The influence of extracellular calcium on the response of fly photoreceptors

Summary This paper presents a systematic investigation of the influence of the extracellular concentration of calcium ([Ca²⁺]₀) on the electrophysiological response of the fly's photoreceptors (R1–R6) to light. The hemisected heads of flies were perfused with a standard medium containing 10^–4 mol/1 CaCl₂ and in this medium the intracellularly recorded response of the cell was virtually identical to the normal response obtained in vivo. All the effects of changing the [Ca²⁺]₀ could be reversed within 5 min by perfusing the eye with the standard medium.Changing the [Ca²⁺]₀ did not influence the frequency with which quantum bumps occurred or the resting membrane potential, but did lead to changes in the latency and amplitude of the response and, most significantly, in the repolarization time (t _r). The plot oft _r versus the [Ca²⁺]₀ revealed that the value oft _r changes significantly in two distinct regions representing a [Ca²⁺]₀ of between 2×10^–8 and 10^–7 mol/l and 10^–4 and 10^–2 mol/l, respectively. Lowering the [Ca²⁺]₀ did not affect the amplitude of the response but did lead to a drastic increase int _r which was accompanied by an increase in latency and peak time. Raising the [Ca²⁺]₀ led to a reduction in the duration and amplitude of the response. The latter effect is evidence of reduction in the sensitivity of the photoreceptor cell which is dependent on the [Ca²⁺]₀.It is postulated that two types of binding site for calcium exist, high affinity binding sites (HABS) and low affinity binding sites (LABS), which modulate the functioning of ion channels in the cell membrane that are activated as a consequence of light absorption. The results indicate that the sensitivity of the photoreceptor cell is determined by the degree of saturation of the LABS.