Choline turnover in phosphatidylcholine of pancreatic islets. Implications for CDP-choline pathway

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Abstract
The CDP-choline pathway is the major route of phosphatidylcholine (PC) biosynthesis in mammalian cells. The incorporation of [14C]choline into PC of isolated pancreatic islets of the rat was time dependent, glucose stimulable, and inhibited by mannoheptulose. Removal of extracellular Ca2+ enhanced glucose-stimulated choline incorporation without affecting basal levels. Glucose stimulated PC synthesis in islets labeled to equilibrium with 32PO4 in the presence or absence of extracellular Ca2+. The water-soluble intermediates of the CDP-choline pathway, phosphorylcholine and CDP-choline, accumulated to a lesser extent under Ca2+-free conditions; however, glucose enhanced the levels of these intermediates in the presence and absence of Ca2+. Thus, glucose stimulates CDP-choline-pathway activity. Ca2+-free conditions may promote flux of choline intermediates through the pathway and retard the hydrolysis of PC. The phospholipase A2-activating agents Δ-9-tetrahydrocannabinol and melittin enhanced [3H]choline incorporation into PC and potentiated incorporation in response to a submaximal secretagogic concentration of glucose (8.5 mM); insulin release paralleled the changes in PC. p-Bromophenacyl bromide and mepacrine reduced islet glucose utilization and glucose-stimulated [3H]choline levels in PC. An inhibitor of CTP: phosphorylcholine cytidylyltransferase, 59-deoxy-59-isobutylthioadenosine, reduced glucose-stimulated [14C]choline incorporation into PC; insulin release was inhibited in a parallel fashion. Thus, islet PC turnover and CDP-choline pathway activity appear to be modulated by glucose metabolism and membrane phospholipid hydrolysis. PC turnover and insulin release appear to be related.

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