INDUCTION OF CA2-MEDIATED, DEPLETION OF INOSITOL-TRISPHOSPHATE-SENSITIVE CA2+ STORES IN RABBIT PANCREATIC ACINAR-CELLS( OSCILLATIONS BY SELECTIVE, U73122)

The effect of the putative inhibitor of phospholipase C activity, U731 22, on the Ca2+ sequestering and releasing properties of internal Ca2 stores was studied in both permeabilized and intact rabbit pancreatic acinar cells. U73122 dose dependently inhibited ATP-dependent Ca2+ up take in the inositol (1,4,5)-trisphosphate-[Ins(1,4,5)P-3]-sensitive, but not the Ins(1,4,5)P-3-insensitive, Ca2+ store in acinar cells perm eabilized by saponin treatment. In a suspension of intact acinar cells , loaded with the fluorescent Ca2+ indicator, Fura-2, U73122 alone evo ked a transient increase in average free cytosolic Ca2+ concentration ([Ca2+](i,av)), which was largely independent of external Ca2+. Additi on of U73122 to cell suspensions prestimulated with either cholecystok inin octapeptide or JMV-180 revealed an inverse relationship in size b etween the U73122- and the agonist-evoked [Ca2+](i,av) transient. More over, thapsigargin-induced inhibition of intracellular Ca2+-ATPase act ivity resulted in a [Ca2+](i,av) transient, the size of which was not different following maximal prestimulation with either U73122 or agoni st. These observations suggest that U73122 selectively affects the Ins (1,4,5)P-3- casu quo agonist-sensitive internal Ca2+ store, whereas th apsigargin affects both the Ins(1,4,5)P-3-sensitive and -insensitive C a2+ store. Digital-imaging microscopy of Fura-2-loaded acinar cells de monstrated that U73122, in contrast to thapsigargin, evoked sustained oscillatory changes in [Ca2+](i). The U73122-evoked oscillations were abolished in the absence of external Ca2+. The ability of U73122 to ge nerate external Ca2+-dependent Ca2+ oscillations suggests that depleti on of the agonist-sensitive store leads to an increase in Ca2+ permeab ility of the plasma membrane and that the Ins(1,4,5)P-3-insensitive Ca 2+ pool is necessary for the Ca2+ oscillations.