Norepinephrine infusion during moderate-intensity exercise increases glucose production and uptake

A role for the increase in circulating norepinephrine (NE) during intense e xercise [IE; greater than or equal to 80% maximum O-2 uptake ((V) over dot O-2max)] in the marked increment in glucose rate of production (Ra) during IE is hypothesized. Seven fit male subjects (27 +/- 2 yr old; body mass ind ex, 23 +/- 1 kg/m(2); (V) over dot O-2max, 63 +/- 5 mL/kg.min) underwent 40 min of postabsorptive moderate-intensity (53% (V) over dot O-2max) cycle e rgometer exercise (126 +/- 14 W), once without [control (CON)I and once wit h NE infusion (0.1 mug/kg.min) from 30-40 min (NE). With infusion, plasma N E reached 15.9 +/- 1.0 nM (8-fold rest, a-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 = 0.0 03). Ra correlated strongly (r(2) = 0.92, P < 0.02) with plasma NE during a nd immediately after infusion. With NE infusion, peak glucose uptake Irate of disappearance (Rd), 6.57 +/- 0.59 us. 4.53 +/- 0.55 mg/kg.min, P < 0.02] and glucose metabolic clearance rate (P < 0.05) were higher than in CON. G lycemia rose minimally during the NE infusion but did not differ between gr oups at any time during exercise. Glucagon-to-insulin ratio increased minim ally, and epinephrine increased approximately 2.5- to 3-fold at peak but di d not differ between groups. Thus, NE infusion during moderate exercise led to increments in Ra and Rd in fit individuals, supporting a possible contr ibutory role for the increase of plasma NE in IE. NE effects on Rd and meta bolic clearance rate during exercise may differ from its effects at rest.