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Am J Physiol Endocrinol Metab 280: E752-E760, 2001;
0193-1849/01 $5.00
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Vol. 280, Issue 5, E752-E760, May 2001

Effects of beta -adrenergic receptor stimulation and blockade on substrate metabolism during submaximal exercise

Ricardo Mora-Rodriguez1,2, Bradley J. Hodgkinson1, Lauri O. Byerley3, and Edward F. Coyle1

1 The Human Performance Laboratory, Department of Kinesiology and Health Education, University of Texas at Austin, Austin, Texas 78712; 2 University of Castilla-la Mancha at Toledo, Toledo 45071, Spain; and 3 Pennington Biomedical Research Center, Baton Rouge, Louisiana 70808

We used beta -adrenergic receptor stimulation and blockade as a tool to study substrate metabolism during exercise. Eight moderately trained subjects cycled for 60 min at 45% of VO2 peak 1) during a control trial (CON); 2) while epinephrine was intravenously infused at 0.015 µg · kg-1 · min-1 (beta -STIM); 3) after ingesting 80 mg of propranolol (beta -BLOCK); and 4) combining beta -BLOCK with intravenous infusion of Intralipid-heparin to restore plasma fatty acid (FFA) levels (beta -BLOCK+LIPID). beta -BLOCK suppressed lipolysis (i.e., glycerol rate of appearance) and fat oxidation while elevating carbohydrate oxidation above CON (135 ± 11 vs. 113 ± 10 µmol · kg-1 · min-1; P < 0.05) primarily by increasing rate of disappearance (Rd) of glucose (36 ± 2 vs. 22 ± 2 µmol · kg-1 · min-1; P < 0.05). Plasma FFA restoration (beta -BLOCK+LIPID) attenuated the increase in Rd glucose by more than one-half (28 ± 3 µmol · kg-1 · min-1; P < 0.05), suggesting that part of the compensatory increase in muscle glucose uptake is due to reduced energy from fatty acids. On the other hand, beta -STIM markedly increased glycogen oxidation and reduced glucose clearance and fat oxidation despite elevating plasma FFA. Therefore, reduced plasma FFA availability with beta -BLOCK increased Rd glucose, whereas beta -STIM increased glycogen oxidation, which reduced fat oxidation and glucose clearance. In summary, compared with control exercise at 45% VO2 peak (CON), both beta -BLOCK and beta -STIM reduced fat and increased carbohydrate oxidation, albeit through different mechanisms.

lipid; glucose; propranolol; epinephrine; stable isotopes


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