Lipoxygenase inhibitors and cyclic AMP-mediated insulin secretion caused by forskolin, theophylline and dibutyryl cyclic AMP.

Forskolin caused a marked and a concentration-dependent elevation of cyclic AMP content in isolated pancreatic islets (EC50, 10 microM). Cyclic AMP level reached a plateau within 30 min after the addition of 10 microM forskolin. In a low glucose (3.3 mM) medium, forskolin induced slight but significant insulin secretion in a concentration-dependent manner (EC50, 0.3 microM). When the glucose concentration was increased to 5.5 mM, marked enhancement of insulin secretion was observed with forskolin (EC50, 0.5 microM). Lipoxygenase inhibitors, such as nordihydroguaiaretic acid, 3-amino-1-(trifluoromethylphenyl)-2-pyrazoline and 1-phenyl-3-pyrazolidinone failed to affect the forskolin-stimulated cyclic AMP generation. The selective cyclooxygenase inhibitor indomethacin also had no influence on forskolin-stimulated cyclic AMP generation. Insulinotropic effects of forskolin, however, were suppressed by these lipoxygenase inhibitors but not by indomethacin. Both nordihydroguaiaretic acid and 1-phenyl-3-pyrazolidinone also prevented the insulinotropic effects of theophylline and dibutyryl cyclic AMP, whereas indomethacin failed to inhibit them. It seems conceivable that a lipoxygenase product(s) is involved in the insulin secretory process distal to cyclic AMP generation, or that alternatively a lipoxygenase product(s) is permissively involved in the insulin secretory process independently from the cyclic AMP-mediated process.