The subcellular localization of the incorporation of 2-(3H)-myoinositol into Upids has been studied in isolated pancreatic islets of the rat. The recovery of lipid-bound myoinositol increased with time in the nuclear, mitochondriai, microsomal, and secretory granule fractions. The utilization of a nitration technique for the more complete separation of mitochondriai and secretory granule elements permitted us to show that the recovery of lipld-bound 2-(3H)-myoinositol increased most rapidly in the secretory granule fraction. A 30-minute exposure of prelabeled islets to a stimulatory concentration of D-glucose (3.0 mg./ml.) resulted in a statistically significant decrease in the amount of lipid-bound 2-(3H)-myoinositol that was recovered from the secretory granule fraction (p < 0.001). In contrast, exposure of islets to the elevated glucose concentration had no statistically significant effect on the recovery of Upid-bound radioactivity from other subcellular fractions. Since the majority of lipid-bound radioactivity associated with the secretory granule fraction could be recovered with the presumptive secretory granule membranes, these data suggest that the hydrolysis of phosphatidylinositol that accompanies glucose-induced insulin secretion from the rat pancreatic islet may be localized to the beta granule and, in particular, to it's limiting membrane.

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