Activation of Ca2+ entry into acinar cells by a non-phosphorylatable inositol trisphosphate

Abstract

IN many cell types, receptor activation of phosphoinositidase C results in an initial release of intracellular Ca²⁺ stores followed by sustained Ca²⁺ entry across the plasma membrane. Inositol 1,4,5-trisphosphate is the mediator of the initial Ca²⁺ release¹, although its role in the mechanism underlying Ca²⁺ entry remains controversial^1–6. We have now used two techniques to introduce inositol phosphates into mouse lacrimal acinar cells and measure their effects on Ca²⁺ entry: microinjection into cells loaded with Fura-2, a fluorescent dye which allows the measurement of intra-cellular free calcium concentration by microspectrofluorimetry, and perfusion of patch clamp pipettes in the whole-cell configuration while monitoring the activity of Ca²⁺-activated K⁺ channels as an indicator of intracellular Ca²⁺. We report here that inositol 1,4,5-trisphosphate serves as a signal that is both necessary and sufficient for receptor activation of Ca²⁺ entry across the plasma membrane in these cells.