Membrane properties of a barnacle photoreceptor examined by the voltage clamp technique.

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1. Electrical properties of the membrane of photoreceptor cells in the lateral ocelli of barnacles, Balanus amphitrite and B. eburneus were investigated by intracellular recording, polarization and voltage-clamp techniques.2. The resting potential of a dark adapted cell was 36.3 +/- 6.6 mV (S.D.) and depended mainly on the external K(+) concentration.3. Current-voltage relations obtained from voltage-clamp experiments in the absence of light were non-linear and varied with time after the onset of a step change in membrane potential; the steady state was reached after about 0.5 sec.4. Illumination resulted in a membrane potential change under current clamp and in a change of membrane current (light-initiated membrane current (L.I.C.): total membrane current with illumination minus current without illumination) under voltage-clamp conditions. Amplitudes and time course of L.I.C. depended on the light intensity as well as membrane potential.5. The L.I.C.-voltage relation was non-linear and corresponded with a slope conductance increase with increasing positive membrane potential.6. The reversal potential of L.I.C. was independent of the light intensity and the time after onset of illumination; the average value obtained in normal saline was +26.9 +/- 5.0 mV.7. The membrane conductance estimated from instantaneous L.I.C.-voltage relations agreed with the chord conductance of the non-linear L.I.C.-voltage relation.8. Decreasing external Na(+) concentration decreased the inward component of L.I.C. but not the outward component.9. Decreasing external Ca(2+) concentration increased the inward as well as the outward component of L.I.C.10. The reversal potential shifted in the negative direction with decreasing external Na(+) concentration (the rate was 10-15 mV for a tenfold change in concentration) and the rate was augmented in the absence of Ca(2+) but did not exceed 21 mV.11. The change of reversal potential with changes of external Ca(2+) concentration was negligible in normal Na(+) media but was significant in the absence of Na(+) (rate as high as 20 mV).12. Alteration of the external K(+) or Cl(-) concentrations did not affect the amplitude or reversal potential of L.I.C.13. The results indicate that illumination increases the membrane permeability mainly to Na(+) ions and that the primary effect of Ca(2+) ions is suppression of the permeability increase; Ca(2+) permeability may increase slightly during illumination.