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The effect of ions on the light-sensitive current of isolated retinal rods from the toad Bufo marinus 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. Reducing the external Na concentration, [Na]o, round the outer segment caused rapid and reversible reductions in the light-sensitive current. With the outer segment in the pipette, reductions of [Na]o round the inner segment had little effect on the light-sensitive current. The current about 15 s after a change in [Na]o was approximately proportional to [Na]2o. The current decreased in elevated external Ca concentration, [Ca]o, and increased in reduced [Ca]o. Between 10 and 0.5 mM-external Ca the current 15 s after a change was approximately inversely proportional to [Ca]o. Reducing [Ca]o from 1 mM to 1 microM or less transiently increased the current by about 15-fold. After a change in [Na]o or [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. 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. The rod current was not changed if the ratio [Na]No/[Ca]o was held constant, N being about 2.5. Between 5 mM and 10 microM-Ca the change in peak current produced by absorption of a single quantum was roughly proportional to the dark current. 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. 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. 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.

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