Differential effects of microtubule-altering agents on beta-cells during development

The effect of microtubule-altering agents on the insulin secretory response to glucose during the perinatal period was investigated with an in vitro perifusion system. Rat pancreatic mince from day 17 of gestation (D17G) to day 6 postnatally (D6PN) were perifused for 60 min in basal glucose followed by 45 min with high glucose (3.5 mg/ml) or with high glucose plus 10 mM arginine (D17G). The two phases of insulin secretion in response to high glucose developed in an age-dependent and asynchronous manner. The first phase matured between D17G and D18G, and maturation of the second phase occurred subsequently. Vinblastine (VB) (20 or 100 microM) had a differential effect on the insulin secretory response. VB did not inhibit stimulated insulin release at D17G. This absence of an inhibitory effect of VB at D17G could not be explained by the absence of polymerized tubulin because microtubules were present in the control beta-cells and, in addition, VB treatment resulted in the formation of paracrystalline deposits. Subsequently in development, and with isolated islets of the adult, VB inhibited stimulated insulin release. Heavy water (deuterium oxide, D2O) inhibited stimulated insulin secretion at D17G but blocked completely insulin release from the near-term beta-cell. The inhibition of insulin secretion by D2O was rapidly reversed when water replaced D2O in the perifusion media. The results indicate that the maturation of the second phase of insulin secretion coincides with the ability of the microtubule-altering agents to modify the insulin secretory response. One possible explanation for these findings is that at D17G the microtubules are not coupled physicochemically to other molecules or structures necessary for their role in insulin secretion to be expressed fully.