Stimulus-secretion coupling in rat pancreas: role of sodium, calcium and cyclic nucleotides studied by X-537A and BrX-537A.

1. The aim of the present work was to study the role of Na+, Ca2+ and cyclic nucleotides in stimulus-secretion coupling in rat pancreatic acinar cells utilizing Ca2+ specific ionophore A23187 as well as ionophores X-537A and BrX-537A which transport both monovalent and divalent cations across biological membranes. 2. X-537A, BrX-537A and A23187 increased amylase secretion by a direct energy dependent action without damaging the cells. Antimycin-A and dinitrophenol increased the spontaneous release of amylase. 3. In the absence of extracellular Ca2+, the secretory response to X-537A and BrX-537A was reduced by 57 and 50% respectively whereas A23187 was without effect. In the absence of extracellular Na+, both basal and ionophore stimulated secretion of amylase were increased. Replacement of Na+ by Li+ or Tris+ in Ca2+-free medium abolished the increase in amylase secretion. 4. X-537A increased bidirectional flux of 45Ca2+. Efflux of 45Ca2+ persisted in Ca2+-free media. In Tris-Ringer and Li-Ringer, basal 45Ca2+ influx and amylase secretion were increased. Moreover X-537A enhanced both 45Ca2+ efflux and amylase secretion. This effect was abolished in media containing no Na+ and Ca2+ suggesting that Na+ concentration gradient across the membrane is critical for digestive enzyme secretion. 5. Theophylline increased basal concentration of cyclic-GMP at 1, 2, 5, 15 and 30 min. X-537A stimulated cyclic-GMP synthesis in presence of theophylline with a peak effect observed at 15 min with X-537A. Incubation of pancreatic fragments in Ca2+-free media containing EGTA (5 x 10(-5) M) decreased basal and X-537A stimulated synthesis of cyclic-GMP. 6. From these data it is concluded that Na+ plays a direct role in Ca2+ mobilization and exocytosis. Like other tissues, cyclic-GMP synthesis is dependent on extracellular Ca2+ and does not seem to play a role in stimulus-secretion coupling.