Effect of a single dose of dexamethasone on glucose homeostasis in healthy horses by using the combined intravenous glucose and insulin test

Sustained dexa methasone administration to horses results in insulin resistance, which may predispose them to laminitis. A single dose of dexamethasone is commonly used as a diagnostic aid, yet the effect of a single dose of dexa methasone on glucose homeostasis in horses is not well defined. The objective of this study was to characterize the change in glucose dynamics over time in response to a single dose of dexa methasone. A combined glucose-insulin tolerance test (CGIT) was performed on 6 adult geldings before and at 2, 24, and 72 h postdexa methasone (40 mu g/kg of BW, i.v.); a minimum of 1 wk of rest was allowed between treatments. Before any treatment, the CGIT resulted in a hyperglycemic phase followed by a hypoglycemic phase. Dexa methasone affected glucose dynamics in 3 ways: 1) at 2 h, dexa methasone shortened the ascending branch of the negative phase ( P < 0.001) of the test, indicating moderate insulin resistance; 2) at 24 h, dexa methasone impaired glucose clearance by extending the positive phase and eliminating the negative phase while insulin was elevated before the CGIT, indicating a decreased response to insulin; and 3) at 72 h, dexa methasone caused a deeper nadir value (P < 0.001) compared with predexa methasone, indicating an increased response to insulin. It was concluded that dexa methasone decreased the response to insulin as early as 2 h and maximally at 24 h. At 72 h, dexa methasone caused an increased response to insulin, which was unexpected.