Ionizable groups and conductances of the rod photoreceptor membrane.

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Abstract
The ionizable groups and conductances of the rod plasma membrane were studied by measuring membrane potential and input impedence with micropipettes that were placed in the rod outer segments [Bufo marinus and Ambystoma mexicanum]. Reduction of the pH from 8.0 to 6.8 or from 7.8 to 7.3 resulted in membrane depolarization in the dark (by 2-3 mV) and an increased size of the light response (also by 2-3 mV). The dark depolarization was accompanied by an increased resting input impedance (by 11-35 M.OMEGA.). When the pH was decreased in a perfusate in which Cl- was replaced by isethionate, the membrane depolarized. When the pH was decreased in a perfusate in which Na+ was replaced by choline, an increase of input impedance was observed (11-50 M.OMEGA.) even though a depolarization did not occur. Apparently the effects of decreased extracellular pH result mainly from a decrease in rod membrane K+ conductance that is presumably caused by protonation of ionizable groups having a pKa between 7.3-7.8. From these results and results obtained by using CO2 and NH3 to affect specifically the internal pH of the cell, it seems unlikely that altered cytoplasmic [H+] is a cytoplasmic messenger for excitation of the rod. When the rods were exposed to perfusate in which Na+ was replaced by choline, the resting (dark) input impedance increased (by 26 M.OMEGA. .+-. 5 M.OMEGA. SE), and the light-induced changes in input impedance became undetectable. Replacement of Cl- by isethionate had no detectable effect on either the resting input impedance or the light-induced changes in input impedance. These results confirmed previous findings that the primary effect of light is to decrease the membrane conductance to Na+ and showed that, if any other changes in conductance occur, they depend on the change in Na+ conductance. The results were consistent with the following relative resting conductances of the rod membranes: GNa+ .simeq. GK+ > 2-5 GCl-.