A Nuclear Magnetic Resonance-Based Demonstration of Substantial Oxidative l-Alanine Metabolism and l-Alanine-Enhanced Glucose Metabolism in a Clonal Pancreatic β-Cell Line

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Abstract
Early experiments indicated that islet β-cells substantially metabolized l-alanine but that insulin secretion was largely unaffected by the amino acid. It was subsequently demonstrated using more intricate studies that l-alanine is a strong stimulus to insulin secretion in the presence of glucose in normal rodent islets and β-cell lines. Using 13C nuclear magnetic resonance (NMR), we have demonstrated substantial oxidative metabolism of l-alanine by the clonal β-cell line BRIN-BD11, with time-dependent increases in production of cellular glutamate and aspartate. Stimulatory effects of l-alanine on insulin secretion were attenuated by the inhibition of β-cell oxidative phosphorylation using oligomycin. Additionally, we detected substantial production of lactate, alanine, and glutamate from glucose (16.7 mmol/l) after 60 min. On addition of 10 mmol/l l-alanine to a stimulus of 16.7 mmol/l glucose, the utilization rate of glucose increased ∼2.4-fold. l-Alanine dramatically enhanced NMR-measurable aspects of glucose metabolism (both oxidative and nonoxidative). The enhanced rate of entry of glucose-derived pyruvate into the tricarboxylic acid (TCA) cycle in the presence of alanine may have stimulated rates of generation of key metabolites, including ATP, which affect the insulin secretory process. Thus l-alanine metabolism, in addition to the enhancing effect on glucose metabolism, contributes to the stimulatory effects of this amino acid on insulin secretion in vitro.