CALCIUM-RELEASE BY CHOLECYSTOKININ ANALOG OPE IS IP3 DEPENDENT IN SINGLE-RAT PANCREATIC ACINAR-CELLS

Cholecystokinin (CCK) and carbachol raise intracellular Ca2+ concentration ([Ca2+](i)) in pancreatic acinar cells by elevating inositol 1,4,5-trisphosphate (IP3). CCK analogues JMV-180 and OPE stimulate fully efficacious enzyme secretion and [Ca2+](i) oscillations but release Ca2+ from intracellular stores by apparently IP3-independent mechanisms in permeabilized acinar cells. In the present study, we investigated whether OPE mobilizes Ca2+ from IP3-sensitive Ca2+ stores and whether IP3 mediates such responses in single intact cells. OPE and JMV-180 similarly elevated IP3 to low levels compared with those elicited by 10 nM CCK. Depletion of IP3-sensitive stores by elevation of intracellular IP3 using carbachol, microinjection of a nonmetabolizable IP3 analogue, or exposure to thapsigargin, in the absence of extracellular Ca2+, depleted the same Ca2+ stores that were sensitive to OPE. In converse experiments, OPE depleted carbachol- or thapsigargin-sensitive stores, indicating that carbachol-, thapsigargin-, IP3-, and OPE-sensitive Ca2+ stores overlap completely and that stores mobilized by OPE are IP3 sensitive. To determine whether IP3 mediates responses to OPE, cells were microinjected with low-molecular-weight heparin, a competitive antagonist of IP3 binding to the IP3 receptor. Heparin competitively inhibited the rise of [Ca2+](i) in response to carbachol, OPE, or JMV-180, whereas de-N-sulfated heparin, an inactive heparin, was without effect. These results indicate that CCK analogues release Ca2+ from IP3-sensitive Ca2+ stores by mechanisms involving the IP3 receptor.