Effects of carbachol and pancreozymin (cholecystokinin-octapeptide) on polyphosphoinositide metabolism in the rat pancreas in vitro.
We studied the possibility that hydrolysis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] may be the initiating event for the increase in [32P]Pi incorporation into phosphatidic acid (PtdA) and phosphatidylinositol (PtdIns) during carbachol and pancreozymin (cholecystokinin-octapeptide) action in the rat pancreas. After prelabelling acini for 2h, [32P]Pi incorporation into PtdA, PtdIns(4,5)P2 and phosphatidylinositol 4-phosphate (PtdIns4P) had reached equilibrium. Subsequent addition of carbachol or pancreozymin caused 32P in PtdIns(4,5)P2 to decrease by 30-50% within 10-15 s, and this was followed by sequential increases in [32P]Pi incorporation into PtdA and PtdIns. Similar changes in 32P-labelling of PtdIns4P were not consistently observed. Confirmation that the decrease in 32P in chromatographically-purified PtdIns(4,5)P2 reflected an actual decrease in this substance was provided by the fact that similar results were obtained (a) when PtdIns(4,5)P2 was prelabelled with [2-3H]inositol, and (b) when PtdIns(4,5)P2 was measured as its specific product (glycerophosphoinositol bisphosphate) after methanolic alkaline hydrolysis and ion-exchange chromatography. The secretogogue-induced breakdown of PtdIns(4,5)P2 was not inhibited by Ca2+ deficiency (severe enough to inhibit amylase secretion and Ca2+-dependent hydrolysis of PtdIns), and ionophore A23187 treatment did not provoke PtdIns(4,5)P2 hydrolysis. The increase in the hydrolysis of PtdIns(4,5)P2 and the increase in [32P]Pi incorporation into PtdA commenced at the same concentration of carbachol in dose-response studies. Our findings suggest that the hydrolysis of PtdIns(4,5)P2 is an early event in the action of pancreatic secretogogues that mobilize Ca2+, and it is possible that this hydrolysis may initiate the Ca2+-independent labelling of PtdA and PtdIns. Ca2+ mobilization may follow these responses, and subsequently cause Ca2+-dependent hydrolysis of PtdIns and exocytosis.
D000681:Amylases / Q000378:metabolism; D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D002217:Carbachol / Q000494:pharmacology*; D002766:Cholecystokinin / Q000494:pharmacology*; D002852:Chromatography, Ion Exchange; D002855:Chromatography, Thin Layer; D004305:Dose-Response Relationship, Drug; D066298:In Vitro Techniques; D010179:Pancreas / Q000187:drug effects / Q000378:metabolism*; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols / Q000378:metabolism*; D010743:Phospholipids / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains
D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols; D010743:Phospholipids; D000001:Calcimycin; D002217:Carbachol; D002766:Cholecystokinin; D000681:Amylases; D002118:Calcium