PATHWAYS OF DEPHOSPHORYLATION OF 1-D-MYO-INOSITOL 1,4,5-TRISPHOSPHATEIN GH3 PITUITARY-TUMOR CELLS

Previous work in H-3!inositol-labelled GH3 pituitary tumor cells stim ulated with thyrotropin-releasing hormone (TRH) reported the existence of at least ten distinct H-3!inositol-containing substances which we re identified as different inositol mono-, bis- and tris-phosphate iso mers 1!. Here a complete kinetic study of the dephosphorylation pathw ays of the second messenger Ins(1,4,5)P3 is reported in GH3 cell homog enates, identifying a new intermediate, Ins(4,5)P2, in the metabolism of the second messenger. In vitro results obtained with exogenous subs trates are compared with in vivo results obtained measuring levels of the endogenous H-3!inositol-labelled isomers that participate in the dephosphorylation pathways of Ins(1,4,5)P3 in resting and TRH-stimulat ed GH3 cells. The effect of Li+ on the activity of the different phosp hatases involved in these pathways is studied as well.