Temporal Relationships Between Plasma Cortisol, Corticosteroid-binding Globulin (CBG), and the Free Cortisol Index (FCI) in Pigs

The free fraction of cortisol in circulation is that which is biologically available to cells. In swine, more than 60% of cortisol is bound to its carrier glycoprotein, porcine corticosteroid-binding globulin (pCBG). The Free Cortisol Index (FCI), which is the ratio of total cortisol to CBG concentrations, has been shown in humans to correlate well with free cortisol. In experiment one, the relationship between plasma free cortisol and the free cortisol index (FCI) was evaluated in 8-wk old pigs over a 24 h period and in response to the administration of saline (n = 4) and ACTH (n = 4). A high (P < 0.001) correlation was found between actual free cortisol and the FCI in both saline (r = 0.73) and ACTH (r = 0.85) treated pigs. A diurnal rhythm was apparent for total cortisol, free cortisol, percent free cortisol, pCBG and FCI with higher values for total cortisol and the FCI occurring in the morning compared to lower pCBG values at that time. For the variables of total cortisol (P < 0.05), free cortisol (P < 0.05), and FCI (P < 0.01), values were elevated during the four hours following administration of ACTH. Concentrations of pCBG differed (P < 0.001) over time and were higher (P < 0.01) for ACTH treated pigs over the 24 h period. In the second study, changes in plasma cortisol, pCBG and the FCI were examined in pigs in response to adrenal stimulation or suppression. Twenty-four 8-wk old pigs allotted in equal numbers, were administered ACTH, dexamethasone, or saline, and blood samples collected every 15 min over a 6 h period. In pigs treated with ACTH, the FCI increased (P < 0.001) in a fashion similar to that of total cortisol, while pCBG values remained stable. In dexamethasone treated pigs, both FCI and total cortisol decreased (P < 0.001) with a subsequent elevation in pCBG concentrations. The results of these two studies affirm the contention that FCI is a suitable estimate of free cortisol in swine, and that a better illustration of the temporal relationships between plasma total cortisol and pCBG may be assessed by the measurement of both.