INTRACELLULAR CA2-INDUCED CA2+ CHANGES IN PHOTORECEPTORS OF THE HONEYBEE DRONE( CONCENTRATION AND LATENCY OF LIGHT)

We have measured Cai at rest and upon light stimulation in the photore ceptors of the honeybee drone microfluorometrically with the fluoresce nt Ca2+ indicator dyes fura-2, fluo-3 and Ca-green 5N. In darkness, Ca , was similar to 90 nM after 5 min of dark adaptation. A saturating li ght step caused Ca-i to rise in the bulk cytoplasm to similar to 750 n M within 1 s. Our measurements with the low affinity dye Ca-green 5N s howed that bright 1-s light flashes cause a rapid increase in Ca-i, wh ich was graded with stimulus intensity. Ca-green 5N fluorescence reach ed a peak in about 200 ms, and then decayed to a slightly lower sustai ned plateau. The fluorescence signal peaked, when the receptor potenti al was repolarizing from its peak to the plateau. This observation is in agreement with the proposal that the peak-to-plateau transition of the receptor potential is caused by the rise in Ca-i. From our Fluo-3 measurements it appears that the latency of the Ca2+ increase is by 3- 4 ms longer than the latency of the receptor potential elicited by bri ght 100-ms light flashes. This result provides no support for the prop osal that Ca2+ mediates the opening of those membrane channels respons ible for the upstroke of the receptor potential.