A sensitive bioassay for the determination of human plasma ACTH levels.

A sensitive bioassay for the measurement of plasma ACTH is presented. The use of silicic acid adsorption of plasma, with a subsequent acid wash and aqueous acetone desorption, was successful in removing those substances which had interfered with the steroidogenic response of dispersed adrenal cells when unextracted plasma was employed. This extraction procedure extracted 72-76% of ACTH present in plasma. Two pg ACTH1-39 could be consistently detected. Alpha-hACTH1-39 and alpha-pACTH1-39 exhibited equal potencies. Beta-MSH was ineffective at dosage levels up to 2 x 10(8) pg. One x 10(8) pg of ACTH1-10, ACTH4-10, or alpha-MSH had a steroidogenic effect equivalent to that of 40 pg ACTH1-39. ACTH 17-39 and ACTH 11-24 were incapable of stimulating steroid production at doses of 1 x 10(8) pg. Excesses of the latter, but not of the former appeared to be able to antagonize the steroidogenic effect of ACTH1-39. Plasma from normal subjects, bioassayed by this extraction procedure, contained 12-186 pg/ml ACTH at 0400-0800: 14-93 pg/ml ACTH at 1000-1300, and less than 10-34 pg/ml ACTH at 1600-2200. Hypoglycemia and vasopressin administration were followed by increases in plasma ACTH concentratrations. Plasma ACTH concentrations in untreated patients with Cushing's disease (sampled over the period 0900-1300) ranged from 65-220 pg/ml. Three patients with Addison's disease (untreated or 12 h following replacement steroid withdrawal) had ACTH concentrations of 223, 370 and 1226 pg/ml. Markedly elevated ACTH concentrations were observed in a patient with Nelson's syndrome (391 and 835 pg/ml). Bioassayable ACTH was not detected in 2 patients with panhypopituitarism.