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

The research investigates the impact of a single dose of dexamethasone on glucose homeostasis in horses and suggests that dexamethasone reduces insulin response initially and increases this response after a prolonged period.

Research Context and Objective

• This study was motivated by the understanding that long-term dexamethasone usage in horses can lead to insulin resistance, possibly predisposing horses to conditions like laminitis. Surprisingly, the effect of a single dose of dexamethasone on glucose homeostasis in horses wasn’t well defined, despite its common use in diagnostics.
• The key objective of this investigation was to characterize how glucose dynamics change over time in response to a single dose of dexamethasone.

Methodology

• A combined glucose-insulin tolerance test (CGIT) was used in this study. It was performed on six adult geldings at different times before and after administering a single dose of dexamethasone (40 micrograms/kg of body weight, given intravenously).
• The times measured were before administration, and then 2, 24, and 72 hours after. The horses rested for at least a week between treatments.
• Prior to any treatment, the CGIT resulted in a hyperglycemic phase (high blood sugar) followed by a hypoglycemic phase (low blood sugar).

Findings

• Dexamethasone was found to modify glucose dynamics in three significant ways:
  • At the 2-hour mark, dexamethasone shortened the hyperglycaemic negative phase of the test, indicating moderate insulin resistance.
  • At the 24-hour mark, dexamethasone impaired the clearance of glucose by prolonging the positive phase and eliminating the negative phase while insulin levels were elevated before the CGIT, indicating a reduced response to insulin.
  • At the 72-hour mark, dexamethasone caused a deeper low in glucose levels compared to before administration of the drug, indicating an increased response to the action of insulin.
• The researchers concluded that dexamethasone decreases the response to insulin as early as 2 hours, with the effect reaching its peak at 24 hours.
• Surprisingly, dexamethasone increased the response to insulin by 72 hours, a response that was not expected from initial hypotheses.