The ionic selectivity and calcium dependence of the light‐sensitive pathway in toad rods.

Abstract

A new method is described for determining the effects of rapid changes in ionic concentration on the light‐sensitive currents of rod outer segments. Replacing Na with another monovalent cation caused a rapid change in current followed by an exponential decline of time constant 0.5‐2 s. From the magnitude of the initial rapid change in current we conclude that Li, Na, and K and Rb ions pass readily through the light‐sensitive channel in the presence of 1 mM‐Ca, whereas Cs crosses with difficulty and choline, tetramethylammonium and tetraethylammonium not at all. The effect of reducing Ca in the external medium indicates that the residual inward current recorded for a few seconds when Na is replaced by an impermeant ion is carried largely by Ca ions. With 1 microM‐Ca in the external medium the relative ability of monovalent cations to carry light‐sensitive current is Li:Na:K:Rb:Cs = 1.4:1:0.8:0.6:0.15. The same order applied in the physiological region but the values are less certain. Large transient inward currents are seen if external Ca is raised form 1 microM to 5 mM or more; these currents which are maximal in an isotonic Ca solution are presumably carried by Ca. The effect of monovalent cations on the number of open light‐sensitive channels was tested by adding the cation to a solution containing 55 mM‐Na. Na ions open light‐sensitive channels with a delay, probably by promoting Na‐Ca exchange; K and Rb close channels by inhibiting exchange; Li and Cs seem inert in the exchange mechanism. The rate at which inward current declines in low [Na]o or high [Ca]o is accelerated by weak background lights and slowed by 3‐isobutyl‐1‐methylxanthine (IBMX), which inhibits the hydrolysis of cGMP. On returning to Ringer solution after a period in low [Na]o the current recovers with a delay of about 1 s which decreases as the Ca concentration of the low [Na]o medium is reduced. We conclude that intracellular Ca has a strong effect on the number of open light‐sensitive channels. None the less, several observations are inconsistent with channel closure being dependent simply on combination with internal Ca.