title: Ca2+-mediated generation of inositol 1,4,5-triphosphate and inositol 1,3,4,5-tetrakisphosphate in pancreatic islets. Studies with K+, glucose, and carbamylcholine.
abstract: The role of Ca2+ in the generation of inositol phosphates was investigated using rat pancreatic islets after steady state labeling with myo-[2-3H]inositol. Depolarizing K+ concentrations (24 mM) evoked early (2 s) increases in inositol 1,4,5-trisphosphate (Ins-1,4,5-P3) and inositol 1,3,4,5-tetrakisphosphate (Ins-1,3,4,5-P4) as measured by high performance anion-exchange chromatography. The increase in Ins-1,4,5-P3 was transient and was followed by a more pronounced rise in Ins-1,3,4-P3. These effects were dependent on the presence of extracellular Ca2+ but were not secondary to release of either neurotransmitters or metabolites of arachidonic acid. K+ also promoted the breakdown of phosphatidylinositol 4,5-bisphosphate (PtdIns-4,5-P2) and of the other phosphoinositides. Glucose (16.7 mM) was less marked in its effects but still promoted rapid increases in Ins-1,3,4,5-P4 (2 s) and Ins-1,4,5-P3 (10 s) and a slower rise in Ins-1,3,4-P3 (30 s). The levels of all three metabolites rose steadily over 10 min stimulation. These responses to glucose could be largely, although not entirely, inhibited by depletion of extracellular Ca2+ or by Ca2+ channel blockade with verapamil (20 microM). Carbamylcholine (0.5 mM) was the most potent stimulus used evoking early rises in Ins-1,4,5-P3 and Ins-1,3,4,5-P4 (2 s) followed by Ins-1,3,4-P3 (10 s), effects which were only partially dependent on extracellular Ca2+. The results suggest that a Ca2+-mediated PtdIns-4,5-P2 hydrolysis accounts for most of the Ins-1,4,5-P3 generated in response to glucose but not carbamylcholine. In addition, glucose may exert effects on inositol phosphate metabolism which are Ca2+ independent.
authors.forename: T J
authors.initials: TJ
authors.lastname: Biden
authors.forename: B
authors.initials: B
authors.lastname: Peter-Riesch
authors.forename: W
authors.initials: W
authors.lastname: Schlegel
authors.forename: C B
authors.initials: CB
authors.lastname: Wollheim
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002217:Carbachol / Q000494:pharmacology*; D005947:Glucose / Q000494:pharmacology*; D066298:In Vitro Techniques; D007294:Inositol / Q000378:metabolism; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000096:biosynthesis*; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D011188:Potassium / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D013403:Sugar Phosphates / Q000096:biosynthesis*; D014316:Tritium
chemical_list: D007295:Inositol Phosphates; D013403:Sugar Phosphates; D014316:Tritium; C081655:inositol-1,3,4,5-tetrakisphosphate; D007294:Inositol; D015544:Inositol 1,4,5-Trisphosphate; D002217:Carbachol; D005947:Glucose; D011188:Potassium; D002118:Calcium