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

The effects of vanadate on exocrine pancreatic function were examined in isolated rat pancreatic acini. Vanadate caused a concentration-dependent stimulation of amylase release above a concentration of 1 mM. Co-incubation of vanadate with vasoactive intestinal polypeptide, 8-bromoadenosine 3':5'-cyclic monophosphate, and the Ca2+ ionophore A23187 produced a synergistic pattern of amylase release, whereas co-incubation with cholecystokinin octapeptide (CCK-8), carbamylcholine, and 12-O-tetradecanoylphorbol 13-acetate produced an additive effect. Vanadate alone had no influence on acinar cyclic AMP content, Ca2+ efflux, or intracellular Ca2+ concentration. However, preincubation with vanadate prevented the plateau phase of CCK-8-induced Ca2+ transient increase from returning to baseline. Moreover, depletion of the intracellular Ca2+ pool by pretreatment of acini with CCK-8 in Ca2+-free medium (plus ethyleneglycol bis[beta-aminoethylether]-N,N'-tetraacetic acid) had no effect on subsequent stimulation by vanadate, although it abolished the response to both CCK-8 and carbamylcholine stimulation. The protein kinase C (PKC) inhibitors staurosporine and calphostin C significantly inhibited vanadate-stimulated amylase release, whereas the protein tyrosine kinase inhibitor genistein had no inhibitory effect. Moreover, vanadate caused a significant translocation of PKC from cytosol to membrane fraction in pancreatic acinar cells. This translocation was inhibited significantly by staurosporine and calphostin C but not by genistein. These results suggest that vanadate acts directly on pancreatic acini and stimulates amylase release by activating PKC without an effect on Ca2+ mobilization, cyclic AMP, or protein tyrosine kinase.