Effect of Growth Hormone on High Plasma Levels of Glucagon-Like Peptide-1 (GLP-1) in Hypophysectomized Rats

Summary

Fasting plasma GLP-1 levels were significantly higher in hypophysectomized (hypox) rats (n = 6) than in intact (normal) rats (n = 7) (54.3 ± 5.2 vs. 33.3 ± 2.4 pmol/L, p < 0.001). To examine the influence of pituitary hormones on plasma GLP-1 levels, concentrations of plasma glucose, insulin and GLP-1 after an oral glucose load to hypox rats that were given either rat growth hormone (rGH) (n = 7), cortisol and thyroxine (n = 7) or no substitution (n = 6) were compared with those of normal rats (n = 7). Plasma glucose levels in the fasting state and after the glucose ingestion were significantly lower in hypox rats, but the hormonal replacements to hypox rats increased their total glucose levels to those of normal rats, although the increasing patterns were different from those in normal rats. Insulin levels both in the fasting state and after the glucose ingestion were significantly decreased in hypox rats and the fasting and total GLP-1 levels were significantly increased in those rats. rGH substitution significantly increased the total insulin levels in hypox rats and decreased the fasting and total GLP-1 levels closely to levels in normal rats, while substitution with cortisol and thyroxine failed to introduce such a significant effect. These results suggested that secretion of GLP-1 might be influenced by the function of GH.

Key words:

Hypophysectomized rat - plasma GLP-1 - GH - oral glucose tolerance test

References

• 1 Altszuler N, Rathgeb I, Winkler B, deBodo R C, Steele R. The effects of growth hormone on carbohydrate and lipid metabolism in the dog.  Ann NY Acad Sci. 1968;  148 441-458
• 2 Davidson M B. Effect of growth hormone on carbohydrate and lipid metabolism.  Endocr Rev. 1987;  8 115-131
• 3 Hiroyoshi M, Tateishi K, Yasunami Y, Maeshiro K, Ono J, Matsuoka Y, Ikeda S. Elevated plasma levels of glucagon-like peptide-1 after oral glucose ingestion in patients with pancreatic diabetes.  Am J Gastroenterol. 1999;  94 976-981
• 4 Jorgensen J OL, Rosenfalck A M, Fisker S, Nyholm B, Fineman M S, Schmitz O, Madsbad S, Holst J J, Christiansen J S. Circulating levels of incretin hormones and amylin in the fasting state and after oral glucose in GH-deficient patients before and after GH replacement: a placebo-controlled study.  Eur J Endocrinol. 2000;  143 593-599
• 5 Levin R J. The effects of hormones on the absorptive, metabolic, and digestive functions of the small intestine.  J Endocrinol. 1969;  45 315-348
• 6 Nauck M A. Is glucagon-like peptide 1 an incretin hormone?.  Diabetologia. 1999;  42 373-379
• 7 Oscarsson J, Olofsson S O, Bondjers G, Eden S. Differential effects of continuous versus intermittent administration of growth hormone to hypophysectomized female rats on serum lipoproteins and their apoproteins.  Endocrinology. 1989;  125 1638-1649
• 8 Penhos J C, Castillo L, Voyles N, Gutman R, Lazarus N, Recant L. Decreased glucose tolerance in the rat following hypophysectomy.  Endocrinology. 1971;  88 1141-1149
• 9 Pierluissi J, Pierlussi R, Ashcroft S JH. Effects of growth hormone on insulin release in the rat.  Diabetologia. 1980;  19 391-396
• 10 Turton M D, O'Shea D, Gunn I, Beak S A, Edwards C MB, Meeran K, Choi S J, Taylor G M, Heath M M, Lambert P D, Wilding J PH, Smith D M, Ghatei M A, Herbert J, Bloom S R. A role for glucagon-like peptide-1 in the central regulation of feeding.  Nature. 1996;  379 69-72