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A. L. F. Gorman^* and John S. McReynolds

Department of Physiology, Boston University School of Medicine, Boston, Mass. 02118, U.S.A.

Department of Physiology, University of Michigan, Ann Arbor, Michigan 48104, U.S.A.

1. The effects of different external ionic conditions and of metabolic inhibitors on the membrane potential of hyperpolarizing photoreceptors in the retina of the scallop Pecten irradians were examined in the presence and absence of light.

2. Changes in extracellular K⁺ have a greater effect on membrane potential in the light than in darkness. The receptor potential is increased in amplitude when [K]_o is reduced and decreased when [K]_o is elevated. It is hyperpolarizing when [K]_o is less than the estimated value for [K]_i and depolarizing when this condition is reversed.

3. The complete replacement of [Na]_o causes a significant hyperpolarization of membrane potential in darkness, whereas it has a much smaller hyperpolarizing effect on the peak of the receptor potential.

4. The ratio of Na⁺ to K⁺ permeabilities (P_Na/P_K) decreases during bright illumination. Our results suggest that P_K is seven times that for P_Na in the dark but is 57 times greater than P_Na in light.

5. The metabolic inhibitors DNP and NaCN cause membrane potential in the dark to hyperpolarize. This hyperpolarization is associated with a decrease in the P_Na/P_K ratio similar to that found during illumination.

6. High [Ca⁺]_o also causes membrane potential in the dark to hyperpolarize. This hyperpolarization is associated with an increase in membrane conductance.

7. The results indicate that the hyperpolarizing receptor potential of the distal photoreceptor is produced by a light-evoked increase in K⁺ permeability.

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