Oral triacylglycerols regulate plasma glucagon-like peptide-1(7-36) and insulin levels in normal and especially in obese rats.
In a previous study of glucose tolerance, plasma insulin levels were greatly elevated in genetically obese Wistar fatty rats but not lean rats fed a diet containing polyunsaturated fatty acids. In the present study, triacylglycerol-regulation of levels of circulating insulin and glucagon-like peptide-1 (7-36) (GLP-1) has been investigated in these rats. In the glucose tolerance test, the two plasma insulin peaks appeared in obese and lean rats intubated with glucose + corn oil, at 15- 30 min and 4 h, whereas only the first peak appeared in rats intubated with glucose alone, although the glucose response did not differ. After intubation of corn oil only, the insulin peak at 15 min was not detected but the peak at 4h was large. The two plasma GLP-1 peaks appeared 15 min and 4 h after intubation of glucose + corn oil similarly to the insulin responses, although the first peak was small and the second peak was very large. A small peak at 15 min was not significant in rats intubated glucose alone and no peak was seen at 4 h. The GLP-1 concentrations were significantly higher in the following order: portal vein > inferior vena cava > tail vein. The plasma GLP-1 increment in response to oral triacylglycerols was significantly higher in obese rats than in lean rats as was the insulin increment. Thus, oral triacylglycerols (possibly polyunsaturated) appeared to act at the gut lumen to stimulate GLP-1 secretion, which may be responsible for the second (4 h) insulin peak.