title: 5-Methyltryptamine stimulates phospholipase C-mediated breakdown of exogenous phosphoinositides by blowfly salivary gland membranes.
abstract: 5-Methyltryptamine, through a GTP-dependent mechanism, stimulated breakdown of endogenous [3H]inositol-labeled phosphoinositides in membranes prepared from blowfly salivary gland homogenates through a phospholipase C exhibiting a pH optimum of approximately 7.0. Unlabeled membranes, prepared from salivary gland homogenates, hydrolyzed exogenous [3H]phosphatidylinositol 4,5-bisphosphate substrate with generation of labeled inositol phosphates. Inositol trisphosphate formation was increased approximately 200% by 10 microM guanosine 5'-(O-thio)-trisphosphate (GTP gamma S) within 30 s. 5-Methyltryptamine, in the presence of 10 microM GTP gamma S, increased the rate of inositol trisphosphate formation by approximately 500% within 30 s. Half-maximal activation of hormone-stimulated breakdown of exogenous substrate required approximately 0.05 microM GTP gamma S. [3H]Phosphatidylinositol was also hydrolyzed during incubation with membranes, resulting in the generation of inositol, glycerol phosphoinositol, and inositol monophosphate. Formation of inositol monophosphate was stimulated approximately 30% by 10 microM GTP gamma S and 10 microM 5-methyltryptamine. Neither inositol nor glycerol phosphoinositol formation was affected by hormone. These results indicate that in a cell-free system from blowfly salivary glands, 5-methyltryptamine, through a GTP-dependent mechanism, directly activates a phospholipase C which mediates phosphoinositide hydrolysis.
mesh_terms: D000818:Animals; D002462:Cell Membrane / Q000201:enzymology; D004175:Diptera / Q000201:enzymology*; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010716:Phosphatidylinositols / Q000378:metabolism*; D012469:Salivary Glands / Q000201:enzymology; D013379:Substrate Specificity; D013873:Thionucleotides / Q000494:pharmacology; D014363:Tryptamines / Q000494:pharmacology*; D010738:Type C Phospholipases / Q000378:metabolism*
chemical_list: D010716:Phosphatidylinositols; D013873:Thionucleotides; D014363:Tryptamines; C027552:5-methyltryptamine; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate; D010738:Type C Phospholipases