Calcium imaging demonstrates colocalization of calcium influx and extrusion in fly photoreceptors

During illumination. Ca2+ enters fly photoreceptor cells through light-acti vated channels that are located in the rhabdomere, the compartment speciali zed for phototransduction. From the rhabdomere. Ca2+ diffuses into the cell body. We visualize this process by rapidly imaging the fluorescence in a c ross section of a photoreceptor cell injected with a fluorescent Ca2+ indic ator in vivo. The free Ca2+ concentration in the rhabdomere shows a very fa st and large transient shortly after light onset. The free Ca2+ concentrati on in the cell body rises more slowly and displays a much smaller transient . After approximate to 400 ms of light stimulation, the Ca2+ concentration in both compartments reaches a steady state, indicating that thereafter an amount of Ca2+. equivalent to the amount of Ca2+ flowing into the cell, is extruded. Quantitative analysis demonstrates that during the steady state, the free Ca2+ concentration in the rhabdomere and throughout the cell body is the same. This shows that Ca2+ extrusion takes place very close to the l ocation of Ca2+ influx, the rhabdomere. because otherwise gradients in the steady-state distribution of Ca2+ should be measured. The close colocalizat ion of Ca2+ influx and Ca2+ extrusion ensures that, after turning off the l ight, Ca2+ removal from the rhabdomere is faster than from the cell body. T his is functionally significant because it ensures rapid dark adaptation.