Modulation of cytosolic free calcium levels by extracellular phosphate and lanthanum.

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Abstract
The effects of extracellular phosphate and lanthanum on cytosolic free Ca2+ [( Ca2+]i) levels were studied in isolated rat pancreatic acini. In the presence of 1.28 mM Ca2+ and 1.0 mM phosphate, the mean resting [Ca2+]i level was 120 nM. Omission of phosphate from incubation medium significantly lowered this value to 94 nM. The gastrointestinal hormone cholecystokinin octapeptide (CCK-8) rapidly enhanced both [Ca2+]i levels and 45Ca2+ efflux, irrespective of the presence or absence of phosphate. Lanthanum (0.1 mM), a compound known to block transmembrane Ca2+ fluxes, attenuated both actions of CCK-8, but only in the absence of extracellular phosphate. There was a concomitant decrease in amylase secretion induced by 0.1 nM CCK-8 but not by 10 nM CCK-8, without a significant change in cellular ATP levels. The inhibitory actions of lanthanum on CCK-8-stimulated [Ca2+]i levels were very rapid and were mimicked only by prolonged incubation of acini in Ca2+-free medium supplemented with EGTA. Omission of phosphate from incubation medium also lowered basal [Ca2+]i levels in IM-9 lymphocytes. These findings suggest that extracellular phosphate may modulate resting [Ca2+]i levels in pancreatic acini and other cell types and that mobilization of intracellular Ca2+ may partly depend on the availability of a lanthanum-sensitive pool of cell-surface Ca2+ that is not readily removed by EGTA.