High uptake of myo-inositol by rat pancreatic tissue in vitro stimulates secretion.

A recent study by Hokin-Neaverson, M., Sadeghian, K., Majumder, A.L., and Eisenberg, F. (1975) Biochem. Biophys. Res. Commun. 67, 1537-1544, demonstrates that free myo-inositol in the pancreas is significantly increased during intense cholinergic stimulation of secretion. Incubation of rat pancreatic tissue in medium with 100 mM myo-inositol increases 10-fold the endogenous content of free myo-inositol and elicits a prompt and sustained 50% increase in the rate of release of amylase activity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals that the electrophoretic pattern of the protein mixture released in the presence of 100 mM myo-inositol is the same as that of the secretory output released in the presence of 10 muM carbamylcholine. Microscopic examination of tissue pieces indicates that there is no significant decrease in zymogen granule content of the pancreatic acinar cells during incubation in medium with 100 mM myo-inositol. Jamieson, J.D., and Palade, G.E. (1967) J. Cell Biol. 34, 597-615, have shown that pulse-labeled secretory proteins in guinea pig pancreas first appear in zymogen granules 1 hour postpulse, becoming maximally accumulated in these storage sites by 2 hours postpulse. myo-Inositol (100 mM) stimulates release of pulse-labeled secretory proteins only if incubation in medium with 100 mM myo-inositol is initiated anytime during the first 80 min postpulse. The findings thus indicate that a high uptake of myo-inositol by rat pancreatic tissue in vitro selectively stimulates the release of just those secretory proteins being packaged in newly forming zymogen granules.