Cation and anion sequences in dark-adapted Balanus photoreceptor.

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Abstract
Anion and cation permeabilities in dark-adapted Balanus photoreceptors were determined by comparing changes in membrane potential in response to replacement of the dominant anion (Cl-) or cation (Na+) by test anions or cations in the superfusing solution. The anion permeability sequence obtained with PI > PSO4 > PBr > PCl > Pisethionate > Pmethanesulfonate. Gluconate, glucuronate and glutamate generally appeared more permeable and propionate less permeable than Cl-. The alkali-metal cation permeability sequence obtained was PK > PRb > PCs > PNa .apprxeq. PLi. This corresponds to Eisenman''s sequence IV which is the same sequence obtained for other classes of nerve cells in the resting state. The values obtained for the permeability ratios of the alkali-metal cations are considered minimal. The membrane conductance measured by passing inward current pulses in the different test cations followed the sequence, GK > GRb > GCs > GNa > GLi. The conductance ratios obtained for a full substitution of the test cation agreed with permeability ratios for all the alkali-metal cations except K+ which was generally higher.