Glucose infusion partially attenuates glucose production and increases uptake during intense exercise

Glucose infusioncan prevent the increase in glucose production (R_a) andincrease glucose uptake (R_d) during exercise of moderate intensity. We postulated that 1)because in postabsorptive intense exercise (>80% maximalO₂ uptake) the eightfold increasein R_a may be mediated by catecholamines rather than byglucagon and insulin, exogenous glucose infusion would not prevent theR_a increment, and 2)such infusion would cause greater R_d. Fit young men were exercised at >85% maximal O₂uptake for 14 min in the postabsorptive state controls (Con),n = 12] or atminute 210 of a 285-min glucose infusion. In seven subjects, the infusion was constant(CI; 4 mg · kg¹ · min¹),and in seven subjects it was varied (VI) to mimic the exercise R_a response in Con. Although glucose suppressedR_a to zero (with glycemia ~6 mM and insulin ~150 pM),an endogenous R_a response to exercise occurred, to peakincrements two-thirds those in Con, in both CI and VI. Glucagon wasunchanged, and very small increases in the glucagon-to-insulin ratiooccurred in all three groups. Catecholamine responses were similar inall three groups, and correlation coefficients of R_a withplasma norepinephrine and epinephrine were significant in all. In allCI and VI, R_d at rest was 2× Con, increased earlierin exercise, and was higher for the 1 h of recovery with glucoseinfusion. Thus the R_a response was only partly attenuated,and the catecholamines are likely to be the regulators. This suggeststhat an acute endogenous R_a rise is possible even in thepostprandial state. Furthermore, the fact that more circulating glucoseis used by muscle during exercise and early recovery suggests thatmuscle glycogen is spared.