PERMEABILITY AND INTERACTION OF CA2-GATED ION CHANNELS DIFFER IN RETINAL ROD AND CONE PHOTORECEPTORS( WITH CGMP)

We studied the ionic permeability of cGMP-dependent currents in membra ne patches detached from the outer segment of retinal cone and rod pho toreceptors. Reversal potentials measured in membranes exposed to symm etric Na+ but with varying cytoplasmic Ca2+ concentrations reveal that the permeability ratio, PCa/PNa, is higher in the cGMP-gated channels of cones (7.6 +/- 0.8) than in those of rods (3.1 +/- 1.0). Ca2+ bloc ks both channels in a voltage-dependent manner. At any Ca2+ concentrat ion, the channel block is maximal near the ionic reversal potential. T he maximal block is essentially identical in channels of cones and rod s with respect to its extent and voltage and Ca2+ dependence. The Ca2 block is relieved by voltage, but the features of this relief differ markedly between rods and cones. Whereas the Boltzmann distribution fu nction describes the relief of block by hyperpolarizing voltages, any given voltage is more effective in relieving the Ca2+ block in cones t han in rods. Similarly, depolarizing voltages more effectively relieve Ca2+ block in cones than in rods. Our results suggest that channels c ontain two binding sites for Ca2+, one of which is similar in the two receptor types. The second site either interacts more strongly with Ca 2+ than the first one or it is located differently in the membrane, so as to be less sensitive to membrane voltage. The channels in rods and cones differ in the features of this second site. The difference in C a2+ permeability between the channels is likely to result in light-dep endent changes in cytoplasmic Ca2+ concentration that are larger and f aster in cones than in rods. The functional differences between channe ls, therefore, may be critically important in explaining the differenc es in the phototransduction signal of the two photoreceptor types.