Vasopressin activation of phosphatidylinositol metabolism in rat anterior pituitary in vitro and its modification by changes in the hypothalamo-pituitary-adrenal axis.

The ability of vasopressin to stimulate the accumulation of 3H-labelled inositol phosphates was studied in vitro using prelabelled rat anterior pituitary quarters. [8-Arginine] vasopressin activates inositol lipid breakdown in this system in a time- and dose-dependent manner; vasopressin (3 X 10(-7) M) resulted in a 1.8-fold stimulation of inositol phosphate accumulation over control accumulation after 10 min. This response to vasopressin is inhibited by the specific V1 antagonist (CH2)5Tyr(Me)AVP. Both oxytocin and the selective V2 agonist DDAVP also show some agonist activity, but are considerably less potent than arginine vasopressin. Corticotrophin-releasing factor alone had no effect on inositol phosphate production, whilst a high dose given in conjunction with vasopressin resulted in a diminution of the response below that found with the same concentration of vasopressin alone. Anterior pituitaries from vasopressin-deficient Brattleboro rats also show a phosphatidylinositol response to vasopressin. Pituitaries from rats that had been adrenalectomized 4 days earlier showed no increase in inositol phosphate accumulation in response to vasopressin. Daily administration of dexamethasone (40 micrograms/day) reversed this effect of adrenalectomy. This reversal was not seen when dexamethasone (40 micrograms/ml) was added to the incubation medium of adrenalectomized rat pituitary quarters. These results confirm that the rat anterior pituitary contains functional vasopressin receptors capable of activating inositol phospholipid metabolism and that this biochemical response is modified by changes in the hypothalamo-pituitary-adrenal axis.