Blood Cholesterol and Hydrocortisone Production in Man: Quantitative Aspects of the Utilization of Circulating Cholesterol by the Adrenals at Rest and under Adrenocorticotropin Stimulation*

Abstract

A kinetic study of the conversion of blood cholesterol into hydrocortisone was carried out in two patients through prolonged infusions of cholesterol-4-¹⁴C. The following points appear to be established by our observations: 1) The infused tracer behaved metabolically like endogenous cholesterol; it could therefore serve as a means of labeling plasma cholesterol for investigating its utilization by the adrenal cortex. 2) At rest, about 80% of hydrocortisone derived from plasma cholesterol, the other 20% thus being synthesized in situ from acetate and other unlabeled precursors. 3) Under ACTH stimulation the participation of plasma cholesterol in the synthesis of hydrocortisone was the same as at rest; the conversion of plasma cholesterol into hydrocortisone was thus proportional to the production of glucocorticosteroids by the adrenal glands. 4) The specific activities of hydrocortisone allowed us to trace its adrenal precursors including adrenal cholesterol. The kinetics of the replacement of adrenal cholesterol by plasma cholesterol underlined the functional heterogeneity of the former. The experimental data were compatible with the following model: A fraction of plasma cholesterol entering the adrenal cell is immediately available for metabolism and conversion into steroid hormones, and another fraction turns over slowly, representing some form of storage.