Norepinephrine Infusion during Moderate-Intensity Exercise Increases Glucose Production and Uptake1

A role for the increase in circulating norepinephrine (NE) during intense exercise [IE; ≥80% maximum O2 uptake (V̇O2max)] in the marked increment in glucose rate of production (Ra) during IE is hypothesized. Seven fit male subjects (27 ± 2 yr old; body mass index, 23 ± 1 kg/m2; V̇O2max, 63 ± 5 mL/kg·min) underwent 40 min of postabsorptive moderate-intensity (53% V̇O2max) cycle ergometer exercise (126 ± 14 W), once without [control (CON)] and once with NE infusion (0.1 μg/kg·min) from 30–40 min (NE). With infusion, plasma NE reached 15.9 ± 1.0 nm (8-fold rest, 2-fold CON). Ra doubled to 4.40 ± 0.44 in CON, but rose to 7.55 ± 0.68 mg/kg·min with NE infusion (P = 0.003). Ra correlated strongly (r2 = 0.92, P < 0.02) with plasma NE during and immediately after infusion. With NE infusion, peak glucose uptake [rate of disappearance (Rd), 6.57 ± 0.59 vs. 4.53 ± 0.55 mg/kg·min, P< 0.02] and glucose metabolic clearance rate (P< 0.05) were higher than in CON. Glycemia rose minimally during the NE infusion but did not differ between groups at any time during exercise. Glucagon-to-insulin ratio increased minimally, and epinephrine increased approximately 2.5- to 3-fold at peak but did not differ between groups. Thus, NE infusion during moderate exercise led to increments in Ra and Rd in fit individuals, supporting a possible contributory role for the increase of plasma NE in IE. NE effects on Rd and metabolic clearance rate during exercise may differ from its effects at rest.