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Synthetic ovine corticotrophin-releasing factor (CRF) and arginine vasopressin (AVP) were injected alone or in combination (for each peptide 1 microgram/kg body weight) in 7-day-old and adult rats and rabbits. Fifteen minutes after the interscapulary injection, blood was collected for plasma adrenocorticotrophin (ACTH), corticosterone and aldosterone evaluation by RIA. The different circulating forms of ACTH were isolated by Sephadex G50 column chromatography with 1% formic acid and measured by RIA using 1-24 ACTH as standard. Such experiments were previously described in lambs and guinea-pigs using the same schedule. In young and adult rabbits the predominant circulating IR-ACTH form was 'big' ACTH; after stimulation with CRF, AVP or CRF + AVP the 'intermediate' IR-ACTH was greatly increased in adults, but no change was observed in young rabbits. In young and adult rats the predominant circulating form was 'intermediate' ACTH in control and injected animals; ACTH increased after CRF alone or in combination with AVP, but not after AVP alone. In both species the 'intermediate' forms of IR-ACTH were not eluted at the same time by chromatography, and calculated molecular weights were different: 14500 in rats and 9500 in rabbits. Plasma corticosterone and aldosterone were increased in rat and rabbit adults after CRF and AVP; however, they remained unchanged in young rabbits and slightly increased only after CRF in young rats in which corticosterone remained at a very low concentration compared with that in adults. Hence, the pituitary-adrenal axis of 7-day-old rabbits and rats in less reactive than that of sheep and guinea-pig of the same age. |
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