A comparison of the effects of glucose and acetylcholine on insulin release and intermediary metabolism in rat pancreatic islets.

In order to help clarify the relationship between hormone release and metabolism in pancreatic islets the secretory and metabolic effects of a fuel and non-fuel stimulant of insulin release were compared. Temporal changes in the levels of glucose-6-P, fructose-1,6-Pz plus triose-P, NADH, Pi, V-AMP, and CAMP were determined in islets of the isolated perfused rat pancreas which was switched from 4 or 4.4 mM glucose to 16 mM glucose or to 1 pM acetylcholine plus 4.4 mM glucose, respectively. Insulin levels in the perfusion effluent were determined concomitantly. Slices of individual islets were microdissected from lyophilized pancreas sections sampled with quickfreezing methods. Intermediates and cofactors were measured with enzymatic fluorometric procedures combined with an oil well method and enzymatic cycling of pyridine nucleotides. Insulin was measured with a radioimmunoassay. Within 30 s after islets were switched to 16 mM glucose, there were increases of insulin release and islet levels of glucose-6-P and possibly NADH. A decrease of Pi and 5’-AMP content was also obvious at this time. Stimulation with 16 mM glucose for longer periods resulted in increased islet content of glucose-6-P, fructose-1,6-P2 plus triose-P, NADH, and decreased content of Pi and Y-AMP. Stimulation of islets with 1 pM acetylcholine plus 4.4 mM glucose resulted in an insulin release profile similar to that observed with 16 mM glucose. With this stimulant there were increases in the islet content of fructose-1,6-P2 plus triose-P, NADH, Pi, 5’-AMP, and CAMP, but decreased content of glucose-6-P. These data are consistent with a concept that glycolysis is activated with either stimulant. The molecular mechanism for activating glycolysis is probably different for each type of stimulant. Since the two stimuli induce similar insulin release profiles but result in different level changes of metabolites and cofactors, a common basis for stimulus secretion coupling is sought elsewhere. It is suggested that both activate secretion by a mechanism which involves an increase in intracellular Ca2+ levels.