Effects of dexamethasone on glucose dynamics and insulin sensitivity in healthy horses.
OBJECTIVE
To determine effects of dexamethasone on glucose dynamics and insulin sensitivity in healthy horses.


METHODS
6 adult Standardbreds.


METHODS
In a balanced crossover study, horses received dexamethasone (0.08 mg/ kg, IV, q 48 h) or an equivalent volume of saline (0.9% NaCl) solution (control treatment) during a 21-day period. Horses underwent a 3-hour frequently sampled IV glucose tolerance test (FSIGT) 2 days after treatment. Minimal model analysis of glucose and insulin data from FSIGTs were used to estimate insulin sensitivity (Si), glucose effectiveness (Sg), acute insulin response to glucose (AIRg), and disposition index. Proxies for Si (reciprocal of the inverse square of basal insulin concentration [RISQI]) and beta-cell responsiveness (modified insulin-to-glucose ratio [MIRG]) were calculated from basal plasma glucose and serum insulin concentrations.


RESULTS
Mean serum insulin concentration was significantly higher in dexamethasone-treated horses than control horses on days 7, 14, and 21. Similarly, mean plasma glucose concentration was higher in dexamethasone-treated horses on days 7, 14, and 21; this value differed significantly on day 14 but not on days 7 or 21. Minimal model analysis of FSIGT data revealed a significant decrease in Si and a significant increase in AIRg after dexamethasone treatment, with no change in Sg or disposition index. Mean RISQI was significantly lower, whereas MIRG was higher, in dexamethasone-treated horses than control horses on days 7, 14, and 21.


CONCLUSIONS
The study revealed marked insulin resistance in healthy horses after 21 days of dexamethasone administration. Because insulin resistance has been associated with a predisposition to laminitis, a glucocorticoid-induced decrease in insulin sensitivity may increase risk for development of laminitis in some horses and ponies.
Tiley
Heather A
HA
Department of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph, ON N1G 2W1, Canada.
Geor
Ray J
RJ
McCutcheon
L Jill
LJ
D000818:Animals; D001786:Blood Glucose / Q000378:metabolism; D018592:Cross-Over Studies; D003907:Dexamethasone / Q000494:pharmacology*; D005230:Fatty Acids, Nonesterified / Q000097:blood; D005260:Female; D005938:Glucocorticoids / Q000494:pharmacology*; D005947:Glucose / Q000378:metabolism*; D005951:Glucose Tolerance Test / Q000662:veterinary; D006736:Horses / Q000378:metabolism*; D006854:Hydrocortisone / Q000097:blood; D007328:Insulin / Q000097:blood; D007333:Insulin Resistance / Q000502:physiology*; D008297:Male; D014280:Triglycerides / Q000097:blood
D001786:Blood Glucose; D005230:Fatty Acids, Nonesterified; D005938:Glucocorticoids; D007328:Insulin; D014280:Triglycerides; D003907:Dexamethasone; D005947:Glucose; D006854:Hydrocortisone