Enhancement of sensitivity in photoreceptors of the honey been drone by light and by Ca2+.

Deeply dark adapted (1 h) photoreceptor cells of the honey bee drone show a light-induced enhancement of sensitivity (facilitation) as an aftereffect of illumination or in the presence of dim backgrounds. The Ca(2+)-dependency of this effect was studied: Reduction of extracellular Ca2+ to 0.1 mM decreases the sensitivity of a dark adapted cell, and the light-induced increase in sensitivity due to repetitive, dim, 20 ms test flashes is slower than in normal saline. After a sensitizing conditioning light, the sensitivity drops faster in low-calcium saline. The light-induced enhancement of sensitivity is mimicked by pressure injections of low amounts of Ca2+ (Ca2+/EGTA-buffers; 0.15 microM free Ca2+) into a dark adapted cell. Injection of EGTA alone decreases the sensitivity. Injection of a solution containing ca1 mM free Ca2+ sequentially decreases and later increases the sensitivity transiently. These results suggest a model in which a progressive increase in intracellular Ca2+ concentration by light first increases (facilitates), and, at higher concentrations, decreases (light adapts) the sensitivity of the cells. One possible site of action for this positive and negative feedback control of cell sensitivity by Ca2+ is the endoplasmic reticulum.