EFFECT OF INHIBITION OF MICROSOMAL CA2-ATPASE ON CYTOPLASMIC CALCIUM AND ENZYME-SECRETION IN PANCREATIC ACINI()

We used thapsigargin (TG), 2,5-di-tert-butyl-l,4-benzohydroquinone (BH Q) and cyclopiazonic acid (CPA), e ach of which inhibits microsomal Ca 2+-ATPase, to evaluate the effects of this inhibition on cytoplasmic f ree calcium ([Ca2+](i)) and secretagogue-stimulated enzyme secretion i n rat pancreatic acini. Using single-cell microspectrofluorimetry of f ura-2-loaded acini we found that all three agents caused a sustained i ncrease in [Ca2+](i) by mobilizing calcium from inositol-(l,4,5)-trisp hosphate-sensitive intracellular calcium stores and by promoting influ x of extracellular calcium. Concentrations of all three agents that in creased [Ca2+](i) potentiated the stimulation of enzyme secretion caus ed by secretagogues that activate adenylate cyclase but inhibited the stimulation of enzyme secretion caused by secretagogues that activate phospholipase C. With BHQ, potentiation of adenylate cyclase-mediated enzyme secretion occurred immediately whereas inhibition of phospholip ase C-mediated enzyme secretion occurred only after several min of inc ubation. In addition, the effects of BHQ and CPA on both [Ca2+](i) and secretagogue-stimulated enzyme secretion were reversed completely by washing whereas the actions of TG could not be reversed by washing. Co ncentrations of BHQ in excess of those that caused maximal changes in [Ca2+](i) inhibited all modes of stimulated enzyme secretion by a mech anism that was apparently unrelated to changes in [Ca2+](i). Finally, in contrast to the findings with TG and BHQ, CPA inhibited bombesin-st imulated enzyme secretion over a range of concentrations that was at l east 10-fold lower than the range of concentrations over which CPA pot entiated VIP-stimulated enzyme secretion.