Functional Role of Calcium in Photoreceptor Cells

Ca‐uptake by disc membranes prepared from frog rod photoreceptor outer segments was examined. Ca‐uptake study revealed two affinity sites which were saturated with 10– 5 M and 10– 3 M of ATP. When disc membranes in 20 m M Tris‐HCl buffer (pH 7.5) were stored at ‐20°C for 6 h, more than 95% of Ca‐uptake activity was lost. Ca‐uptake activity was, however, preserved if the disc membrane suspension was mixed with 1–10m M ATP and stored at ‐20°C. Furthermore the reactivation of Ca‐uptake was observed if disc membranes, which had lost Ca‐uptake ability by storing at 4°C for 3 h, were mixed with 10 m M ATP and then frozen at ‐20°C for 5 h or 28 h (ATP‐induced ATP‐dependent Ca‐uptake). When the contents of ATP bound to disc membranes were measured during a brief aging at 37°C, the decrement of bound ATP content was correlated well with the decreasing of Ca‐uptake activity. Carbonylcyanide m ‐chlorophenylhydrazone (CCCP), a conductor of protons, inhibited Ca‐uptake activity and half maximal inhibition was achieved at 2 × 10– 8 M. When 10– 6 M of CCCP was added to the 45 Ca‐accumulated disc membranes, rapid release of 45 Ca from the disc membranes was observed. These results suggest that ATP may play a role in the Ca‐pump regulation in disc membranes and a [H + ] gradient across disc membrane may be linked to Ca‐uptake mechanism.