RHODOPSIN REGENERATION, CALCIUM, AND THE CONTROL OF THE DARK CURRENT IN VERTEBRATE RODS

Abstract— The dark current of retinal rods is suppressed for an extended period when their rhodopsin is bleached. An 8% bleach completely suppresses the current for 8 min and after 35 min it is fully recovered. The dark current can recover fully from a bleaching flash without any rhodopsin being regenerated. Moreover the recovery can be hastened either by lowering the activity of calcium ions surrounding the rods or by regenerating the rhodopsin. The recovery of the dark current following these bleaches showed zero order kinetics, regardless of whether the recovery was hastened by low calcium, 11- cis retinaldehyde or not. If all the rhodopsin is bleached in the retina, the dark current does not recover; the addition of 11- cis retinaldehyde, but not all- trans retinaldehyde, to the bleached retina causes the dark current to begin its recovery as early as 10 min after the addition with zero order kinetics (1.3% min^-1). In two of three similar experiments, the dark current recovered 100%. When the recovery rate of the dark current from the retina showing the earliest response is compared with the rate of the regeneration of rhodopsin in the plasma and disc membranes, the dark current begins its recovery after the plasma membrane rhodopsin is fully regenerated and the disc rhodopsin is half regenerated. When the disc rhodopsin is fully regenerated, the dark current is recovered 75%, and 20 min later it is completely recovered.