Measurement of free glucocorticoids: quantifying corticosteroid binding capacity and its variation within and among mammal and bird species

Brendan Delehanty; Gregory D Bossart; Cory Champagne; Daniel E Crocker; Kyle H Elliott; Patricia A Fair; Dorian Houser; Amy E M Newman; Rudy Boonstra

doi:10.1093/conphys/coaa057

Measurement of free glucocorticoids: quantifying corticosteroid binding capacity and its variation within and among mammal and bird species

Conserv Physiol. 2020 Jul 28;8(1):coaa057. doi: 10.1093/conphys/coaa057. eCollection 2020.

Authors

Brendan Delehanty¹, Gregory D Bossart², Cory Champagne³, Daniel E Crocker⁴, Kyle H Elliott⁵, Patricia A Fair⁶, Dorian Houser³, Amy E M Newman⁷, Rudy Boonstra¹

Affiliations

¹ Centre for the Neurobiology of Stress, Department of Biological Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, ON M1C 1A4, Canada.
² Georgia Aquarium, 225 Baker Street Atlanta, GA 30313, USA.
³ National Marine Mammal Foundation, 2240 Shelter Island Dr Suite 200, San Diego, CA 92106, USA.
⁴ Department of Biology, Sonoma State University, 1801 E. Cotati Ave., Rohnert Park, CA 94928, USA.
⁵ Department of Natural Resource Sciences, McGill University, Sainte Anne-de-Bellevue, Québec, H9X 3V9, Canada.
⁶ Department of Public Health Sciences, Medical University of South Carolina, 135 Cannon Street, Charleston, SC 29425, USA.
⁷ Department of Integrative Biology, University of Guelph, 50 Stone Rd E, Guelph, Ontario N1G 2W1, Canada.

Abstract

Plasma glucocorticoid (CORT) levels are one measure of stress in wildlife and give us insight into natural processes relevant to conservation issues. Many studies use total CORT concentrations to draw conclusions about animals' stress state and response to their environment. However, the blood of tetrapods contains corticosteroid-binding globulin (CBG), which strongly binds most circulating CORT. Only free CORT (CORT not bound by CBG) leaves the circulation and exerts biological effects on CORT-sensitive tissues. Measuring free CORT concentrations provides insight to an animal's stress response that cannot be revealed by simply measuring total CORT. To calculate free CORT concentrations in plasma or serum samples, one needs three measurements: the binding affinity of CBG for CORT (which varies by species), the total CORT concentration in the sample and the maximum corticosteroid binding capacity (MCBC) of CBG in the sample. Here, we detail the measurement of CBG binding capacity. We compare and contrast the three main methods to measure MCBC: charcoal, cell harvester and dialysis. Each is defined by the means by which free and bound CORT are separated. We weigh the relative merits and challenges of each. We conclude that sample volume, species and taxon binding specificity, and availability of equipment are the primary considerations in selecting the appropriate separation method. For most mammals, the charcoal method is recommended. For birds, the harvester method has critical advantages over the charcoal method. The dialysis method is widely regarded as the gold standard and has lower equipment costs but is more time-intensive and costly in terms of radioactive isotope needed and is less suited to processing large numbers of samples. The binding capacity of CBG varies tremendously within and among the bird and marine mammal species studied, and we discuss the implication of this variation for understanding the role of stress in wildlife.

Keywords: cortisol; dialysis; free hormone hypothesis; harvester; interspecific variation; marine mammals; stress; wildlife.