2 OPPOSITE EFFECTS OF ATP ON THE APPARENT SENSITIVITY OF THE CGMP-GATED CHANNEL OF THE CARP RETINAL CONE

Effects of ATP on the activity of cGMP-gated channels from carp cone p hotoreceptors were studied. In 29% of the patches examined (N = 45), A TP (1 mM) enhanced a current evoked by cGMP (20 mu M, up to about 100% ), in 33%, ATP suppressed it by up to about 90%, and in the remaining 38%, ATP had no effect. ATP showed similar effects on a current evoked by 8-bromoguanosine 3',5'-cyclic monophosphate (2 mu M, enhancing in 42% of the patches, suppressing in 25%, no effect in 33%, N = 12), sug gesting that the effects were not through modulation of the phosphodie sterase. Both of the effects, enhancement and suppression, were produc ed by a change in apparent affinity for cGMP, since (1) the maximum cu rrent evoked by cGMP of the saturating concentration (greater than or equal to 1 mM) was not affected, and (2) the K-1/2 value decreased by approximately 45% (N = 2) or increased by approximately 25% (N = 2). A lower pH (approximately 6) facilitated the enhancing effect. ATP-gamm a-S (1 mM) showed a suppressing effect in 80% of the patches and no ef fect in 20% of the patches (N = 10). However, ATP-gamma-S did not show an enhancing effect. Thus, ATP had two opposite effects through diffe rent mechanisms on the apparent sensitivity of the channel to cGMP; in creasing and decreasing.