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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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The succinic dehydrogenase (SDHase) and adenosinetriphosphatase (ATPase) activities were determined in the corpora lutea, ovarian interstitial tissue and endometria of pseudopregnant (PP) rabbits and rabbits in which superovulation was produced by treatment with pituitary gonadotrophins (SO). Statistical analysis of the results showed that the weight, SDHase, ATPase and percentage solids of the corpora lutea, and the SDHase of the endometria of each of the two groups varied significantly (P<0·01) between days. The day values for the SDHase of the endometria, however, were particularly high in SO rabbits at the 2-day stage and low in the 12- to 16-day stages as compared with the PP group. Other differences found were of less statistical significance.

The results are considered in relation to the poor survival of embryos formed from eggs of SO rabbits. The lesser weight of the corpora lutea and the lesser SDHase activity of the endometria of the SO group, as compared with the PP group during the latter half of the experimental period, may be significant in relation to the poorer survival of embryos.

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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.