Effect of Cholera Toxin on Rabbit Jejunal Carbohydrate-Metabolizing Enzymes

Abstract

Cholera enterotoxin is a well studied, remarkable polypeptide which causes intestinal secretion by increasing the mucosal concentration of cAMP. Little is known about the effect of cholera toxin on intestinal metabolism other than alterations of intracellular cyclic nucleotide levels. This study was designed to investigate the effect of cholera toxin on key enzymes mediating carbohydrate metabolism in rabbit jejunum. Biopsies of proximal jejunum were maintained for up to 12 hr in organ culture, and the effect of purified cholera toxin on mucosal cAMP, fructose diphosphatase, pyruvate kinase, and fructose diphosphate aldolase was examined. After exposure to 10 μg per ml of cholera toxin, mucosal cAMP increased significantly by 4 hr of incubation. Fructose diphosphatase activity, an enzyme unique to gluconeogenesis, also increased significantly after incubation with toxin. Pyruvate kinase, an enzyme unique to glycolysis, was reciprocally decreased after exposure to cholera toxin, whereas activity of fructose diphosphate aldolase, an enzyme which catalyzes a reversible reaction common to both glycolysis and gluconeogenesis, remained unchanged. To further characterize the mechanism of these changes, the phosphodiesterase inhibitor, theophylline, was used instead of cholera toxin to increase cAMP. After incubation with 10-3 m theophylline, mucosal cAMP levels were noted to be increased within 15 min of exposure. Similar but more rapid alterations were noted in carbohydrate-metabolizing enzymes after theophylline exposure when compared with cholera toxin. Fructose diphosphatase activity increased and pyruvate kinase activity was reciprocally decreased, whereas fructose diphosphate aldolase remained unchanged. We conclude that cholera enterotoxin has a profound effect on the activities of key carbohydrate-metabolizing enzymes in intestinal mucosa characterized by enhancement of gluconeogenesis and reciprocal depression of glycolysis. Moreover, the mechanism of these changes appears to be mediated by toxin- stimulated increase in mucosal cAMP concentration. Lastly, the organ culture system is a useful method to study the effect of bacterial enterotoxins on intestinal mucosa.