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This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).
Equine Cushing's Disease (Pituitary pars intermedia dysfunction (PPID)) is a common condition of older horses but its pathophysiology is complex and poorly understood. In contrast to pituitary-dependent hyperadrenocorticism in other species, PPID is characterised by elevated plasma ACTH but not elevated plasma cortisol. In this study, we address this paradox and the hypothesis that PPID is a syndrome of ACTH excess in which there is dysregulation of peripheral glucocorticoid metabolism and binding. In 14 PPID horses compared with 15 healthy controls, we show that: in plasma, cortisol levels and cortisol binding to CBG were not different; in urine, glucocorticoid and androgen metabolites were increased up to four-fold; in liver, 11ß-hydroxysteroid dehydrogenase type 1 (11ß-HSD1) expression was reduced; in peri-renal adipose tissue 11ß-HSD1 and carbonyl reductase 1 expression was increased; and tissue cortisol levels were not measurably different. The combination of normal plasma cortisol with markedly enhanced urinary cortisol metabolite excretion and dysregulated tissue-specific steroid-metabolising enzymes suggests that cortisol clearance is increased in PPID horses. We infer that the ACTH excess may be compensatory and pituitary pathology and autonomous secretion may be a secondary rather than primary pathology. It is possible, that successful therapy in PPID may be targeted either at lowering ACTH or, paradoxically, at reducing cortisol clearance.
Author(s): Morgan RA, Keen JA, Homer N, Nixon M, McKinnon-Garvin A, Moses-Williams J, Davis D, Hadoke PWF, Walker BR
Publication type: Article
Publication status: Published
Journal: Endocrinology
Year: 2018
Volume: 159
Issue: 11
Pages: 3791-3800
Print publication date: 01/11/2018
Online publication date: 04/10/2018
Acceptance date: 28/09/2018
Date deposited: 25/10/2018
ISSN (print): 0013-7227
ISSN (electronic): 1945-7170
Publisher: Oxford University Press
URL: https://doi.org/10.1210/en.2018-00726
DOI: 10.1210/en.2018-00726
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