Light-induced Ca 2+ release in the visible cones of the zebrafish

We used suction-pipette recording and fluo-4 fluorescence to study light-induced Ca 2+ release from the visible double cones of zebrafish. In Ringer, light produces a slow decrease in fluorescence which can be fitted by the sum of two decaying exponentials with time constants of 0.5 and 3.8 s. In 0Ca 2+ –0Na + solution, for which fluxes of Ca 2+ across the outer segment plasma membrane are greatly reduced, light produces a slow increase in fluorescence. Both the decrease and increase are delayed after incorporation of the Ca 2+ chelator BAPTA, indicating that both are produced by a change in Ca 2+ . If the Ca 2+ pool is first released by bright light in 0Ca 2+ –0Na + solution and the cone returned to Ringer, the time course of Ca 2+ decline is much faster than in Ringer without previous light exposure. This indicates that the time constants of 0.5 and 3.8 s actually reflect a sum of Na + /Ca 2+ -K + exchange and light-induced release of Ca 2+ . The Ca 2+ released by light appears to come from at least two sites, the first comprising 66% of the total pool and half-released by bleaching 4.8% of the pigment. Release of the remaining Ca 2+ from the second site requires the bleaching of nearly all of the pigment. If, after release, the cone is maintained in darkness, a substantial fraction of the Ca 2+ returns to the release pool even in the absence of pigment regeneration. The light-induced release of Ca 2+ can produce a modulation of the dark current as large as 0.75 pA independently of the normal transduction cascade, though the rise time of the current is considerably slower than the normal light response. These experiments show that Ca 2+ can be released within the cone outer segment by light intensities within the physiological range of photopic vision. The role this Ca 2+ release plays remains unresolved.