACTIVITY IN SINGLE RETINAL ROD OUTER SEGMENTS

cGMP mediates vertebrate phototransduction by directly gating cationic channels on the plasma membrane of the photoreceptor outer segment. T his second messenger is produced by a guanylate cyclase and hydrolyzed by a light-activated cGMP-phosphodiesterase. Both of these enzyme act ivities are Ca2+ sensitive, the guanylate cyclase activity being inhib ited and the light-activated phosphodiesterase being enhanced by Ca2+. Changes in these activities due to a light-induced decrease in intrac ellular Ca2+ are involved in the adaptation of photoreceptors to backg round light. Mie describe here experiments to characterize the guanyla te cyclase activity and its modulation by Ca2+ using a truncated rod o uter segment preparation, in order to evaluate the enzyme's role in li ght adaptation. The outer segment of a tiger salamander rod was drawn into a suction pipette to allow recording of membrane current, and the remainder of the cell was sheared off with a probe to allow internal dialysis. The cGMP-gated channels on the surface membrane were used to monitor conversion of GTP, supplied from the bath, into cGMP by the g uanylate cyclase in the outer segment. At nominal 0 Ca2+, the cyclase activity had a K-m, of 250 mu M MgGTP and a V-max of 25 mu M cGMP s(-1 ) in the presence of 1.6 mM free Mg2+ in the presence of 0.5 mM free M g2+, the K-m was 310 mu M MgGTP and the V-max was 17 mu M cGMP s(-1). The stimulation by Mg2+ had an EC(50) of 0.2 mM Mg2+ for MgGTP at 0.5 mM. Ca2+ inhibited the cyclase activity. In a K+ intracellular solutio n, with 0.5 mM free Mg2+ and 2.0 mM GTP, the cyclase activity was 13 m u M cGMP s(-1) at nominal 0 Ca2+; Ca2+ decreased this activity with a IC50 Of similar to 90 nM and a Hill coefficient of similar to 2.0.