Cation selectivity of light-sensitive conductance in retinal rods.
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.
D000818:Animals; D002024:Bufo marinus; D002118:Calcium / Q000494:pharmacology*; D004553:Electric Conductivity; D066298:In Vitro Techniques; D008027:Light; D010786:Photoreceptor Cells / Q000187:drug effects / Q000502:physiology*; D012964:Sodium / Q000494:pharmacology*
D012964:Sodium; D002118:Calcium