A QUANTITATIVE ESTIMATE OF THE MAXIMUM AMOUNT OF LIGHT-INDUCED CA2+ RELEASE IN DROSOPHILA PHOTORECEPTORS

Simultaneous measurements of the light-induced current (LIC) and cytos olic Ca2+ (using INDO-1) were made in Drosophila photoreceptors. In th e presence of 1.5 mM Ca-o(2+), the UV light used to measure INDO-1 flu orescence saturated the LIC and induced a large Ca2+ rise. In the abse nce of extracellular Ca2+ and with Na+ replaced by N-methyl-D-glucamin e, the light-induced Ca2+ rise was virtually abolished. A residual ris e of about 20 nM is regarded as an upper estimate of Ca2+ released fro m internal stores. To estimate the Ca2+ flux required to generate such a rise, Ca2+ influx signals in response to weak light steps (500 ms L ED stimulus) were measured in the presence of external Ca2+. The relat ionship between [Ca-in] and the total charge carried during the LIC ha d a slope of 2.7 nM pC(-1). Assuming that 50% of the LIC is carried by Ca2+ and that the single-channel Ca2+ 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 Ca2+ rise of 20 nM within 5 00 ms. By contrast, the current activated by the UV measuring light wa s equivalent to the activation of at least 5000 quantum bumps, making it unlikely that InsP(3)-induced Ca2+ release could have been the caus al event for excitation under these conditions.