Effects of dexamethasone administration on insulin resistance and components of insulin signaling and glucose metabolism in equine skeletal muscle.

OBJECTIVE: To determine the effects of dexamethasone treatment on selected components of insulin signaling and glucose metabolism in skeletal muscle obtained from horses before and after administration of a euglycemic-hyperinsulinemic clamp (EHC). ANIMALS: 6 adult Standardbreds. PROCEDURES: In a balanced crossover study, horses received either dexamethasone (0.08 mg/kg, IV, q 48 h) or an equivalent volume of saline (0.9% NaCl) solution, IV, for 21 days. A 2-hour EHC was administered for measurement of insulin sensitivity 1 day after treatment. Muscle biopsy specimens obtained before and after the EHC were analyzed for glucose transporter 4, protein kinase B (PKB), glycogen synthase kinase (GSK)-3alpha/beta protein abundance and phosphorylation state (PKB Ser(473) and GSK-3alpha/beta Ser(21/9)), glycogen synthase and hexokinase enzyme activities, and muscle glycogen concentration.
RESULTS: Dexamethasone treatment resulted in resting hyperinsulinemia and a significant decrease (70%) in glucose infusion rate during the EHC. In the dexamethasone group, increased hexokinase activity, abrogation of the insulin-stimulated increase in glycogen synthase fractional velocity, and decreased phosphorylation of GSK-3alpha Ser(21) and GSK-3B Ser(9) were detected, but there was no effect of dexamethasone treatment on glucose transporter 4 content and glycogen concentration or on PKB abundance and phosphorylation state. CONCLUSIONS AND CLINICAL RELEVANCE: In horses, 21 days of dexamethasone treatment resulted in substantial insulin resistance and impaired GSK-3 phosphorylation in skeletal muscle, which may have contributed to the decreased glycogen synthase activity seen after insulin stimulation.