[ABSTRACT]

Vertebrate photoreceptors respond to light by a membrane hyperpolarization which is thought to result from the decrease of a Na-selective conductance in the outer segment. One hypothesis is that light increases intracellular free Ca which reversibly blocks the Na conductance; at least part of this Ca is then extruded to the cell exterior by a Na-Ca exchanger at the plasma membrane. We describe experiments here which show that the light-sensitive conductance in rods is also highly permeable to K. While external Na acts to keep the conductance open, external K tends to keep it closed, both actions probably involving the Na-Ca exchanger. The conductance is also permeable to the monovalent cations Li, Rb and Cs and the divalent cations Ca, Sr and Ba. The ability of both Na and K to go through the light-sensitive conductance explains the long-standing puzzle as to why the reversal potential for the light response is at 0 to + 10 mV.