| Abstract: | In the present study, an activation mechanism for phospholipase D (PLD) in 3H]palmitic acid-labeled pheochromocytoma PC12 cells in response to carbachol (CCh) was investigated. PLD activity was assessed by measuring the formation of 3H]phosphatidylethanol (3H]PEt), the specific marker of PLD activity, in the presence of 0.5% (vol/vol) ethanol. CCh caused a rapid accumulation of 3H]-PEt, which reached a plateau within 1 min, in a concentration-dependent manner. The 3H]PEt formation by CCh was completely antagonized by atropine, demonstrating that the CCh effect was mediated by the muscarinic acetylcholine receptor (mAChR). A tumor promoter, phorbol 12-myristate 13-acetate (PMA), also caused an increase in 3H]-PEt content, which reached a plateau at 30-60 min after exposure, but an inactive phorbol ester, 4 alpha-phorbol 12,13-didecanoate, did not. Although a protein kinase C (PKC) inhibitor, staurosporine (5 microM), blocked PMA-induced 3H]PEt formation by 77%, it had no effect on the CCh-induced formation. These results suggest that mAChR-induced PLD activation is independent of PKC, whereas PLD activation by PMA is mediated by PKC. NaF, a common GTP-binding protein (G protein) activator, and a stable analogue of GTP, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), also stimulated 3H]PEt formation in intact and digitonin-permeabilized cells, respectively. GTP, UTP, and CTP were without effect. Furthermore, guanosine 5'-O-(2-thiodiphosphate) significantly inhibited CCh- and GTP gamma S-induced 3H]PEt formation in permeabilized cells but did not inhibit the formation by PMA, and staurosporine (5 microM) had no effect on 3H]PEt formation by GTP gamma S.(ABSTRACT TRUNCATED AT 250 WORDS) |
