DIFFERENCES IN INSULIN-SECRETION BETWEEN THE RAT AND MOUSE - ROLE OF CAMP

Although information regarding insulin secretion usually Is considered equivalent when generated in the mouse or the rat, it is established that the kinetics of insulin secretion fi:om mouse and rat pancreatic beta cells differ. The mechanisms underlining these differences are not understood. The in vitro perfused pancreas and isolated islets of the mouse or rat were employed in this study to investigate the role of cyclic adenosine monophosphate (cAMP), a major positive modulator of beta-cell function, as one differentiating signal for the uniquely different insulin release from the beta cells of these commonly used rodents. Glucose-stimulated first-phase insulin release from the perfused pancreas of the rat was higher than the mouse when calculated per gram of pancreas or as fractional secretion, but this phase was identical in the two species when results were adjusted for total body weight. Whether related to insulin content, pancreatic weight or body weight, the rat pancreas responded to glucose with a progressively increasing second-phase insulin release compared to the mouse pancreas, which secreted a flat second-phase of lesser magnitude. Isolated islets from rat and mouse were comparable in insulin content whereas the basal cAMP level of mouse islets was less than half that of the rat. At submaximal stimulation with glucose or glucose + IBMX or forskolin, mouse islets exhibited lower cAMP levels to a given stimulus than the rat. In rat islets cAMP levels increased to approximately 1000 fmol per islet, although insulin secretion maximized by 100-150 fmol. Insulin release at the same 100-150 fmol cAMP per mouse islet was one-third that of the rat and secretion had not maximized in mouse islets at 800 fmol. Despite their similar insulin contents, mouse islets consistently secreted less insulin for a given level of cAMP per islet than the rat. The lower capacity of mouse islets to achieve comparable cAMP levels was not the result of increased catabolic rate because the ''half-time'' disappearance of islet cAMP after a stimulus was similar (similar to 1 min) for both species. It is concluded that, compared to the mouse, beta cells of the rat pancreas elicit a more pronounced second-phase insulin secretion that is due, at least in part, to a greater production of, and sensitivity to, cAMP.