Involvement of Vasopressin in the Down-Regulation of Pituitary Corticotropin-Releasing Factor Receptors after Adrenalectomy

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Abstract
The role of vasopressin (VP) in the regulation of pituitary corticotropin-releasing factor (CRF) receptors was studied by examining the effects of adrenalectomy and VP infusion on pituitary CRF receptors in genetically VP-deficient rats (di/di) and Long-Evans control rats. Binding studies with [125I]Tyr-ovine CRF in 30,000 .times. g anterior pituitary membrane-rich fractions revealed similar characteristics for the CRF receptors in Long-Evans and di/di rats, with Kd values of 2.4 .+-. 0.6 and 1.9 .+-. 0.2 nM, respectively, and receptor concentrations of 278 .+-. 31 and 286 .+-. 43 fmol/mg, respectively. Two day after adrenalectomy, the pituitary CRF receptor concentration decreased by 72 .+-. 4.2% in Long-Evans rats, but by only 20.3 .+-. 5.6% in di/di rats. CRF receptor affinity was unchanged after adrenalectomy (Kd = 1.7 .+-. 0.5 nM; n = 8). To determine whether VP deficiency is responsible for the smaller decrease in CRF receptor in di/di rats, the effect of exogenous VP infusion (100 ng/min) by sc osmotic minipumps was studied in adrenalectomized di/di rats. Two days after adrenalectomy, pituitary CRF receptors were reduced by 21 .+-. 8% in control di/di rats, whereas a 77.7 .+-. 1.8% decrease was observed in VP-infused di/di rats, comparable to the effect of adrenalectomy in Long-Evans rats. VP infusion also caused a significant 35 .+-. 2% decrease in CRF receptors in the pituitaries of sham-operated di/di rats, with no changes in CRF receptor affinity. In Sprague-Dawley rats, VF or CRF infusion (100 ng/min) decreased pituitary CRF receptors by 14 .+-. 1.9% and 46 .+-. 3%, respectively. However, the combined infusion of both peptides caused a 65% .+-. 4.2 decrease, similar to that observed after adrenalectomy. In vitro incubation of quartered pituitaries with VP or CRF for 4 h reduced CRF receptors by 23.1 .+-. 8.2% and 38.2 .+-. 3.8%, respectively, while simultaneous preincubation with both peptides was followed by a decrease of 55.3% .+-. 5.3%. These findings indicate that increased hypothalamic release of VP contributes to the down-regulation of pituitary CRF receptors after adrenalectomy.

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