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Catecholamine-Induced Adipocyte Lipolysis in Human Hyperthyroidism¹

Department of Medicine and Research Center, Karolinska Institute, Huddinge University Hospital, Huddinge, Sweden

Address all correspondence and requests for reprints to: Peter Arner, M.D., Department of Medicine, Huddinge Hospital, S-141 86 Huddinge, Sweden.

Increased lipid mobilization in thyrotoxicosis is attributed to amplification of catecholamine action in fat cells by thyroid hormones. We investigated the adrenergic regulation of lipolysis in isolated sc abdominal fat cells obtained from 14 patients with thyrotoxicosis and 18 control subjects. Ten of the hyperthyroid subjects were also reinvestigated after antithyroid treatment. The thyrotoxic state was associated with a 3-fold increase in maximum norepinephrine-induced lipolysis (P < 0.005), unaltered sensitivity to dobutamine (selective ß₁-adrenoceptor agonist) and clonidine (selective ₂-adrenoceptor agonist), but 15 times enhanced sensitivity to terbutaline (selective ß₂-adrenoceptor agonist; P < 0.01). Moreover, thyrotoxicosis was accompanied by a 3-fold increase in ß₂-adrenoceptor number (P < 0.005), but unchanged ß₁-adrenoceptor levels. Further, the lipolytic effects of dibutyryl cAMP (activating protein kinase A and thereby hormone-sensitive lipase) and forskolin (activating adenylate cyclase) were about 60% enhanced (P < 0.005). No change in the maximum activity of the hormone-sensitive lipase could be demonstrated in the hyperthyroid state compared to that in the euthyroid state. The observed abnormalities in lipolysis and ß₂-adrenoceptor number were normalized after antithyroid treatment. It is concluded that in human hyperthyroidism, the interactions between thyroid hormone and catecholamines in adipocytes involve abnormalities at both receptor and postreceptor levels. The former mechanism seems to be a selective increase in the expression of the ß₂-adrenoceptors. The latter mechanism involves increased ability of cAMP to activate hormone-sensitive lipase, but not a change in maximum enzyme capacity.

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