The effects of fatty acids on phosphoinositide synthesis and myo-inositol accumulation in exocrine pancreas

The effects of arachidonic acid (20:4) on phosphoinositide turnover were examined in rat pancreatic acinar cells prelabeled with myo-3H]inositol. Arachidonic acid (50 microM) increased the accumulation of myo-3H]inositol, but not that of 3H]inositol monophosphate, 3H]inositol bisphosphate, or 3H]inositol trisphosphate. By contrast, 10 microM carbamoylcholine increased the accumulation of all four compounds. A combination of arachidonic acid plus carbamoylcholine caused a selective and marked accumulation of myo-3H]inositol, which was abolished by 10 mM LiCl. Arachidonic acid (10-100 microM) produced a concentration-dependent inhibition of myo-3H]inositol incorporation into phosphoinositides and markedly depressed carbamoylcholine-induced increases in myo-3H]inositol incorporation into inositol phospholipids. Several other unsaturated and saturated fatty acids failed to elicit a synergistic response with carbamoylcholine in stimulating myo-3H]inositol accumulation and did not retard the incorporation of myo-3H]inositol into phosphoinositides. The fact that eicosapentaenoic acid (20:5), but not arachidic acid (20:0), mimicked the depressant effect of arachidonate on phosphoinositide labeling suggests that the degree of unsaturation of the fatty acid, rather than chain length, is important for inhibition of phosphoinositide synthesis. The arachidonate-induced decrease in myo-3H]inositol incorporation was accompanied by a reduction in the steady state level of 32P]phosphatidylinositol 4,5-bisphosphate. The mass of arachidonic acid liberated in response to carbamoylcholine was measured by gas chromatography-mass spectrometry, and the time course of stimulated arachidonate accumulation paralleled that of inositol phosphate accumulation and amylase release. These observations suggest that in exocrine pancreas, endogenous arachidonic acid serves as a negative feedback regulator of phosphoinositide turnover.