Fast Stages of Photoelectric Processes in Biological Membranes

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Abstract
The functioning of visual rhodopsin as a photoelectric generator was demonstrated with a direct method. Photoreceptor discs [bovine retina] were incorporated into a phospholipid-impregnated collodion film. Illumination of the resulting system with continuous light induced formation of an electric potential (the disc-free side positive) that was measured with 2 electrodes separated by the film. A photopotential exceeding 40 mV was shown. It dissipated before the light source was switched off. A 15 ns 530 nm laser flash induced the formation of a photopotential of up to 35 mV whose appearance was preceded with a small oppositely directed electrogenic phase. This negative photoresponse took < 200 ns. The positive photoresponse was composed of at least 2 phases (t1/2 [half-time] about 500 .mu.s and several ms). The latter correlated with formation of metarhodopsin II. A 347 nm laser flash added after a 530 nm flash resulted in a photoelectric effect similar to that initiated by a 530 nm flash but of opposite direction. The 347 nm response was completely abolished by hydroxylamine preventing the accumulation of metarhodopsin II. The response at 530 nm was hydroxylamine-resistant. Both the amplitude and the decay time of the flash-induced potential were maximal in the response to the 1st flash, each subsequent flash being less effective than the preceding one. Flashes caused acceleration of the photopotential decay. The latter effect was due to an increase of membrane conductance that developed faster than in 50 ms. Addition of 11-cis retinal after illumination improved the amplitude of the photoresponse but not the conductance. The light-induced increase in conductance was insensitive to hydroxylamine. Apparently a function of visual rhodopsin consists of generating a potential difference across the photoreceptor disc membrane which responds with an increase in membrane permeability to a rise of the membrane potential. A possible role of an electric breakdown of the membrane, induced by the rhodopsin-generated local or partially delocalized electric field, was discussed.