Effect of ions on retinal rods from Bufo marinus.

Abstract

SUMMARY1. The effect of ions on the light-sensitive current of isolated retinal rods from the toad Bufo marines was studied by sucking the inner segment into a tightly fitting pipette. The outer segment projected into flowing solution whose composition could be changed rapidly. [Ca]o the current did not approach its final value monotonically but with a characteristic overshoot or underswing, followed by a slow relaxation of current which may reflect the time course of change in internal Na. 6. Reducing [Na]o from 110 to 70 mm or less prolonged the response to a flash; very long responses were observed in solutions containing Li rather than Na and also in rods that had been returned to Ringer solution after exposure to low Ca. All these effects might be explained if Ca extrusion in exchange for Na determines the reactivation of current after a flash. 8. Between 5 mm and 10 /LM-Ca the change in peak current produced by absorption of a single quantum was roughly proportional to the dark current.9. Responses in the absence of external Na were not normally seen if the solution contained 0-1 mm-Ca or more. Responses of normal polarity were regularly observed in 0 Na, 0 Ca EGTA solutions containing 1-6 mM-Mg. Removal of Mg from such solutions gave inverted responses.10. Other conditions which promote responses of normal and inverted polarity in Na-free solutions are described briefly. We conclude that Li, Ca, Mg and perhaps K can pass through the light-sensitive channel.A. L. HODGKIN AND OTHERS 11. The above results suggest that external Na has two distinct effects: (1) it provides ions to carry inward current, and (2) it keeps the light-sensitive conductance open by maintaining the internal Ca concentration, [Ca]i, at a low level.

Sodium‐calcium exchange in the outer segments of bovine rod photoreceptors.

“…Several observations are consistent with the notion that Na-Ca exchange similarly operates in the outer segments ofphysiologically functional rod cells both in the dark as well as in the light. The rapid changes of the dark current in functioning rod cells upon changes of the external Na/Ca ratio are consistent with the notion that the internal Ca2+ concentration in these cells is controlled by Na-Ca exchange (Fain & Lisman, 1981;Hodgkin et al 1984;Yau & Nakatani, 1984a). Similarly, the Na+ dependence of the light-induced Ca2+ release from r.o.s.…”

“…Na+ and K+ fluxes through a conductance mechanism were observed in the preparation of isolated intact r.o.s., but Ca2+ fluxes through a conductance mechanism were only detected at high (20 mM) external Ca2+ concentrations (Schnetkamp, 1985b). Under physiological conditions part of the dark current through the light-sensitive con-40 Na-Ca EXCHANGE IN RODS ductance may be carried by Ca2+ (Hodgkin et al 1984). The maximal Caa2+ flux through the Na-Ca exchanger would be of comparable magnitude as that due to a Ca2+ current of 60 pA.…”

“…Yau & Nakatani (1984b) recently reported a maximal current of 30 pA in illuminated toad rod outer segments, which the authors attribute to electrogenic Na-Ca exchange operating at a stoicheiometry of 3 Na/Ca. If the dimensions of a toad rod outer segment are taken to be 6-5 x 60,um (Hodgkin et al 1984) a Ca2+ flux of 25 pmol cm-2 s-1 can be calculated. It appears that the maximal Ca2+ flux of Na-Ca exchange is quite similar in isolated bovine r.o.s.…”

“…Changes in the external Ca2+ concentration result in rapid and large changes in a Na+ current flowing into the outer segment in the dark (Yau, McNaughton & Hodgkin, 1981;Hodgkin, McNaughton, Nunn & Yau, 1984;MacLeish, Schwartz & Tachibana, 1984), and in apparent changes in the range of cations able to carry the dark current Woodruff, Fain Yau & Nakatani, 1984 a;Hodgkin, McNaughton & Nunn, 1985). These effects are generally thought to be mediated by changes in the intracellular Ca2+.…”

Properties of ion channels closed by light and opened by guanosine 3',5'‐cyclic monophosphate in toad retinal rods.

“…All recordings were made from isolated rod photoreceptors obtained by mechanical dissociation of isolated retina as described previously (Baylor et al 1979a; Hodgkin, McNaughton, Nunn & Yau, 1984). Under dim red light, a toad was killed by pithing the brain and spinal cord, and one eye was removed and hemisected.…”