Dual role for extracellular calcium in blowfly phototransduction

Abstract

1. The effect of inhibiting Ca2+-entry during phototransduction in the compound eye of Calliphora vicina has been investigated in a newly developed semiintact preparation. Simultaneous measurements were made of the receptor potential and the light-induced mitochondrial activation while changing the composition of the extracellular fluid. 2. Lowering the extracellular calcium concentration with EGTA: (i) slows down the kinetics of excitation until the peak of the receptor potential is completely suppressed; and (ii) induces a collapse of the plateau phase after 1 s, although illumination continues. Both effects are reversible. 3. Superfusing the preparation with Ringer solution containing the Ca2+-antagonist Co2+ initially only affects the peak. Prolonged superfusion with Co2+ reduces both the peak and the plateau phase. The latter effect of Co2+ is similar to introduction of Co2+ into the cell via the recording pipette, suggesting that these effects are due to Co2+-entry. These effects of Co2+ are irreversible. 4. The results with EGTA and Co2+ are compared with the reported effect of the Ca2+-antagonist La2+, which induces a collapse of the plateau phase while leaving the peak relatively unaffected. It is suggested that two [Ca2+]e-dependent processes contribute to blowfly phototransduction. Both processes are sensitive to EGTA, whereas Co2+ affects only one and La2+ the other. 5. In the presence of EGTA or Co2+ the light-induced mitochondrial activation was inhibited. This result supports the hypothesis that upon illumination a rise in the intracellular calcium concentration activates mitochondrial respiration.