Light induced changes of internal pH in a barnacle photoreceptor and the effect of internal pH on the receptor potential.

Abstract

Intracellular pH (pHi) was measured in Balanus eburneus photoreceptors using pH-sensitive glass micro-electrodes. The average pHi of 12 photoreceptors which were dark adapted for at least 30 min was 7.3 .+-. 0.07 (S.D.). Illumination reduced the recorded pHi by as much as 0.2 pH unit. The change in pHi was graded with light intensity. When the cells were exposed to CO2 in the dark, pHi declined monophasically. Saline equilibrated with 2% CO2; 98% O2 produced a steady reduction in pHi of .apprx. 0.25 unit in 2-3 min. The buffering capacity of the receptor cell cytoplasm calculated from such experiments is .apprx. 15 slykes. In the presence of HCO3-, CO2 saline produced smaller, biphasic changes in pHi. The membrane depolarization produced by a bright flash (depolarizing receptor potential) was reversibly reduced in the presence of external CO2 or by injection of H+. Iontophoretic injection of HCO3- increased the amplitude of the receptor potential. In individual cells there was a close correlation between the amplitude of the receptor potential and pHi. Saline equilibrated with CO2 reduced the light induced current (recorded under voltage-clamp) by 40-50% without affecting its reversal potential. Exposure of the receptor to 95% CO2 saline for several minutes (pHo 5.5) not only abolished the receptor potential but also reversibly decreased the K conductance of the membrane in the dark. These effects were not reproduced by pHo 5.5 buffered saline or by a 5 min exposure to saline equilibrated with N2. Changes in pHi induced by light may modulate the sensitivity of the receptor under physiological conditions.