Low lactate dehydrogenase and high mitochondrial glycerol phosphate dehydrogenase in pancreatic beta-cells. Potential role in nutrient sensing

Abstract

Nutrient metabolism was examined with regard to insulin secretion in purified rat islet beta- and non-beta-cells, beta-cell lines, and hepatocytes. Lactate dehydrogenase (LDH) activity (nanomoles.min-1.mg protein-1) was remarkably low in the glucose-sensitive INS-1 cell line (15.7) and in beta-cells (22.3). Thus, beta-cell LDH was respectively 8-, 122-, 17-, and 136-fold lower than in islet non-beta, liver, HIT-T15, and RINm5F cells. Plasma membrane lactate transport activity was 3-10-fold lower in beta- or INS-1 cells than in the other cell types. Conversely, mitochondrial glycerol phosphate dehydrogenase was strongly expressed only in beta- and INS-1 cells. The significance of these findings to nutrient recognition was explored using INS-1 cells as a model of native beta-cells. Glucose-stimulated lactate output and glucose utilization were, respectively, 12- and 5-fold lower in INS-1 than in RINm5F cells. Each process was entirely blocked by respiratory chain inhibitors in INS-1 cells, whereas glucose utilization was barely affected and lactate output stimulated in RINm5F cells. Glucose oxidation represented 73% of total utilization in INS-1 cells, but only 9% in RINm5F cells. Absolute rates of glucose oxidation, and the extent of mitochondrial NAD(P) reduction, were similar in the two cell types, and glucose stimulated insulin secretion 1.9-fold in INS-1 and 1.4-fold in RINm5F cells. The mitochondrial substrates, monomethyl succinate, pyruvate, and leucine, each triggered secretion in INS-1 cells. The balance of LDH, plasma membrane lactate transport, and mitochondrial glycerol phosphate dehydrogenase activities therefore appear to be important in beta- and INS-1 cell glucose recognition to ensure that mitochondrial oxidation is the principle fate of pyruvate and NADH produced by glycolysis. The resultant close coupling of glycolysis with mitochondrial oxidation explains the absence in beta-cells of Crabtree and Pasteur effects.