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Evaluation of a radio immunoassay for plasma acth

Journal of Endocrinology 78(3): 309-320

Evaluation of a radio immunoassay for plasma acth

A radioimmunoassay for plasma ACTH was described and evaluated. Rabbit antiserum produced by immunization with [Asp25,Ala26,Gly27,]-.alpha.h-corticotropin-(1-28)-octacosa-peptide (a sequence analog of .**GRAPHIC**. bovine .gamma.-globulin conjugate was used. The antiserum is specific for the NH2-terminal portion of the ACTH molecule and cross-reactivity of human, porcine and rat ACTH in the system was demonstrated. Reasonable agreement was found between estimates obtained by bioassay and radioimmunoassay of the ACTH content of rat pituitary gland incubation media, indicating a close relationship between the sequence of ACTH recognized by the antibodies and the sequence possessing the steroidogenic activity. Measurement of the amount of ACTH in the plasma required the preliminary extraction and concentration of the hormone. Over a range of concentrations, 3.5-3600 pg/ml, extraction recovery was independent of the initial concentration of ACTH in the plasma. Extraction gave rise to no changes in the immunological properties of standard ACTH. The concentration of immunoreactive ACTH in rat plasma was 48 .+-. 36 (SEM [standard error of the mean]) pg/ml in the morning and 106 .+-. 99 pg/ml in the afternoon. Exposure to ether for 5 min and subsequent laparotomy gave rise to a significant increase in the concentration of immunoreactive ACTH in the plasma. The resting level of ACTH and the ACTH response to stress were both significantly higher 1 and 7 days after adrenalectomy. Injection [i.v.] of a hypothalamic extract elicited a considerable rise in the concentration of immunoreactive ACTH in the plasma, but no response was seen after oral administration of this partially purified extract. The sensitivity, precision and specificity of this ACTH radioimmunoassay make it a useful tool for studying pituitary-adrenal physiology.

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Accession: 005395570

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