Colour dependence of the early receptor potential and late receptor potential in scallop distal photoreceptor.

1. Intracellular voltage and current responses to short (blue) and long (red) wave-length lights were measured in the distal hyperpolarizing photoreceptor (‘off receptor’) of the isolated and perfused scallop (Pecten irradians) retina. 2. The early receptor potential (e.r.p.) was isolated by holding membrane potential at the reversal potential for the late receptor potential (l.r.p.) or by working at temperatures (< 5·0 °C) that abolished the l.r.p. 3. The e.r.p., measured using intense flashes of white light, consisted of a positive phase followed by a negative phase, but was converted to a monophasic, negative-going wave following pre-adaptation with red light and to a monophasic, positive-going wave following pre-adaptation with blue light. 4. The spectral sensitivity curve for the negative e.r.p. was maximum at 500 nm, whereas the spectral sensitivity curve for the positive e.r.p. was maximum at 575 nm. 5. The positive or negative e.r.p.s approached their maximum amplitude exponentially when tested with red or blue flashes of increasing intensity. The results suggest that the positive (or negative) e.r.p. is proportional to the number of photopigment molecules photo-isomerized. 6. The photosensitivity maximum of rhodopsin calculated at 500 nm, using the exponential constant and the spectral sensitivity data, was estimated to be 2·1 × 10−16 cm2 photon−1, whereas the photosensitivity maximum of metarhodopsin calculated at 575 nm was estimated to be 2·6 × 10−16 cm2 photon−1. 7. In cells pre-adapted with white light, stimulation with blue light caused a hyperpolarizing l.r.p. which was followed by a prolonged hyperpolarizing after-potential (p.h.a.). Stimulation with red light under similar conditions caused an initial hyperpolarization which was followed by a small depolarization during the stimulus, but no after-potential. 8. The duration of the p.h.a. was increased by pre-adaptation with a red light, which caused the maximum net transfer of metarhodopsin to rhodopsin; however, its decay was always complete in 5 min or less. 9. The photo-isomerization of metarhodopsin by red light suppressed the p.h.a. and caused an after-depolarizing response that decayed in less than 1 min. 10. The spectral sensitivity curve for the induction of the p.h.a. was maximum at 500 nm and corresponded to the spectral sensitivity for the negative e.r.p. and for the l.r.p. studied in the dark-adapted retina, whereas the spectral sensitivity curve for the suppression of the p.h.a. and for the induction of the after-depolarization was maximum at 575 nm and corresponded to the spectral sensitivity for the positive e.r.p. 11. In photoreceptors clamped to the resting potential in normal ASW, the photo-isomerization of rhodopsin, in the absence of light absorption by metarhodopsin, activated a persistent outward current that had the same time course of decay as the p.h.a. The photo-isomerization of metarhodopsin suppressed the persistent outward current and activated an inward current whose decay took longer than the decay of the after-depolarizing response. 12. In the absence of external Ca2+ and Na+ ions, the persistent outward current produced by light absorption by rhodopsin, and the inward current produced by light absorption by metarhodopsin, both reversed at the K+ equilibrium potential. The results show that the induction of the prolonged hyperpolarizing after-potential and the after-depolarizing response involve only the movement of K+ ions through the same light-dependent K+ channels that determine the hyperpolarizing l.r.p. of the distal cells.