Are 5-hydroxytryptamine-preloaded beta-cells an appropriate physiologic model system for establishing that insulin stimulates insulin secretion?

The release and oxidation of 5-hydroxytryptamine from 5-hydroxytryptamine-preloaded beta-cells has been used as a surrogate marker for insulin secretion. Findings made using this methodology have been used to support the concept that insulin stimulates its own release. In the present studies, the effects of 5-hydroxytryptamine on stimulated insulin secretion from isolated perifused rat islets was determined. When added together with stimulatory glucose, 5-hydroxytryptamine (0.5 mm) significantly reduced both phases of 8 mm glucose-induced secretion and reduced the first phase of 15 mm glucose-induced release by 60% without any effect on sustained insulin release rates. Preloading of beta-cells with 0.5 mm 5-hydroxytryptamine for 3 h resulted in a more severe impairment of 15 mm glucose-induced secretion. First and second phase release rates were reduced by 70 and 55%, respectively. In addition, this pretreatment protocol also abolished 200 microm tolbutamide-induced insulin secretion from perifused islets. These findings confirm that 5-hydroxytryptamine is a powerful inhibitor of stimulated insulin secretion. The responses of 5-hydroxytryptamine-preloaded beta-cells may not accurately reflect the biochemical events occurring during the physiologic regulation of insulin secretion. The suggestion that insulin stimulates its own secretion based exclusively on amperometric measurements should be reconsidered.