Effect of growth hormone on high plasma levels of glucagon-like peptide-1 (GLP-1) in hypophysectomized rats

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 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.