Stimulation of Insulin Secretion from Isolated Rat Islets by SaRI 59–801: Relation to cAMP Concentration and Ca2+ Uptake

Abstract

The mechanism of stimulation of insulin release from isolated rat islets by 0.3 mM SaRI 59–801 (DL-α-dimethylaminomethyl-2-[3-ethyl-5-methyl-4-isoxazoyl]-1Hindole- 3-methanol) was investigated, considering cAMP concentration and Ca2+ uptake. Ten millimolar theophylline or 1 mM 1-methyl-3-isobutylxanthine, which inhibit cAMP phosphodiesterase, each greatly increased the stimulation of insulin release by 59–801. Forskolin (0.1 mM), an activator of adenylate cyclase, or 1 mM dibutyryl cAMP also potentiated 59–801, suggesting that 59–801 does not elevate islet cAMP but is potentiated by other compounds that do. Measurement of cAMP in islets by radioimmunoassay confirmed that it was not significantly elevated by 59–801 but was increased sevenfold by forskolin or 1-methyl-3-isobutylxanthine. SaRI 59–801 was not effective in the absence of Ca2+ and presence of 1 mM EGTA. Agents that block entry of Ca2+ into β-cells, verapamil, nifedipine, or CoCl2, inhibited the release of insulin in response to 59–801. Studies of 45Ca2+ uptake by isolated islets revealed an increased uptake in the presence of 59–801 and blockage of this effect by 50 μM verapamil. Thus, the stimulation of insulin secretion by 59–801 appears to involve a stimulation of Ca2+ uptake rather than an increase of cAMP concentration. The mechanism of stimulation of Ca2+ uptake by 59–801 requires further investigation.