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Muscle Sympathetic Nerve Activity in Patients with Acromegaly

Departments of Internal Medicine (B.C., G.L., V.R., R.G., L.S.) and Endocrinology (P.M., A.C., G.L.), University Federico II, 80131 Naples, Italy

Address correspondence and requests for reprints to: Luigi Saccà, M.D., Department of Internal Medicine, University Federico II, Via Pansini 5, 80131 Naples, Italy. E-mail: sacca{at}unina.it

Muscle sympathetic nerve activity was measured in nine acromegalic patients (age, 35 ± 4 yr; body mass index, 28 ± 2 kg/m²) and eight healthy subjects (age, 32 ± 3 yr; body mass index, 25 ± 2 kg/m²) by combining the forearm arterial-venous difference technique with the tracer method [infusion of tritiated norepinephrine (NE)]. Muscle NE release was quantified both at rest and during physiological hyperinsulinemia while maintaining euglycemia (90 mg/dL) by means of the euglycemic clamp.

Arterial plasma NE was similar in the two groups at rest (197 ± 28 and 200 ± 27 pg/mL^-1) and slightly increased during insulin infusion. Forearm NE release was 2.33 ± 0.55 ng·liter^-1·min^-1 in healthy subjects and 2.67 ± 0.61 ng·liter^-1·min^-1 in acromegalic subjects in the basal state and increased to a similar extent during insulin infusion in both groups (3.13 ± 0.71 and 3.32 ± 0.75 ng·L^-1· min^-1, P < 0.05 vs. basal), indicating a normal stimulatory effect of insulin on muscle sympathetic activity. In contrast, insulin-stimulated forearm glucose uptake was markedly lower in acromegalic patients (2.3 ± 0.4 mg·L^-1·min^-1) than in control subjects (7.9 ± 1.3 mg·L^-1·min^-1, P < 0.001), indicating the presence of severe insulin resistance involving glucose metabolism.

Our data demonstrate that patients with long-term acromegaly have normal sympathetic activity in the skeletal muscle in the basal, postabsorptive state and normal increments in NE spillover in response to the sympatho-excitatory effect of insulin. Thus, the presence of severe insulin resistance in acromegaly is not accounted for by adrenergic mechanisms.

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