The Concentration of Arginine Vasopressin in Pituitary Stalk Plasma of the Rat after Adrenalectomy or Morphine*

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Abstract
We have investigated the role of adrenal steroids and the opiates in regulating arginine vasopressin (AVP) secretion into the pituitary stalk blood of the rat. The portal plasma concentration of AVP in urethane-anesthetized male rats was 532 .+-. 68 pg/ml (mean .+-. SEM), while the peripheral plasma AVP concentration in intact urethane-anesthetized rats was 20.7 .+-. 5.7 pg/ml. Column chromatography on Sephadex G-25 of an extract of a pool of portal plasma revealed that the material being assayed comigrated with synthetic AVP. Bilateral adrenalectomy (ADX) 5 days before the collection of portal blood elevated portal plasma AVP concentrations approximately 6-fold (655 .+-. 124 pg/ml in controls vs. 4090 .+-. 504 pg/ml in adrenalectomized animals). Dexamethasone administration (15 .mu.g/kg .cntdot. day) for 5 days prevented the ADX-induced increase in portal plasma AVP concentrations without significantly changing portal plasma AVP concentrations in intact rats. Portal plasma concentrations of .beta.-endorphin were not changed by ADX or dexamethasone treatment. The iv infusion of morphine sulfate (3 mg/kg) dramatically decreased the concentration of AVP in the portal plasma of the rat (501 .+-. 101 pg/ml before morphine vs. 185 .+-. 50 pg/ml after morphine). The inhibitory effect of morphine was reversed by naltrexone (1.0 mg/kg), whereas naltrexone alone did not alter AVP secretion. Morphine administration also decreased systemic plasma AVP concentrations in urethane-anesthetized rats (27.1 .+-. 6.6 pg/ml in controls vs. 3.3 .+-. 1.3 pg/ml in morphine-treated rats). Naltrexone treatment reversed this effect. These results suggest that AVP secretion into pituitary stalk blood is under the inhibitory influence of the adrenal steroids, and the increased concentration of AVP found in portal blood may be partially responsible for the elevated levels of ACTH after ADX. Furthermore, morphine-induced activation of the pituitary-adrenal axis is apparently independent of hypothalamic AVP secretion.

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