Assay of Corticotrophin in Dexamethasone-Treated Rats

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Abstract
Dexamethasone administered in the drinking water consistently and completely suppresses corticotropin secretion in the rat. Rats thus treated with dexamethasone overnight conveniently replace hypophy-sectomized rats in the assay of corticotropin by the plasma corticosterone response. By intrajugular injection doses in the range 0.0005-0.002 IU (1962)/100 g rat are assayable with an average index of precision ([lambda]) of 0.185. The precision is better than that generally reported for assays by corticosterone estimations in rats or mice either hypophysectomized or treated by dexamethasone injection. Assayed by this method the Second (1956) International unit of corticotrophin was found to contain 2.65 (limits 2.C6-3.40) Third (1962) International units. It is recommended that the results of assays by the intra-vascular administration of corticotropin preparations be reported in terms of the Third (1962) International unit.