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

• Published in: American journal of physiology: endocrinology and metabolism Vol. 280; no. 5; p. E752
• Main Authors: Mora-Rodriguez, Ricardo, Hodgkinson, Bradley J, Byerley, Lauri O, Coyle, Edward F
• Format: Journal Article
• Language: English
• Published: 01.05.2001
• ISSN: 0193-1849; 1522-1555

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 -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 O 2 peak 1 ) during a control trial (CON); 2 ) while epinephrine was intravenously infused at 0.015 µg · kg 1 · min 1 ( -STIM); 3 ) after ingesting 80 mg of propranolol ( -BLOCK); and 4 ) combining -BLOCK with intravenous infusion of Intralipid-heparin to restore plasma fatty acid (FFA) levels ( -BLOCK+LIPID). -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 (R d ) of glucose (36 ± 2 vs. 22 ± 2 µmol · kg 1 · min 1 ; P  < 0.05). Plasma FFA restoration ( -BLOCK+LIPID) attenuated the increase in R d 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, -STIM markedly increased glycogen oxidation and reduced glucose clearance and fat oxidation despite elevating plasma FFA. Therefore, reduced plasma FFA availability with -BLOCK increased R d glucose, whereas -STIM increased glycogen oxidation, which reduced fat oxidation and glucose clearance. In summary, compared with control exercise at 45% O 2 peak (CON), both -BLOCK and -STIM reduced fat and increased carbohydrate oxidation, albeit through different mechanisms. lipid; glucose; propranolol; epinephrine; stable isotopes