Serum regulates cortisol bioactivity by corticosteroid-binding globulin-dependent and independent mechanisms, as revealed by combined bioassay and physicochemical assay approaches.

Context: Corticosteroid-binding globulin (CBG) is the principal carrier of natural glucocorticoids in the circulation, and we hypothesized that it modulates glucocorticoid bioactivity (GBA). Alterations in CBG, the presence of noncortisol, naturally occurring glucocorticoids and the use of potent, synthetic glucocorticoids, all make it difficult to assess adrenal activity in-vivo; these problems can be addressed by a glucocorticoid bioassay.

Design And Subjects: A bioassay was developed for serum GBA and a physicochemical ultrafiltration-liquid chromatography-tandem mass spectrometry assay for free serum cortisol (FreeF). We studied individuals homozygous and heterozygous for a nonfunctioning CBG variant (CBG G237V) and healthy controls.

Results: FreeF concentrations were similar in healthy controls, and those with absent functional CBG, but surprisingly we found low GBA in CBG null individuals. This may suggest that CBG delivers cortisol to target cells. However, further experiments revealed that dilution of serum in the bioassay caused release of cortisol from CBG, resulting in elevated GBA measurements in all but the CBG G237V homozygotes. Furthermore, we identified a specific and potent inhibitory effect of high concentration serum on glucocorticoid sensitivity of the recipient cells used in the bioassay. Analysis of inflammatory synovial fluid, a filtrate of serum with lower CBG concentration, revealed elevated free cortisol compared to noninflammatory synovial fluid, a change not attributable to interconversion between cortisol and cortisone.

Conclusions: Our findings reveal that dilution of CBG enhances cortisol release, and so bioactivity, and also that serum potently induces glucocorticoid resistance in target cells.