Alterations in β-adrenergic sensitivity and platelet α2-adrenoceptors in obese women: effect of exercise and caloric restriction

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Abstract
1. Peripheral adrenergic responses were studied in eight obese women before and after 15 days of caloric restriction (2500 kJ/day) and in eight sex- and age-matched lean controls. 2. .beta.-Adrenergic sensitivity (defined as the dose of isoprenaline required to increase resting heart rate by 25 beats/min) was evaluated before and after the diet. Density and affinity (determined as the apparent dissociation constant) of platelet .alpha.2-adrenergic receptors, and plasma adrenaline and noradrenaline levels, were measured after overnight bed-rest and after 9 min of standardized exercise performed before and after the low caloric diet. 3 Before the diet basal antecubital venous plasma noradrenaline concentrations were lower in obese women when compared with lean women (0.94 .+-. 0.06 vs 1.27 .+-. 0.17 nmol/l, P < 0.02). Isoprenaline sensitivity did not differ between lean and obese women. 4. At rest, platelet .alpha.2-adrenoceptor density was lower in overweight than in lean women (129 .+-. 21 vs 168 .+-. 16 fmol/mg of protein, P < 0.02). Exercise significantly increased platelet .alpha.2-adrenoceptor density and decreased affinity in lean women. This decrease correlated with the rise in plasma noradrenaline. 5. In obese women exercise did not modify platelet .alpha.2-adrenoceptor density or affinity, despite a significant increase in plasma catecholamines. However, the increase in plasma noradrenaline during exercise was lower in obese women. 6. The low caloric diet produced a .beta.-adrenergic supersensitivity. 7. We conclude that (a) obesity does not alter .beta.-adrenergic sensitivity; (b) 15 days of caloric restriction produces a .beta.-adrenergic supersensitive state; (c) obese women show an inability to desensitize platelet .alpha.2-adrenoceptors in response to exercise.