The calcium gradient along the rod outer segment

Vertebrate photoreceptor outer segments renew themselves by growing new membrane near the base and shedding old membrane at the tip. Parallel to the resulting age gradient along the outer segment there have also been observed gradients of membrane composition, rhodopsin phosphorylation, cGMP regeneration, responsiveness to light and others. This chapter describes the calcium gradient which has been found to exist along the outer segment. The concentration of calcium which increases towards the tip is due to an increase in buffered calcium. Since calcium is involved in a network of regulatory processes this gradient has implications for the transduction cascade as it affects the light response, as well as on disc shedding and other functions of the outer segments.