TY - JOUR T1 - INDO-1 Measurements of Absolute Resting and Light-Induced Ca<sup>2+</sup> Concentration in <strong><em>Drosophila</em></strong>Photoreceptors JF - The Journal of Neuroscience JO - J. Neurosci. SP - 2924 LP - 2933 DO - 10.1523/JNEUROSCI.16-09-02924.1996 VL - 16 IS - 9 AU - Roger C. Hardie Y1 - 1996/05/01 UR - http://www.jneurosci.org/content/16/9/2924.abstract N2 - Absolute Ca2+ levels in dissociatedDrosophila photoreceptors were measured using the ratiometric indicator dye INDO-1 loaded via patch pipettes, which simultaneously recorded whole-cell currents. In wild-type photoreceptors, the ultraviolet (UV) excitation light used to measure fluorescence elicited a massive Ca2+ influx that saturated the dye (&gt;10 μmCa2+), but lagged the electrical response by 2.8 msec. Resting Ca2+ levels in the dark, measured during the latent period before the response, averaged 160 nm in normal Ringer’s (1.5 mm Ca2+). Ca2+ increases in response to weak illumination were estimated (1) by using a weak adapting stimulus before the UV excitation light and measuring Ca2+ during the latent period; and (2) by using ninaE mutants with greatly reduced rhodopsin levels. Ca2+ rose linearly as a function of the time integral of the light-sensitive current with a slope of 2.7 nm/pC. In the transient receptor potential (trp) mutant, which lacks a putative light-sensitive channel subunit, the slope was only 1.1 nm/pC, indicating a 2.5-fold reduction in the fractional Ca2+ current. From these data, it can also be estimated that &gt;99% of the Ca2+ influx is effectively buffered by the cell. In Ca2+-free Ringer’s, resting cytosolic Ca2+ was reduced (to 30–70 nm), but contrary to previous reports, significant light-induced increases (∼250 nm) could be elicited. This rise was reduced to &lt;20 nm when extracellular Na+was replaced withN-methyl-d-glucamine, suggesting that it could be attributed to Na+ influx altering the Na/Ca exchanger equilibrium. It is concluded that any light-induced release from internal stores amounts to &lt;20 nm. ER -