Angiotensin-stimulated production of inositol trisphosphate isomers and rapid metabolism through inositol 4-monophosphate in adrenal glomerulosa cells.

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RESUMO

The production and metabolism of inositol phosphates in rat adrenal glomerulosa cells prelabeled with [3H]inositol and stimulated with angiotensin II were analyzed by high-performance anion-exchange chromatography. Exposure to angiotensin II was accompanied by a rapid and substantial decrease in the phospholipid precursor, phosphatidylinositol (PtdIns) 4,5-bisphosphate with only a slight and transient increase in the level of the biologically active product, inositol 1,4,5-trisphosphate (Ins-1,4,5-P3), to a peak at about 5 sec. Inositol 1,3,4-trisphosphate (Ins-1,3,4-P3), the putative metabolite of Ins-1,4,5-P3, was also formed rapidly and maintained an elevated steady-state level during stimulation by angiotensin II. Inositol 1,4-bisphosphate (Ins-1,4-P2) exhibited a simultaneous and prominent increase that could not be accounted for solely by direct breakdown of PtdIns 4-phosphate, indicating that large amounts of Ins-1,4,5-P3 must also have been produced and metabolized. The rapid formation of a substantial amount of inositol 4-monophosphate (Ins-4-P), with no significant change in the level of inositol 1-monophosphate (Ins-1-P) during the first minute of stimulation, was a notable feature of the glomerulosa cell response to angiotensin II. These observations indicate that PtdIns-4,5-P2 catabolism in the angiotensin-stimulated glomerulosa cell initially proceeds via Ins-1,4,5-P3 through Ins-1,3,4-P3 and Ins-1,4-P2 to form Ins-4-P rather than Ins-1-P and that direct hydrolysis of PtdIns by phospholipase C does not occur during the initial phase of angiotensin action. In glomerulosa cells stimulated by angiotensin II in the presence of Li+, the progressive accumulation of both Ins-4-P, and after a short lag period, Ins-1-P indicated that dephosphorylation of both isomers was inhibited by Li+. The increase of Ins-P isomers in the presence of Li+ was associated with increased and progressive accumulation of Ins-1,4-P2 and Ins-1,3,4-P3 but not of Ins-1,4,5-P3. These data demonstrate that sustained and massive breakdown of PtdIns phosphates begins within seconds during cell activation by angiotensin II. The Ca2+-mobilizing metabolite, Ins-1,4,5-P3, is rapidly converted to Ins-1,3,4-P3 and degraded through Ins-1,4-P2 and Ins-4-P, in contrast to the previous view that conversion to Ins-1-P is the major route of PtdIns 4,5-bisphosphate metabolism.

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