Effect of short- and long-term β-adrenergic blockade on lipolysis during fasting in humans

Stable isotope tracers and indirect calorimetry were used to evaluate the importance of β-adrenergic stimulation of lipolysis and triglyceride-fatty acid cycling during fasting in healthy human volunteers. Each subject was studied after 12 and 84 h of fasting both with and without propranolol infusion (protocol 1) and when oral propranolol treatment was given throughout fasting (protocol 2). In protocol 1, the rates of appearance of glycerol and palmitic acid increased from 3.04 ± 0.19 and 1.78 ± 0.17 μmol · kg lean body mass^-1 · min^-1, respectively, after 12 h of fasting to 5.28 ± 0.31 and 3.47 ± 0.15 μmol · kg lean body mass^-1 · min^-1, respectively, after 84 h of fasting (P < 0.005). The rate of triglyceride-fatty acid cycling increased from 97 ± 8 to 169 ± 5 μmol/min (P < 0.005). Intravenous propranolol infusion decreased the rate of lipolysis after both 12 and 84 h of fasting, but the magnitude of the antilipolytic effect was much greater after 84 h (P < 0.005). In protocol 2, the rate of lipolysis and triglyceride-fatty acid cycling was still increased by fasting despite β-adrenergic blockade with oral propranolol. This study demonstrates that β-adrenergic stimulation contributes to the mobilization of fat during fasting. However, other mechanism(s) can increase lipolysis and triglyceride-fatty acid cycling when β-adrenergic receptors are continuously blocked.