Abstract

The regulation of phosphatidylinositol turnover by alpha-adrenergic agonists in rat parotid acinar cell aggregates was examined with respect to kinetics and agonist-antagonist interactions. Phosphatidylinositol turnover was followed by the changes in the specific activities of [32P]phosphatidic acid and [32P]phosphatidylinositol. The specific activity of phosphatidic acid increased rapidly (within 1 min) after addition of epinephrine (10(-5) M), reached a maximal level within 12-16 min, and then decreased. Incorporation of 32P into phosphatidylinositol exhibited a lag phase of about 5 min and then increased continuously for an additional 40 min. The absolute amounts of phosphatidic acid and phosphatidylinositol did not change. The concentrations of epinephrine needed to stimulate 32P incorporation into phosphatidic acid and phosphatidylinositol, measured at 15 and 30 min, respectively, were similar; Ka values of 2.05 +/- 0.46 X 10(-6) M for phosphatidic acid and 2.98 +/- 0.30 X 10(-6) M for phosphatidylinositol were found. The effects of agonists on 32P labeling of phosphatidylinositol, in order of potency, were epinephrine greater than or equal to norepinephrine greater than phenylephrine much greater than normetanephrine. When various adrenergic antagonists were evaluated for their ability to inhibit 10(-5) M epinephrine-stimulated 32P incorporation into both phosphatidic acid and phosphatidylinositol, the order of antagonist potency was prazosin greater than or equal to phenoxybenzamine greater than phentolamine greater than or equal to yohimbine greater than much greater than propranolol. These findings indicate that phosphatidylinositol-phosphatidic acid turnover in the rat parotid gland is mediated by the alpha 1-adrenergic receptor system.