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Stimulation of enzyme secretion in the pancreas on injection of a single dose of the cholinergic drug, pilocarpine, was associated with an increased incorporation of 2-(3)Hmyoinositol into a lipid, which was previously characterized as phosphatidylinositol. Stimulation of enzyme secretion by hourly injection of the pancreozymin congener, caerulein, led to more increased phosphatidylinositol synthesis than with a single injection of pilocarpine. The amylase level of the pancreas remained at a low level as long as caerulein was injected, indicating continued stimulation of enzyme secretion even though increased phosphatidylinositol synthesis ceased after 6 h. Feeding gave the same stimulation of phosphatidylinositol synthesis as caerulein. The major synthesis of phosphatidylinositol in controls and the stimulation of phosphatidylinositol synthesis by pilocarpine was entirely confined to the microsome fraction throughout the experiments (up to 18 h). This shows that there is no flow of microsomal membrane (smooth- or rough-surfaced endoplasmic reticulum) to other membranous structures throughout the secretory cycle and beyond. It is concluded that the stimulation of phosphatidylinositol synthesis by pancreatic secretagogues is confined to microsomal elements and does not play any role in membrane flow.
Gerber et al. (1973) studied this question.