Role of calcium in regulating the cyclic GMP cascade of phototransduction in retinal rods

Both cGMP and Ca²⁺ appear to be involved in the process of phototransduction in vertebrate rods, but their precise roles have been the subject of debate. To investigate the role of Ca²⁺ we have artificially increased the calcium buffering capacity of the rod by using a patch pipet to incorporate the calcium buffer 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) into the rod cytoplasm. In the presence of buffer the Na⁺-Ca²⁺ exchange current became greatly slowed, suggesting that the cytoplasmic calcium concentration (Ca(i)) had indeed been buffered substantially. Although the presence of buffer had negligible effect on the rising phase of the light response, it profoundly altered the later behavior. Responses to brief flashes became prolonged and exhibited an overshoot, apparently because the shut-off process was modified. The normal acceleration of time-to-peak with brighter flashes (an early sign of light adaptation) disappeared. Responses to steady adapting illumination took much longer than normal to settle to a steady level, although the final level represented a similar fractional suppression of current. With superimposed test flashes the presence of such adapting illumination caused a more rapid recovery, whereas the presence of calcium buffer slowed the recovery. The results are consistent with the idea that the rapid drop in Ca(i), which has recently been shown to accompany the light response, is involved in terminating the light response, and that Ca(i) is thereby involved in setting the operating point and sensitivity of phototransduction. From comparison with other work we infer that Ca(i) appears to act, at least in part, by means of control of cGMP phosphodiesterase activity.