THE EFFECTS OF PROTONS ON 3',5'-CGMP-ACTIVATED CURRENTS IN PHOTORECEPTOR PATCHES

Macroscopic 3',5'-guanine cyclic monophosphate (cGMP)-activated curren ts from photoreceptor outer segment membranes were examined as the pH on the cytoplasmic face of inside/out patches was reduced. In the abse nce of divalent cations, protons reduced the current in both direction s without affecting the shape of the current-voltage relation consiste nt with a voltage-independent block. When Ca2+ was added to the bath, increasing the [H+] relieved the Ca2+ block and eliminated the Ca2+-in duced reversal potential shifts seen at pH 7.4. These results suggest that protons alter Na+/Ca2+ permeability of the channel and relieve Ca 2+ block of the sodium transport.