The Journal of General Physiology
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Published 1 October 2001. doi:10.1085/jgp.118.4.377
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© The Rockefeller University Press, 0022-1295/2001//377/ $5.00
Journal of General Physiology, Volume 118, Number 4, 2001

Original Article

Prolongation of Actions of Ca2+ Early in Phototransduction by 9-Demethylretinal

Hugh R. Matthewsa, M.C. Cornwallb, and R.K. Crouchc

a Physiological Laboratory, University of Cambridge, Downing Street, Cambridge CB2 3EG, United Kingdom
b Department of Physiology, Boston University Medical School, Boston, MA 02215
c Department of Ophthalmology, Medical University of South Carolina, Charleston, SC 29401
Physiological Laboratory, Downing Street, Cambridge CB2 3EG, United Kingdom.44-223-333840

hrm1{at}cam.ac.uk

During adaptation Ca2+ acts on a step early in phototransduction, which is normally available for only a brief period after excitation. To investigate the identity of this step, we studied the effect of the light-induced decline in intracellular Ca2+ concentration on the response to a bright flash in normal rods, and in rods bleached and regenerated with 11-cis 9-demethylretinal, which forms a photopigment with a prolonged photoactivated lifetime. Changes in cytoplasmic Ca2+ were opposed by rapid superfusion of the outer segment with a 0Na+/0Ca2+ solution designed to minimize Ca2+ fluxes across the surface membrane. After regeneration of a bleached rod with 9-demethlyretinal, the response in Ringer's to a 440-nm bright flash was prolonged in comparison with the unbleached control, and the response remained in saturation for 10–15s. If the dynamic fall in Ca2+i induced by the flash was delayed by stepping the outer segment to 0Na+/0Ca2+ solution just before the flash and returning it to Ringer's shortly before recovery, then the response saturation was prolonged further, increasing linearly by 0.41 ± 0.01 of the time spent in this solution. In contrast, even long exposures to 0Na+/0Ca2+ solution of rods containing native photopigment evoked only a modest response prolongation on the return to Ringer's. Furthermore, if the rod was preexposed to steady subsaturating light, thereby reducing the cytoplasmic calcium concentration, then the prolongation of the bright flash response evoked by 0Na+/0Ca2+ solution was reduced in a graded manner with increasing background intensity. These results indicate that altering the chromophore of rhodopsin prolongs the time course of the Ca2+-dependent step early in the transduction cascade so that it dominates response recovery, and suggest that it is associated with photopigment quenching by phosphorylation.

Key Words: retinal rod • calcium • photoreceptor • rhodopsin • adaptation

© 2001 The Rockefeller University Press


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