Contributions of cortisol and corticosterone to metabolic regulation in humans
Kyle, Catriona Jane
Both cortisol and corticosterone circulate in human plasma however corticosterone has been relatively neglected in human research to date. There is evidence of distinct regulation within different tissues with the transmembrane transporter ABCB1, highly expressed in the brain, exporting cortisol but not corticosterone. This may account for the relative accumulation of corticosterone in the CNS. In contrast, ABCC1, highly expressed in adipose tissue and skeletal muscle, exports corticosterone but not cortisol, suggesting cortisol is the principal glucocorticoid acting in these tissues. We tested the hypotheses that: (i) corticosterone physiology in humans is different to that of cortisol; (ii) inhibition of ABCC1 increases binding of corticosterone to corticosteroid receptors in adipose tissue and skeletal muscle but has no central CNS effect; and (iii) corticosterone is superior to cortisol as a basis for glucocorticoid replacement therapy with fewer metabolic side effects. We compared paired salivary and plasma samples from 10 healthy individuals. Plasma corticosterone showed a similar diurnal variation to cortisol but salivary corticosterone was low and did not correlate with plasma concentrations. A placebo-controlled randomised crossover study was carried out in 14 healthy individuals comparing receptor occupancy of glucocorticoids centrally and peripherally with and without ABCC1 inhibition. Receptor occupancy was assessed through displacement with MR and GR antagonists potassium canrenoate and mifepristone. Centrally, ABCC1 inhibition caused increased activation of the HPA axis after MR and GR antagonism. Peripherally, we were unable to show displacement from adipose tissue or skeletal muscle. A further placebo-controlled randomised crossover study is still ongoing in 16 patients with congenital adrenal hyperplasia, comparing metabolic effects of placebo, cortisol and corticosterone infusions over 6 hours. We present interim data for n=8. ACTH and 17-OHP were suppressed with corticosterone. Metabolic parameters were similar between placebo, cortisol and corticosterone phases. These data suggest corticosterone physiology is distinct compared with cortisol in humans. We have shown ABCC1 inhibition alters the HPA axis after receptor antagonism which suggests ABCC1 may play more of a key role centrally than previously thought. Corticosterone suppresses ACTH and 17-OHP in the short term in congenital adrenal hyperplasia, highlighting the possibility of its use as an alternative glucocorticoid replacement therapy in the future.