title: Breakdown of polyphosphoinositides and not phosphatidylinositol accounts for muscarinic agonist-stimulated inositol phospholipid metabolism in rat parotid glands.
abstract: The molecular mechanisms underlying the ability of muscarinic agonists to enhance the metabolism of inositol phospholipids were studied using rat parotid gland slices prelabelled with tracer quantities of [3H]inositol and then washed with 10 mM unlabelled inositol. Carbachol treatment caused rapid and marked increases in the levels of radioactive inositol 1-phosphate, inositol 1,4-bisphosphate, inositol 1,4,5-trisphosphate and an accumulation of label in the free inositol pool. There were much less marked changes in the levels of [3H]phosphatidylinositol, [3H]phosphatidylinositol 4-phosphate and [3H]phosphatidylinositol 4,5-bisphosphate. At 5 s after stimulation with carbachol there were large increases in [3H]inositol 1,4-bisphosphate and [3H]inositol 1,4,5-trisphosphate, but not in [3H]inositol 1-phosphate. After stimulation with carbachol for 10 min the levels of radioactive inositol 1,4-bisphosphate and inositol 1,4,5-trisphosphate greatly exceeded the starting level of radioactivity in phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate respectively. When carbachol treatment was followed by addition of sufficient atropine to block all the muscarinic receptors the radioactive inositol phosphates rapidly returned towards control levels. The carbachol-evoked changes in radioactive inositol phosphate and phospholipid levels were blocked in the presence of 2,4-dinitrophenol (an uncoupler of oxidative phosphorylation). The results suggest that muscarinic agonists stimulate a polyphosphoinositide-specific phospholipase C and that these lipids are continuously replenished from the labelled phosphatidylinositol pool. [3H]Inositol 1-phosphate in the stimulated glands probably arises via hydrolysis of inositol 1,4-bisphosphate and not directly from phosphatidylinositol.
mesh_terms: D019297:2,4-Dinitrophenol; D000818:Animals; D002217:Carbachol / Q000494:pharmacology*; D004140:Dinitrophenols / Q000494:pharmacology; D006868:Hydrolysis; D066298:In Vitro Techniques; D007294:Inositol / Q000378:metabolism; D007295:Inositol Phosphates / Q000378:metabolism; D050356:Lipid Metabolism; D010306:Parotid Gland / Q000187:drug effects / Q000378:metabolism*; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols / Q000378:metabolism*; D051381:Rats; D013268:Stimulation, Chemical; D013997:Time Factors; D010738:Type C Phospholipases / Q000378:metabolism
chemical_list: D004140:Dinitrophenols; D007295:Inositol Phosphates; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols; D007294:Inositol; D002217:Carbachol; D010738:Type C Phospholipases; D019297:2,4-Dinitrophenol