Adrenaline but not noradrenaline is a determinant of exercise-induced lipid mobilization in human subcutaneous adipose tissue

Abstract

The relative contribution of noradrenaline (norepinephrine) and adrenaline (epinephrine) in the control of lipid mobilization in subcutaneous adipose tissue (SCAT) during exercise was evaluated in men treated with a somatostatin analogue, octreotide. Eight lean and eight obese young men matched for age and physical fitness performed 60 min exercise bouts at 50% of their maximal oxygen consumption on two occasions: (1) during i.v. infusion of octreotide, and (2) during placebo infusion. Lipolysis and local blood flow changes in SCAT were evaluated using in situ microdialysis. Infusion of octreotide suppressed plasma insulin and growth hormone levels at rest and during exercise. It blocked the exercise-induced increase in plasma adrenaline while that of noradrenaline was unchanged. Plasma natriuretic peptides (NPs) level was higher at rest and during exercise under octreotide infusion in lean men. Under placebo, no difference was found in the exercise-induced increase in glycerol between the probe perfused with Ringer solution alone and that with phentolamine (an alpha-adrenergic receptor antagonist) in lean subjects while a greater increase in glycerol was observed in the obese subjects. Under placebo, propranolol infusion in the probe containing phentolamine reduced by about 45% exercise-induced glycerol release; this effect was fully suppressed under octreotide infusion while noradrenaline was still elevated and exercise-induced lipid mobilization maintained in both lean and obese individuals. In conclusion, blockade of beta-adrenergic receptors during exercise performed during infusion of octreotide (blocking the exercise-induced rise in adrenaline but not that of noradrenaline) does not alter the exercise-induced lipolysis. This suggests that adrenaline is the main adrenergic agent contributing to exercise-induced lipolysis in SCAT. Moreover, it is the combined action of insulin suppression and NPs release which explains the lipolytic response which remains under octreotide after full local blockade of fat cell adrenergic receptors. For the moment, it is unknown if results apply specifically to SCAT and exercise only or if conclusions could be extended to all forms of lipolysis in humans.

Figure 1. Plasma noradrenaline and adrenaline concentrations at rest, during exercise and the recovery period under intravenous infusion of placebo or octreotide (30 ng min⁻¹ kg⁻¹) in lean subjects

Data are expressed as mean ±s.e.m. *P < 0.05 significant when compared to values obtained with octreotide infusion.

Figure 4. Dialysate glycerol concentration from SCAT measured at rest, during exercise and the recovery period under intravenous infusion of placebo (A) or octreotide (30 ng min⁻¹ kg⁻¹; B) in lean subjects

Probes were perfused with Ringer solution alone or with Ringer solution plus phentolamine (100 μmol l⁻¹). Data are expressed as mean ±s.e.m.

Figure 3. Mean changes in dialysate glycerol concentrations (DGC) in subcutaneous adipose tissue (SCAT) promoted by exercise in obese subjects under intravenous infusion of (A) placebo and (B) octreotide (30 ng min⁻¹ kg⁻¹)

One probe was perfused with Ringer solution alone (Ringer), the second with phentolamine (100 μmol l⁻¹) (phentolamine) and the third one with Ringer solution plus phentolamine (100 μmol l⁻¹) plus propranolol (100 μmol l⁻¹) (phentolamine + propranolol). Data are expressed as mean ±s.e.m. P= 0.03 significant when compared to control (Ringer) values and P= 0.04 significant when compared to phentolamine values.

Figure 2. Dialysate glycerol concentration from subcutaneous adipose tissue (SCAT) measured at rest, during exercise and the recovery period under intravenous infusion of placebo or octreotide (30 ng min⁻¹ kg⁻¹) in lean subjects

Probes were perfused with Ringer solution alone. Data are expressed as mean ±s.e.m. *P < 0.05 significant when compared to values obtained with octreotide infusion.

Figure 5. Dialysate glycerol concentration from subcutaneous adipose tissue (SCAT) measured at rest, during exercise and the recovery period under intravenous infusion of placebo (A) or octreotide (30 ng min⁻¹ kg⁻¹; B) in lean subjects

One probe was perfused with Ringer solution plus phentolamine (100 μmol l⁻¹) and the second one with Ringer solution plus phentolamine (100 μmol l⁻¹) plus propranolol (100 μmol l⁻¹). Data are expressed as mean ±s.e.m. *P < 0.05 significant when compared to values obtained in the probe with phentolamine plus propranolol and the Ringer probe with phentolamine alone.

Figure 6. Ethanol ratio in SCAT measured at rest, during exercise and the recovery period under intravenous infusion of placebo or octreotide (30 ng min⁻¹ kg⁻¹) in lean subjects

Each column represents the mean values before infusion and during infusion.