Human plasma contains both cortisol (F) and corticosterone (B). B has been largely neglected in humans as it is 1020 times less abundant however previous studies suggest B and F have differential tissue-specific actions. Compared with F, B has an enhanced response to ACTH, relatively higher concentrations in brain and CSF and differential transmembrane trafficking by ATP-binding cassette (ABC) transporters. Plasma F is bound to corticosteroid-binding globulin (CBG) and albumin with only 510% unbound. Our unpublished data suggest F has greater affinity than B for CBG and plasma B:F ratio is higher in the free than total pool. Salivary glucocorticoid measurement reflects unbound steroid in plasma and we hypothesised that salivary B:F ratio is similarly high compared with the total plasma pool.
With ethical approval, we collected salivary samples from 6 healthy male volunteers 5 times over 24 h (mean±S.D. age 32.8±11.9 years, BMI 23.8±4.2 kg/m2). B and F concentrations were measured by ELISA, using antibodies without significant cross-reactivity. Multiple pooled morning and evening samples from 1 volunteer were analysed by LCMS/MS for validation of ELISA results.
In healthy male samples, mean±S.D. B on waking was 3.94±1.24 nmol/l and F was 8.17±3.67 nmol/l whereas bedtime concentrations were 3.64±1.68 and 0.63±0.23 nmol/l, respectively. The mean B:F ratio was higher at bedtime than on waking (0.55±0.23 vs 6.28±3.49, P=0.01). LCMS/MS analysis confirmed high morning salivary B with lack of diurnal variation compared with F.
B and F are subject to differential peripheral handling. The B:F ratio is higher in saliva than in plasma, consistent with lower protein binding of B than F in plasma and/or selective steroid trafficking by ABC transporters in salivary glands. Diurnal B variation is absent compared with F, so that B concentrations are higher than F in the evening, consistent with differential adrenal control of B and F.