Sitagliptin improved glucose assimilation in detriment of fatty-acid utilization in experimental type-II diabetes: role of GLP-1 isoforms in Glut4 receptor trafficking

BACKGROUND: The distribution of glucose and fatty-acid transporters in the heart is crucial for energy consecution and myocardial function. In this sense, the glucagon-like peptide-1 (GLP-1) enhancer, sitagliptin, improves glucose homeostasis but it could also trigger direct cardioprotective actions, including regulation of energy substrate utilization. METHODS: Type-II diabetic GK (Goto-Kakizaki), sitagliptin-treated GK (10 mg/kg/day) and wistar rats (n = 10, each) underwent echocardiographic evaluation, and positron emission tomography scanning for [(18)F]-2-fluoro-2-deoxy-d-glucose ((18)FDG). Hearts and plasma were isolated for biochemical approaches. Cultured cardiomyocytes were examined for receptor distribution after incretin stimulation in high fatty acid or high glucose media. RESULTS: Untreated GK rats exhibited hyperglycemia, hyperlipidemia, insulin resistance, and plasma GLP-1 reduction. Moreover, GK myocardium decreased (18)FDG assimilation and diastolic dysfunction. However, sitagliptin improved hyperglycemia, insulin resistance, and GLP-1 levels, and additionally, enhanced (18)FDG uptake and diastolic function. Sitagliptin also stimulated the sarcolemmal translocation of the glucose transporter-4 (Glut4), in detriment of the fatty acyl translocase (FAT)/CD36. In fact, Glut4 mRNA expression and sarcolemmal translocation were also increased after GLP-1 stimulation in high-fatty acid incubated cardiomyocytes. PI3K/Akt and AMPKα were involved in this response. Intriguingly, the GLP-1 degradation metabolite, GLP-1(9-36), showed similar effects. CONCLUSIONS: Besides of its anti-hyperglycemic effect, sitagliptin-enhanced GLP-1 may ameliorate diastolic dysfunction in type-II diabetes by shifting fatty acid to glucose utilization in the cardiomyocyte, and thus, improving cardiac efficiency and reducing lipolysis.