Perifusion of Isolated Rat Islets in Vitro: Participation of the Microtubular System in the Biphasic Release of Insulin

A simple perifusion system for in vitro studies on the rate of insulin secretion of isolated rat islets is described. A biphasic pattern of insulin secretion was produced by glucose whereas tolbutamide stimulated only the first phase of insulin secretion. The perifusion system was used to determine the effect of anti-mitotic agents on the biphasic pattern of insulin secretion. Yinblastine and colchicine destroy microtubules whereas deuterium oxide (D2O) produces stabilization and interference with the function of microtubules. Vinblastine (10-4 M) and D2O (100 per cent) inhibited completely the first and second phase of glucose-induced insulin secretion. The inhibitory effect of D2O was reversible and a biphasic pattern of secretion occurred following the replacement of D2O with water. Colchicine (10-3 M) produced significant inhibition of only the second phase of glucose-induced insulin secretion. The first phase of tolbutamide-induced insulin secretion was inhibited by vinblastine and D2O and the inhibitory effect of D2O was reversible. Colchicine did not inhibit the first phase of tolbutamide-induced insulin release. These findings indicate that the microtubular system is involved in both phases of insulin secretion. The first phase of secretion may be due to the release of beta granules already associated with the microtubular system and the second phase could be the result of stored and newlysynthesized granules becoming associated with the system.