Phototransduction and calcium exchange along the length of the retinal rod outer segment

THE aim of this study was to investigate whether there are differences in calcium exchange between the ends of a retinal rod outer segment (OS) which could be correlated with the known differences of light responsiveness and which could explain the latter. We found that there is little, if any, difference in the exchange current density. However, the time constant of the exchange current decay after a saturating flash of light lengthened from base to tip and more calcium was extruded at the tip than the base of the OS. Our analysis of the results leads to the conclusion that there is little, if any, difference in free calcium concentration between the ends of the outer segment, but that there is a difference in the bound calcium and that this can explain the difference in light responsiveness along the length of the rod.