Effect of 12-O-tetradecanoyl-phorbol-13-acetate on Ca2+ efflux and protein discharge in pancreatic acini.

When pancreatic acini are exposed to the peptide caerulein (an analogue of the hormone cholecystokinin) or the cholinergic agonist carbamylcholine, they exhibit a rapid release of intracellular Ca2+ and a rise in the level of cGMP, accompanied by discharge of secretory proteins. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulates protein discharge without affecting the level of cGMP, while N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) elevates cGMP, but does not elicit discharge (Gunther, G. R., and Jamieson, J. D. (1979) Nature 280, 318-320). To clarify the relationships among these events, the effect of tPA and MNNG on Ca2+ efflux in guinea pig acini was determined. Cells were incubated in medium containing 45Ca2+ and transferred to medium with nonradioactive Ca2+ before adding the agents to be tested. Caerulein (10(-9) M), carbamylcholine (10(-5) M), and the Ca2+ ionophore A23187 (5 micrograms/ml) all caused the release of 40-60% of the cell-associated 45Ca2+ within 3 min. In contrast, acini incubated with 10(-7) M TPA, 10(-4) M MNNG, or no added agents retained over 80% of their 45Ca2+ after 22 min. TPA caused significant secretory discharge within the first 15 min and did not block CA2+ efflux stimulated by other secretagogues. TPA-induced discharge requires energy and the presence of sufficient Ca2+ in the extracellular medium. These observations are consistent with a model in which the rapid release of Ca2+ into the cytoplasm triggers an increase in the level of cGMP and, by a separate series of events, protein discharge. TPA may act at a later point on the pathway to discharge, but before an obligatory step requiring Ca2+.