Nitrate-Induced Hypothyroidism is Associated with a Reduced Concentration of Growth Hormone-Releasing Factor in Hypothalamic Tissue of Rats

Abstract

Dietary nitrate significantly inhibits the growth of male and female rats. To test the possibility that the growth hormone-releasing factor (GRF) content in hypothalamic tissue is deranged under these conditions, male and female rats were fed a diet containing 3% KNO3 for 6 weeks, compared to a normal diet (4 X 5 animals). The food intake of rats fed nitrate was reduced significantly (23 and 28% resp.). Weight gain was also decreased by 35 and 41% in male and female rats. The mean Sm-C/IGF-I concentration was 1.61 and 1.03 rU/ml in male and female control rats, whereas the concentrations in nitrate-exposed rats were 0.92 and 0.64, respectively (P less than 0.01). The GRF content of hypothalamic tissue also decreased significantly from 407 and 533 ng/g protein in controls to 174 and 229 in treated male and female rats. Nitrate exposure is characterized by hypothyroidism, food intake depression, low Sm-C/IGF-I concentrations in plasma and a decreased hypothalamic GRF content. Independent of the peripheral changes, the content of Sm-C/IGF-I in the brain remains constant. The results of the study demonstrate that thyroid hormone deficiency leads to an inhibition of GH axis already at the hypothalamic level.