Effects of Chronic β-Receptor Stimulation on Sympathetic Nervous System Activity, Energy Expenditure, and Thyroid Hormones*

Abstract

The effects of hyper- and hypothyroidism on sympathetic nervous system activity and energy expenditure are well recognized. The impact of altered sympathetic nervous system activity on energy expenditure and thyroid hormone metabolism has not been well studied. We investigated the effects of orally administered terbutaline sulfate,a βreceptor agonist (5 mg, three times per day for 2 weeks), on the activity of the sympathetic nervous system, energy expenditure, and thyroid hormone metabolism in six normal men, aged 21–36 yr. The cardiovascular, metabolic, and thermogenic responses to an infusion of the βadrenergic agonist isoproterenol were clearly blunted after 2 weeks oftreatment with terbutaline sulfate, indicating down-regulation of βreceptors and/ordevelopment of reduced sensitivity. There were no significant changes in the cardiovascular, metabolic, or thermogenic responses to an infusion of the α-adrenergic agonist phenylephrine. Basal metabolic rate was significantly increased by the chronic administration of terbutaline sulfate [5.040 ± 0.167 (±SE) vs. 5.421 ± 0.234 kJ/ min; P < 0.05]. There was a highly significant change in the serum T₃ to T₄ ratio (19.4 ± 1.0 vs. 24.4 ± 1.0; P < 0.001). This was a result of increased serum T₃ concentrations (136 ± 9 vs. 160 ± 14 ng/dl; P < 0.05) and decreased serum T₄ concentrations (7.2 ± 0.8 vs. 6.7 ± 0.8 µg/dl; P = NS). Chronic µ-receptor stimulation with terbutaline sulfate increases the basal metabolic rate and T₃ concentrations. These changes occurred despite down-regulation of µ-receptors and/or decreased sensitivity in response to chronic terbutaline administration.