Specific Receptors for Vasopressin in the Pituitary Gland: Evidence for Down-Regulation and Desensitization to Adrenocorticotropin-Releasing Factors

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Abstract
Specificity of binding of 3H-labeled arginine vasopressin ([3H]AVP), down-regulation of receptors, and desensitization were studied in anterior pituitary glands of both Wistar and Brattleboro rats. Studies using both crude membrane fractions and isolated cells of anterior pituitaries revealed the presence of a single population of binding sites with a Kd of .apprx. 1 nM. The receptor recognized the following peptides, with AVP = lysine vasopressin = vasotocin > oxytocin = 1-deamino-(8-D-AVP) > d-(CH2)5-Try-(Me)-Val4-AVP > 1-deaminopenicillamine-(Val4-D-Arg8)VP. Neither corticotropin-releasing factor (CRF) nor any of the neuropeptides tested, including AVP ring and tail fragments, competed for tracer binding. Increased extracellular vasopressin levels due to chronic injections or long term adrenalectomy decreased receptor density by 80%, while oxytocin was less effective than AVP. Comparing binding data in Brattleboro homozygotes and heterozygotes revealed that AVP levels within the physiological range could down-regulate pituitary receptors as well. This could not be caused by occupation of sites by endogeneous vasopressin, since injection of large doses of peptide decreased tracer binding by less than 10%. Loss of pituitary receptors reduced enhancement by AVP of CRF-induced cAMP accumulation, intrinsic CRF-like activity and synergistic effect of AVP on ACTH secretion elicited by CRF. Evidence was provided for the presence of highly specific vasopressin receptors in the anterior pituitary, which may undergo homologous down-regulation and desensitization in terms of cAMP production and ACTH release.