DIRECT PHOSPHATIDYLINOSITOL BREAKDOWN FROM A DISTINCT HORMONE-SENSITIVE POOL OCCURS ON DEPLETION OF CYTOSOLIC INOSITOL IN BOVINE ADRENOCORTICAL (ZFR) CELLS

previous studies of [H-3]inositol-prelabelled bovine adrenocortical fa sciculata cells have shown that on prolonged stimulation with All, [H- 3]phosphoinositides are catabolised from two distinct hormone-sensitiv e subpools. The first (30% of labelled [H-3]phosphoinositides) is used immediately, is continually degraded, and is resynthesised as long as cytosolic inositol is available, The second (45%) is only used after cytosolic [H-3]inositol is depleted (30 min in the presence of Li+) an d so compensatory resynthesis is no longer possible. We have investiga ted the time-dependent effects of All on levels of H-3]inositol, and i ndividual species of [H-3]phosphoinositides and [H-3]phosphoinositols in [H-3]inositol-prelabelled fasciculata cells in the absence and pres ence of Li+. Our findings show that while cytosolic inositol is not li miting, the first pool is catabolised by a polyphosphoinositide-specif ic phospholipase C and delayed losses of [H-3]PtdIns could be fully ex plained through compensatory resynthesis of the [H-3]polyphosphoinosit ides. However once [H-3]inositol depletion had occurred (only in the p resence of Li+), a dramatic loss of [H-3]PtdIns was still observed eve n though there was a rapid loss of accumulated [H-3]Ins(1,4,5)P-3, [H- 3]Ins(1,4)P-2, [H-3]Ins(1,3,4)P-3 and [H-3]Ins(1,3)P-2. increases in [ H-3]Ins4P could be accounted for by degradation of previously accumula ted [H-3]Ins(1,4,5)P-3 and [H-3]Ins(1,4)P-2, but radioactivity continu ed to accumulate in [H-3]Ins1P/ Ins3P and far exceeded that lost from [H-3]Ins(1,3,4)P, and [H-3]Ins(1,3)P-2. Thus catabolism of the second hormone-sensitive pool (on depletion of cytosolic inositol) involved t he direct action of phosphoinositidase C on [H-3]PtdIns to form [H-3]I ns1P.