Calcium transients in the rhabdomeres of dark- and light-adapted fly photoreceptor cells

The light response of fly photoreceptor cells is modulated by changes in fr ee Ca2+ concentration. Fly phototransduction and most processes regulating it take place in or very close to the rhabdomere. We therefore measured the kinetics and the absolute values of the free Ca2+ concentration in the rha bdomere of fly photoreceptor cells in vivo by making use of the natural opt ics of the fly's eye. We show that Ca2+ flowing into the rhabdomere after l ight stimulation of dark-adapted cells causes fast Ca2+ transients that rea ch peak values higher than 200 mu M in <20 msec. Approximately 500 msec lat er, the free Ca2+ concentration has declined again to similar to 20 mu M. T he duration of the Ca2+ transients becomes still shorter, and their size re duced, when the photoreceptor cell is light-adapted. This reduction in dura tion and size of the Ca2+ transients is graded with the intensity of the ad apting light. The kinetics and absolute values of the free calcium concentr ation found to occur in the rhabdomere are suitable to mediate the fast fee dback signals known to act on the fly phototransduction cascade.