[A study on the possible mechanism for the second control site of insulin secretion in islets]

Glucose stimulation of insulin release involves closure of ATP-sensitive K+ channels (K(+)-ATP channels) in beta cells. However, by using diazoxide to open K(+)-ATP channels, it has been reported that another mechanism exists, by which glucose can control insulin release independently from changes in K(+)-ATP channel activity. These data suggested that there is a second control site in islets when insulin is secreted. To probe the possible existence of this site, mouse islets were used to investigate the energy state of islets in insulin secretion when the activity of K(+)-ATP channels was eliminated by diazoxide. In the present study, isolated islets were cultured in PRMI 1640 medium for 18 hours. The islets were then divided into 6 groups randomly, with 10 islets per group, placed in 1 ml KREBS medium containing 30 mmol/L K(+) and 250 mmol/L diazoxide with 0, 3, 6, 10, 15, 20 mmol/L glucose respectively and incubated in 37 degrees C water for 60 minutes. A portion of the supernatant was withdrawn at the end of the incubation for insulin assay, the islets were treated with trichloracetic acid and diethyl ether for the measurements of ATP, ADP, GTP and UTP. The above findings demonstrated that insulin secretion increases with the rise of glucose concentration. In the same time, the level of ATP increaseded and the level of ADP decreased gradully. The ratio of ATP/ADP in islets correlated with the insulin secretion very well. The level of GTP and UTP also increased with the rise of glucose concentration. It is suggested that glucose can control insulin secretion independently from its action of K(+)-ATP channels and the existence of a second control site as an another mechanism for insulin release. The ratio of ATP/ADP is an important factor in this mechanism; GTP and UTP also participated in the control of insulin secretion.