A quantitative estimate of the maximum amount of light-induced Ca 2+ release in Drosophila photoreceptors

Simultaneous measurements of the light-induced current (LIC) and cytosolic Ca 2+ (using INDO-1) were made in Drosophila photoreceptors. In the presence of 1.5 mM Ca 0 2+, the UV light used to measure INDO-1 fluorescence saturated the LIC and induced a large Ca 2+ rise. In the absence of extracellular Ca 2+ and with Na + replaced by N-methyl- d -glucamine, the light-induced Ca 2+ rise was virtually abolished. A residual rise of about 20 nM is regarded as an upper estimate of Ca 2+ released from internal stores. To estimate the Ca 2+ flux required to generate such a rise, Ca 2+ influx signals in response to weak light steps (500 ms LED stimulus) were measured in the presence of external Ca 2+. The relationship between [Ca in ] and the total charge carried during the LIC had a slope of 2.7 nM pC −1. Assuming that 50% of the LIC is carried by Ca 2+ and that the single-channel Ca 2+ current carried by the InsP 3 receptor is 0.04 pA, it was estimated that about 350 InsP 3 receptors should have been sufficient to generate a Ca 2+ rise of 20 nM within 500 ms. By contrast, the current activated by the UV measuring light was equivalent to the activation of at least 5000 quantum bumps, making it unlikely that InsP 3 -induced Ca 2+ release could have been the causal event for excitation under these conditions.