Calcium efflux across the plasma membrane of rat parotid acinar cells is unaffected by receptor activation or by the microsomal calcium ATPase inhibitor, thapsigargin

The rate of Ca2+ extrusion across the plasma membrane of rat parotid acinar cells was determined by measuring the decay of the intracellular calcium concentration, Ca2+]i, following the addition of EGTA to agonist stimulated cells. In the presence of extracellular Ca2+, the muscarinic cholinergic receptor agonist, methacholine, rapidly increased Ca2+]i (peaking within 5 s), which then decreased to a higher steady state level. This elevated steady state level was dependent on extracellular Ca2+ concentration. Likewise, thapsigargin, a non-phorbol ester tumor promoter that does not increase inositol phosphates, gradually increased Ca2+]i, peaking within 1 min and then declining to a new elevated plateau level which was also dependent on extracellular Ca2+. Ca2+]i, elevated by methacholine or thapsigargin, was rapidly decreased by the addition of EGTA by a process the kinetics of which depended on the value of Ca2+]i before the addition of EGTA. That is, Ca2+]i increased as a function of the extracellular Ca2+ concentration and also the apparent half-time for Ca2+ extrusion following the addition of EGTA to cells was increased as the Ca2+]i increased. This presumably reflects the saturable nature of the Ca2+ extrusion mechanism. The steady state Ca2+]i in cells stimulated with methacholine or thapsigargin in nominally Ca2+ free medium was similar to the steady state Ca2+]i in unstimulated cells in normal, Ca2(+)-containing medium. Under these similar Ca2+]i conditions, stimulated and unstimulated cells showed a similar time course of decay upon addition of EGTA. In addition, neither methacholine nor phorbol myristate acetate decreased the sustained elevation of Ca2+]i induced by ionomycin.(ABSTRACT TRUNCATED AT 250 WORDS)