title: G-protein involvement in central-nervous-system muscarinic-receptor-coupled polyphosphoinositide hydrolysis.
abstract: Potentiation of muscarinic-agonist-stimulated polyphosphoinositide (PPI) hydrolysis was demonstrated in a rat cerebral-cortical membrane preparation prelabelled with myo-[3H]inositol. Accumulation of myo-[3H]inositol 1,4-bisphosphate ([3H]IP2) was used to assess brain [3H]phosphatidylinositol 4,5-bisphosphate hydrolysis as its immediate metabolite, myo-[3H]inositol 1,4,5-trisphosphate, was rapidly hydrolysed to [3H]IP2. Inclusion of ATP (100 microM) and Mg2+ (5 mM) in the assay medium was necessary to demonstrate the effect of GTP analogues on carbachol-stimulated brain [3H]PPI turnover. Carbachol (100 microM) induced only a small increment in [3H]IP2 accumulation (142% of control) in 1 min. However, its effect was markedly enhanced, to 800% and 300% of control, by 100 microM-guanosine 5'-[gamma-thio]triphosphate (GTP[S]) and guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG) respectively. GTP[S] and p[NH]ppG also stimulated [3H]IP2 accumulation by over 500% and 200% of control, respectively. The GTP-analogue-potentiated carbachol effect was antagonized by 10 microM-atropine, whereas the GTP-analogue stimulation was unaffected. This report confirms the involvement of a G (GTP-binding) protein(s) in brain PPI metabolism and provides new evidence for the role of G protein(s) in the coupling of stimulated muscarinic receptors to PPI hydrolysis in the central nervous system.
authors.affiliation: Clarke Institute of Psychiatry, University of Toronto, Ontario, Canada.
authors.forename: A S
authors.initials: AS
authors.lastname: Chiu
authors.forename: P P
authors.initials: PP
authors.lastname: Li
authors.forename: J J
authors.initials: JJ
authors.lastname: Warsh
mesh_terms: D000818:Animals; D002217:Carbachol / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism; D002540:Cerebral Cortex / Q000187:drug effects / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate / Q000031:analogs & derivatives* / Q000494:pharmacology; D006868:Hydrolysis; D007295:Inositol Phosphates / Q000378:metabolism; D008297:Male; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000378:metabolism*; D013873:Thionucleotides / Q000494:pharmacology*; D010738:Type C Phospholipases / Q000378:metabolism
chemical_list: D002352:Carrier Proteins; D007295:Inositol Phosphates; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols; D011976:Receptors, Muscarinic; D013873:Thionucleotides; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate; D002217:Carbachol; D010738:Type C Phospholipases