Effect of NO−Precursor (L−Arginine) and NO−Synthase Inhibitor (L−NAME) on the Secretion of Thyroid Hormone and Thyrotropin (TSH) in Female Rats

Background. Nitric oxide (NO) plays an important role in various biological processes. It has been suggested that NO may modulate the functions of the endocrine system. The aim of this study was to examine the role of nitric oxide in total triiodothyronine (T3), free thyroxine (fT4) and TSH secretion in adult female Wistar rats. Material and Methods. The animals were treated with L−NAME (10 mg/kg IP) and L−arginine (100 mg/kg IP) daily for 7 consecutive days. On the day 8th plasma TSH concentration was estimated by immunoenzymatic micro− particle method, while thyroid hormones, T3 and fT4, were determined using radioimmunoassay method. Results. The plasma TSH concentration was very significantly diminished, following L−arginine treatment. On the other hand L−arginine was without effect on T3 and fT4 plasma level. L−NAME administrated for 7 days did not produce any significant changes in the plasma concentration of T3, fT4 and TSH. Conclusions. These findings may indicate that long−term L−arginine exposure decreases TSH secretion under ba− sal condition in female rats. However, the results of this study need further investigation that might be able to con− firm whether L−arginine action on TSH secretion is indeed mediated via NO (Adv. Clin. Exp. Med. 2004, 13, 1, 23–26).