Glucose infusion attenuates endogenous glucose production and enhances glucose use of horses during exercise

We examined the effects of increased glucose availability on glucose kinetics and substrate utilization in horses during exercise. Six conditioned horses ran on a treadmill for 90 min at 34 +/- 1% of maximum oxygen uptake. In one trial [glucose (Glu)], glucose was infused at a mean rate of 34.9 +/- 1.1 mu mol.kg(-1).min(-1), whereas in the other trial [control (Con)] an equivalent volume of isotonic saline was infused. Plasma glucose increased during exercise in Glu (90 min: 8.3 +/- 1.7 mM) but was largely unchanged in Con (90 min: 5.1 +/- 0.4 mM). In Con, hepatic glucose production (HGP) increased during exercise, reaching a peak of 38.6 +/- 2.7 mu mol.kg-1.min-1 after 90 min. Glucose infusion partially suppressed (P < 0.05) the rise in HGP (peak value 25.8 +/- 3.3 mu mol.kg(-1).min(-1)). In Con, glucose rate of disappearance (Rd) rose to a peak of 40.4 +/- 2.9 mu mol.kg(-1).min(-1) after 90 min; in Glu, augmented glucose utilization was reflected by values for glucose R-d that were twofold higher (P < 0.001) than in Con between 30 and 90 min. Total carbohydrate oxidation was higher (P < 0.05) in Glu (187.5 +/- 8.5 mu mol.kg(-1).min(-1)) than in Con (159.2 +/- 7.3 mu mol.kg(-1).min(-1)), but muscle glycogen utilization was similar between trials. We conclude that an increase in glucose availability in horses during low-intensity exercise I) only partially suppresses HGP, 2) attenuates the decrease in carbohydrate oxidation during such exercise, but 3) does not affect muscle glycogen utilization.