After intravenous glucose/insulin infusion there is an increase in oxygen consumption and energy expenditure that has been referred to as thermogenesis. To examine the contribution of the beta and alpha adrenergic nervous system to this thermogenic response, 12 healthy volunteers participated in three studies: (a) euglycemic insulin (plasma insulin ~ 100 μU/ml) clamp study (n = 12); (b) insulin clamp study after beta adrenergic blockade with intravenous propranolol for 1 h (n = 12); (c) insulin clamp study after alpha adrenergic blockade with phentolamine for 1 h (n = 5). During the control insulin clamp study total glucose uptake, glucose oxidation and nonoxidative glucose uptake averaged 7.85±0.47, 2.62±0.22, and 5.23±0.51 mg/kg.min. After propranolol infusion, insulin-mediated glucose uptake was significantly reduced, 6.89±0.41 (P < 0.02). This decrease was primarily the result of a decrease in glucose oxidation (1.97±0.19 mg/kg.min, P < 0.01) without any change in nonoxidative glucose metabolism. Phentolamine administration had no effect on total glucose uptake, glucose oxidation, or nonoxidative glucose disposal. The increments in energy expenditure (0.10±0.01 vs 0.03±0.01 kcal/min) and glucose/insulin-induced thermogenesis (4.9±0.5 vs. 1.5±0.5%) were reduced by 70% during the propranolol/insulin clamp study. The increments in energy expenditure (0.12±0.03 kcal/min) and thermogenesis (5.0±1.5%) were not affected by phentolamine. These results indicate that activation of the beta adrenergic receptor plays an important role in the insulin/glucose-mediated increase in energy expenditure and thermogenesis. In contrast, the alpha adrenergic receptor does not appear to participate in this response.
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