SUBCELLULAR GRADIENTS OF INTRACELLULAR FREE CALCIUM-CONCENTRATION IN ISOLATED LACRIMAL ACINAR-CELLS

The spatial distribution of intracellular free calcium concentration ([Ca2+]i) was measured in small clusters of isolated rat lacrimal acinar cells by imaging the fluorescence of the Ca2+-sensitive dye fura-2. In the absence of extracellular Ca2+, stimulation with acetyl choline (ACh) caused an increase in [Ca2+]i, due to release of intracellular Ca2+ Stores, which was maximal at the luminal pole of the cell. In contrast, the organellar Ca2+-ATPase inhibitor 2,5-di(tert-butyl)-hydroquinone caused an increase in [Ca2+]i, which was most marked in the basolateral region of the cell. When the cells were stimulated with ACh in a medium containing Ca2+, the gradients of [Ca2+]i (with [Ca2+]i most elevated at the luminal pole) were maintained for the duration of agonist stimulation. The possible implications of these results concerning the location and identity of intracellular Ca2+ stores, and the location of the sites that underlie agonist-stimulated Ca2+ influx, are considered. In particular, it seems likely that intracellular inositol-1,4,5-trisphosphate (InsP3) binding sites may be concentrated in the luminal region of the cell. It is not clear, however, whether this implies that there is a distinct luminally located InsP3-sensitive organelle.