Ca 2 ’ - mediated Generation of Inositol 1 , 4 , 5 - Trisphosphate and Inositol 1 , 3 , 4 , 5 - Tetrakisphosphate in Pancreatic Islets

The role of Ca2+ in the generation of inositol phosphates was investigated using rat pancreatic islets after steady state labeling with my0-[2-~H]inositol. Depolarizing K+ concentrations (24 mM) evoked early ( 2 s) increases in inositol 1,4,5-trisphosphate (Ins1,4,5-P,) and inositol 1,3,4,5-tetrakisphosphate (Ins1,3,4,5-P4) as measured by high performance anionexchange chromatography. The increase in Ins-1,4,5P3 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-1,3,4P, (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 MM). 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.