Immunoreactive vasopressin and oxytocin were measured in the hypothalamo-hypophysial portal blood of both Long–Evans and homozygous Brattleboro rats. Adrenalectomy caused an increase in vasopressin immunoreactivity in portal blood in the Long–Evans strain, whilst administration of dexamethasone to these adrenalectomized animals resulted in a reduction in portal vasopressin immunoreactivity to levels below those seen in sham-operated animals. This vasopressin immunoreactivity co-eluted with synthetic vasopressin on high-pressure liquid chromatography (HPLC), and diluted in parallel in radioimmunoassay. In Brattleboro rats, however, although vasopressin-like immunoreactivity was detected, the portal concentration did not vary with the adrenal status of the animal, nor did it show the characteristics of standard vasopressin on HPLC or in immunoassay. Oxytocin was present in the portal blood of both Long–Evans and Brattleboro rats at similar very high concentrations, but did not vary in response to adrenalectomy. These results are consistent with a role for vasopressin, but not oxytocin, in the hypothalamic response to adrenalectomy and glucocorticoid feedback. Neither vasopressin immunoreactivity nor oxytocin appear to subserve this role in the homozygous Brattleboro rat.
J. Endocr. (1988) 117, 27–34