Determination of cortisol concentration in human erythrocytes

In order to investigate cytoplasmic free cortisol (F) concentrations, F levels in human red blood cells (RBC-F) were determined. Mean basal levels of plasma unbound free F and RBC-F in 10 normal healthy subjects were 16.6 and 14.8 ng/ml, respectively. RBC-F in the normal subjects showed well-defined diurnal rhythm, suppressibility by dexamethasone, and responsiveness to ACTH stimulation similar to plasma free F. There was an excellent linear relationship between RBC-F and corresponding plasma free F concentrations; RBC-F (ng/ml) = 1.25 X plasma free F (ng/ml) - 3.4, r = 0.94, n = 90. Mean basal RBC-F concentration in nine pregnant women was 13.9 ng/ml. In three patients with Cushing's syndrome and six patients with adrenal insufficiency, RBC-F showed parallel changes to plasma free F, which were apparently different from those in normal subjects, patients with simple obesity, and a patient with Cushing's disease in complete remission. These results indicated that RBC-F ran nearly parallel to plasma free F under various conditions.