CA2- DIFFERENCES IN CA2+ SELECTIVITY OF THE CGMP-GATED ION CHANNELS AND CA2+ CLEARANCE RATES( FLUX IN RETINAL ROD AND CONE OUTER SEGMENTS )

In intact rod and cone photoreceptors of various vertebrate species, d epolarization in the dark to greater than or equal to+20 mV specifical ly activates the cGMP-dependent conductance in the outer segment. This activation reflects a voltage-dependent decrease in cytoplasmic Ca2and the consequent activation of a Ca2+-dependent guanylyl cyclase. Th e conductance activation in cones is much faster in time course and la rger in extent than that in rods. Simulations of the experimental resu lts suggest that these differences arise from differences in Ca2+ home ostasis in the rod and cone outer segments. Direct measurements demons trate that, indeed, the Ca2+ permeability of the cGAMP-gated channels is higher in cones than in rods. Also, as was previously known, the ra te of Ca2+ efflux from cone outer segments is higher than that in rods . Therefore, a given light-dependent change in membrane current should cause a much larger and much quicker decrease in Ca2+ concentration i n cones than in rods. The activity of every Ca2+-dependent biochemical event in the outer segment should, hence, change more rapidly and to a larger extent in cones than in rods. We propose that these kinetic a nd stoichiometric differences in the function of Ca2+-dependent proces ses is important in explaining the difference in the transduction sign al of the two receptor types.