The long-term effect of low protein diet on the somatostatin hypothalamic neuronal system and the pituitary growth hormone cells in growing ewe

 

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Summary

Three-month old Polish Lowland female lambs were fed isocaloric diets containing 14.2% (standard) or 8.1% of proteins for twenty weeks. Changes in body weight were characteristic for normal growth performance in all animals, but daily body gain calculated for the whole experimental period was significantly lower in lambs fed a low protein diet (87.9 ± 13.5 and 158 ± 13.8 g/day, respectively). Two series of blood collections (4 hrs with 15 min interval) were performed at age of 21 and 26 weeks in order to analyze the growth hormone (GH) concentration in the peripheral blood. The results obtained by radioimmunoassay showed that at both ages the mean concentration of GH was significantly higher in the group fed a low protein diet (4.84 ± 2.23 and 3.68 ± 1.86 vs 1.46 ± 0.72 and 1.48 ± 0.44 ng/ml, respectively) and this difference was associated with significant elevation of the pulse amplitude (3.83 ± 4.23 and 4.54 ± 3.06 vs 1.48 ± 1.11 and 1.31 ± 0.68 ng/ml, respectively). Using immunocytochemistry, the somatostatin in the hypothalamus and the GH in the pituitary cells were analyzed in all animals slaughtered at age of 8 months. Lowering the content of dietary proteins diminished markedly the content of immunoreactive somatostatin in the median eminence (ME) and augmented the concentrations of the immunoproduct in the somatostatin perikarya. In the pituitary gland, a marked increase of the number of GH-producing cells was observed. The results obtained indicate that chronic restriction of dietary proteins, irrespective of sufficient energy supply, augment the secretion of GH via a decrease in the hypothalamic somatostatin output due to the suppression of its axonal transport.