The bolus intraperitoneal administration of 5-bromo-2'-deoxyuridine (BrdU), at a dose in the range of those currently used for in vivo cell-kinetic studies, was found to provoke a marked rise in the plasma concentrations of adrenocorticotrophic hormone (ACTH), corticosterone and aldosterone in rats. This secretagogue effect of BrdU was annulled by the chronic pretreatment of animals with dexamethasone. The prolonged administration of endothelin-1 (ET-1) raised the blood level of aldosterone (but not of ACTH or corticosterone), and did not alter the response to BrdU. The pretreatment of rats with BQ-123 or BQ-788, two specific antagonists of ET-1 receptor subtypes A and B, did not affect the plasma concentrations of ACTH, corticosterone and aldosterone, but did partially reverse the effects of BrdU. In view of these findings we concluded that BrdU activates the pituitary-adrenal axis in rats, with its main mode of action being pituitary ACTH release; and the suppressive actions of BQ-123 and BQ-788 are independent of their antagonism on ET-1 receptors, and may be due to their interference with the intra-cellular mechanism(s) mediating the secretagogue action of BrdU, This effect of BrdU may have particular relevance to in vivo studies using BrdU labelling to assess cell kinetics of tissues (e.g. lymphatic tissue) affected profoundly by adrenal steroid hormones.
