TY - JOUR T1 - [<sup>3</sup>H]Inositol Polyphosphate Metabolism in Muscarinic Cholinoceptor-Stimulated Airways Smooth Muscle: Accumulation of [<sup>3</sup>H]inositol 4,5 Bisphosphate <em>Via</em> a Lithium-Sensitive Inositol Polyphosphate 1-Phosphatase JF - Journal of Pharmacology and Experimental Therapeutics JO - J Pharmacol Exp Ther SP - 974 LP - 982 VL - 280 IS - 2 AU - Barbara J. Lynch AU - Miratul M. K. Muqit AU - Trevor R. Walker AU - Edwin R. Chilvers Y1 - 1997/02/01 UR - http://jpet.aspetjournals.org/content/280/2/974.abstract N2 - Agonist-stimulated phosphoinositide hydrolysis is the principal mechanism underlying pharmacomechanical coupling in airways smooth muscle. In bovine tracheal smooth muscle, activation of muscarinic cholinoceptors results in sustained phospholipase C-mediated PtdIns(4,5)P2 hydrolysis but transient Ins(1,4,5)P3 accumulation, which implies agonist-stimulated metabolism of Ins(1,4,5)P3. To investigate the metabolic fate of Ins(1,4,5)P3 in bovine tracheal smooth muscle, we developed a [3H]inositol-labeling protocol wherein more than 98% of the [3H]inositol polyphosphates that accumulated over a 0 to 30-min incubation with 100 μM carbachol in the presence of 5 mM LiCl were derived from [3H]Ins(1,4,5)P3 and wherein the Ins(1,4,5)P3 3-kinase (EC 2.7.1.127) and 5-phosphatase (EC 3.1.3.56) pathways generated a set of mutually exclusive [3H]inositol polyphosphate isomers. Under these conditions, the 5-phosphatase pathway was shown to be the dominant route for [3H]Ins(1,4,5)P3 metabolism at all time intervals measured, especially at early times (0–300 sec), where it accounted for more than 85% of [3H]Ins(1,4,5)P3 metabolism. We also observed accumulation of a novel agonist and LiCl-sensitive [3H]InsP2 isomer identified as [3H]Ins(4,5)P2. The presence of a LiCl-sensitive inositol polyphosphate 1-phosphatase (EC 3.1.3.57) was demonstrated, and high LiCl concentrations (30 mM) caused a significant enhancement of [3H]Ins(1,4)P2 accumulation and a corresponding decline in [3H]Ins4P levels. Because nearly identical bell-shaped LiCl concentration-response curves were obtained for [3H]Ins4P and [3H]Ins(4,5)P2 accumulation, and [3H]Ins(4,5)P2 was not generated under conditions expected to stimulate phospholipase D, these data suggest that the most likely precurser of [3H]Ins(4,5)P2 is [3H]Ins(1,4,5)P3. This is the first demonstration of Ins(4,5)P2 accumulation in a non-neuronal cell type, and the foregoing data suggest a novel route of formationvia an Ins(1,4,5)P3 1-phosphatase, which would represent an additional pathway for [3H]Ins(1,4,5)P3 removal. The American Society for Pharmacology and Experimental Therapeutics ER -