Stimulatory effects of bilirubin on amylase release from isolated rat pancreatic acini

Considered to be an etiologic factor of acute pancreatitis, hypersecretion of pancreatic juice and digestive enzymes is often associated with hyperbilirubinemia. We explored the intracellular mechanisms through which bilirubin affects pancreatic exocrine secretory function by examining the effect of bilirubin on isolated rat pancreatic acini. Bilirubin stimulated amylase release in a concentration- and time-dependent manner, significantly increasing amylase release at concentrations >5 mg/100 ml and after 15 min of incubation. Coincubation of bilirubin with vasoactive intestinal polypeptide, 8-bromo-cAMP, or A-23187 had a synergistic effect on amylase release, whereas coincubation with CCK-8, carbamylcholine, or 12- O -tetradecanoylphorbol 13-acetate had an additive effect. Bilirubin did not affect acinar cAMP content or Ca2+efflux. Intracellular Ca2+ pool depletion had no influence on bilirubin-evoked amylase release. The protein kinase C (PKC) inhibitors staurosporine and calphostin C partially but significantly inhibited bilirubin-stimulated amylase release, whereas the PKA inhibitor H-89 did not. The tyrosine kinase (TK) inhibitor genistein, phospholipase A2 (PLA2) inhibitor indoxam, and PLC inhibitor U-73122 also inhibited amylase release. Bilirubin significantly translocated PKC activity from the cytosol to the membrane fraction and activated TK in cytosol and membrane fractions. These results indicate that bilirubin stimulates amylase release by activating PKC and TK in rat pancreatic acini and that PLC and PLA2 partly mediate this process.