Effect of phorbol esters on glucagon secretion from a glucagon-secreting clonal cell line. Synergistic effects of A23187 and theophylline

The cell line In-R1-G9 is one of the clones from the hamster insulinoma cell line, In-111-R1, and it produces glucagon. Phorbol esters markedly enhanced glucagon secretion and the stimulatory effect was found to be correlated to their biological activity as tumor promoters. At a concentration of 200 nM, 12-O-tetradecanoylphorbol 13-acetate (TPA) stimulated glucagon secretion 13-fold more than the control in 10 min. The effect of TPA was not influenced by actinomycin D, cycloheximide, colchicine or vincristine. Depletion of calcium from the incubation medium inhibited TPA-induced glucagon secretion by approximately 50% and dibucaine also suppressed glucagon secretion to 67.4%. An addition of A23187 to TPA induced 150% enhancement over the TPA-stimulated glucagon level, and the maximum secretory response was observed when the cells were stimulated with the simultaneous addition of TPA, A23187 and theophylline.