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U Leonhardt, U Ritzel, G Schafer, W Becker and G Ramadori

Leptin, the product of the ob gene, is an important circulating signal for the regulation of body weight. In the present study the role of immunoreactive leptin (leptin-IR) was investigated in functional thyroid disease. Serum leptin-IR levels of 23 hypothyroid and 19 hyperthyroid patients were compared with 21 controls. Leptin-IR was quantified by a specific RIA. In hyperthyroid patients, leptin-IR was not different from controls. Serum leptin-IR levels were significantly increased in hypothyroid patients (21.0 +/- 2.7 micrograms/l vs controls 10.8 +/- 2.1 micrograms/l, P = 0.0044). When serum leptin of hypothyroid patients was compared with euthyroid controls of the same body mass index the difference was still significant (P = 0.0333 by paired Student's t-test). This might indicate that elevation of the serum leptin level does not merely reflect changes in body weight secondary to hypothyroidism, but might be increased to overcome the gain of body weight caused by hypothyroidism.

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U Leonhardt, E Gerdes, U Ritzel, G Schafer, W Becker and G Ramadori

In this study, plasma leptin concentrations were measured in rats artificially rendered hyper- or hypothyroid by administration of thyroxine or TRH, by administration of methimazole, or by thyroidectomy. Compared with those in untreated controls, leptin immunoreactivity was not affected in the hyperthyroid state, but was significantly increased in hypothyroid animals. Methimazole administration for longer time periods caused a stepwise increase in plasma leptin immunoreactivity. Greatest leptin concentrations were seen after 28 days of methimazole. Seven days after withdrawal of the methimazole, leptin concentrations no longer differed from those observed in control animals. In hypothyroid animals, expression of leptin mRNA was increased in both retroperitoneal and epididymal adipose tissue, whereas no difference was seen for subcutaneous or mesenteric fat. Incubation of rat leptin with plasma of eu- or hypothyroid rats and subsequent HPLC analysis of leptin plasma peaks gave no indication of an altered hormone stability. We conclude that, in hypothyroid rats, leptin concentrations may be increased as a result of stimulated leptin synthesis in retroperitoneal and epididymal adipose tissue.

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U Ritzel, U Leonhardt, M Ottleben, A Ruhmann, K Eckart, J Spiess and G Ramadori

Glucagon-like peptide-1 (GLP-1) is the most potent endogenous insulin-stimulating hormone. In the present study the plasma stability and biological activity of a GLP-1 analog, [Ser]GLP-1(7-36)amide, in which the second N-terminal amino acid alanine was replaced by serine, was evaluated in vitro and in vivo. Incubation of GLP-1 with human or rat plasma resulted in degradation of native GLP-1(7-36)amide to GLP-1(9-36)amide, while [Ser]GLP-1(7-36)amide was not significantly degraded by plasma enzymes. Using glucose-responsive HIT-T15 cells, [Ser]GLP-1(7-36)amide showed strong insulinotropic activity, which was inhibited by the specific GLP-1 receptor antagonist exendin-4(9-39)amide. Simultaneous i.v. injection of [Ser]GLP-1(7-36)amide and glucose in rats induced a twofold higher increase in plasma insulin levels than unmodified GLP-1(7-36)amide with glucose and a fivefold higher increase than glucose alone. [Ser]GLP-1(7-36)amide induced a 1.5-fold higher increase in plasma insulin than GLP-1(7-36)amide when given 1 h before i.v. application of glucose. The insulinotropic effect of [Ser]GLP-1(7-36)amide was suppressed by i.v. application of exendin-4(9-39)amide. The present data demonstrate that replacement of the second N-terminal amino acid alanine by serine improves the plasma stability of GLP-1(7-36)amide. The insulinotropic action in vitro and in vivo was not impaired significantly by this modification.