Objective: Overweight and heightened sympathetic activity are more common in hypertensive than normotensive subjects. beta-adrenoceptor down-regulation has been described in hypertension. We tested the hypothesis that chronic sympathetic overactivity impairs beta-adrenergic-mediated thermogenesis and thereby favours gain of weight in hypertension.
Participants: The study included 13 hypertensive subjects aged 35.3 +/- 7.9 years and 25 normotensive subjects of control of similar age.
Methods: To measure beta-adrenergically mediated haemodynamic, metabolic and thermogenic responsiveness, increasing doses of isoproterenol diluted in 2.5 ml saline were injected as intravenous boluses (0.1, 0.25, 0.5, 1.0 and 2.0 microg/m). On a separate day, isoproterenol was infused continuously intravenously in increasing doses (10, 20 and 40 ng/kg per min), each dose for 30 min.
Results: The sitting heart rate and body mass were greater in hypertensives (P = 0.000, and P = 0.005, respectively). The heart rate responses to 1 and 2 microg/m isoproterenol bolus (P = 0.01 and P = 0.03, respectively) were reduced in hypertensives. The energy expenditure (P = 0.002) and oxygen consumption (P = 0.0004) increase with 40 ng/kg per min isoproterenol infusion, and glucose and phosphate responses at both 20 (P = 0.01 and P = 0.05) and 40 (P = 0.001 and P = 0.02) ng/kg per min isoproterenol infusion were attenuated in hypertensives. The baseline heart rate negatively correlated with heart rate (P = 0.015) response to isoproterenol bolus and blood pressure (P = 0.02) response to isoproterenol infusion. The urinary noradrenaline negatively correlated with heart rate response to isoproterenol bolus (P = 0.001), and with systolic blood pressure (P = 0.02) and energy expenditure responsiveness to isoproterenol infusion (P = 0.04). Furthermore, plasma noradrenaline negatively correlated with heart rate responsiveness to isoproterenol bolus (P = 0.004).
Conclusions: These results show a generalized decrease of beta-adrenergic responsiveness in stage 1 hypertension and support the concept that sympathetic overactivity, via down-regulation of beta-adrenoceptor-mediated thermogenic responses, may facilitate the development of obesity in hypertension.