Protons, Calcium, and the Rate-limiting Step for Shutoff of Phototransduction in Retinal Cones

Abstract

Abstract: : Purpose: To investigate how changes in the external pH alter the shutoff dynamics of the light response of salamander L- and S-cones. Method: Membrane current was recorded by drawing a cone inner segment into a suction electrode, leaving the outer segment protruding into a stream of flowing solution whose composition could be changed. Results: In L-cones lowering the external pH from 7.6 to 6.6 rapidly and reversibly reduced the dark current 20% and shortened the integration time of the dim flash response 2-fold. The Pepperberg time constant shortened by 25%, indicating accelerated shutoff of the rate-limiting intermediate in the excitation cascade. Partial block of the dark current with L-cis diltiazem gave similar changes, as did exposure of the L-cone to dim background light. The shortening of the light response likely resulted from a drop in internal Ca₂₊ concentration secondary to the reduced Ca₂₊ influx. While the light response of S-cones also shortened in dim background light and during partial block of the dark current with L-cis diltiazem, lowering the pH from 7.6 to 6.6 had no effect on the dark current and response kinetics. Conclusion: In both L- and S-cones the rate-limiting step for the shutoff of the transduction cascade is apparently Ca₂₊ sensitive. The cGMP-gated channels of L- and S-cones are different proteins; that of L-cones is blocked by protons, whereas that of S-cones is not. Supported by NIH grant EY01543

Keywords: 517 photoreceptors • 384 dark/light adaptation • 394 electrophysiology: non-clinical