Isoproterenol infusion test in anorexia nervosa: assessment of pre- and post-beta-noradrenergic receptor activity

Anorexia nervosa is associated with alterations in sympathetic activity and metabolism that persist during and after weight recovery. This study assessed beta-adrenergic receptor activity, an important modulator of vascular and metabolic function, in anorexic patients studied when underweight and at intervals during recovery, in comparison with healthy volunteer women. An increase in heart rate, in response to increasing doses of isoproterenol, served as an index of postsynaptic activity. Anorexic patients, during refeeding and weight gain, needed a significantly higher dose of isoproterenol to increase basal heart rate by 25 beats/minute, compared with underweight anorexic patients. Down-regulated postsynaptic cardiac beta-adrenoceptors during weight gain may protect against refeeding-induced exaggerated sympathetic activity. Because presynaptic beta-adrenoceptors serve as a positive feedback loop for synaptic catecholamine secretion, the increase in plasma norepinephrine concentrations during the isoproterenol infusion served as an index of presynaptic activity. We found that increasing doses of isoproterenol were associated with a linear increase in plasma norepinephrine in each healthy volunteer. In contrast, anorexic patients at any state had a significantly more erratic secretion of plasma norepinephrine in response to increasing doses of isoproterenol. Altered regulation of presynaptic adrenoceptors may explain, in part, the large variance and little consensus between previous studies as to whether anorexic patients have reduced or normal plasma norepinephrine levels.