Escherichia coli heat stable enterotoxin receptors & guanylyl cyclase activity in the intestinal brush border membrane of hamsters & guinea pigs

Background & objectives : Although Escherichia coli heat stable enterotoxin (STa) causes diarrhoea in laboratory animals, no studies were done to find out the species specific variation of distribution of the STa receptors in laboratory animals. The present investigation evaluates the density of STa receptors and the guanylyl cyclase (GC) activity in the small intestinal e pithelial cells of hamsters and guinea pigs. Methods : Brush border membrane (BBM) was prepared from the small intestine s of hamsters and guinea pigs. Receptor binding assay, GC assay and autorad iography were performed to determine the density of STa receptors, the CC a ctivity and molecular weights of the STa binding proteins respectively. Results : The receptor densities, per mg BBM protein at equilibrium, were f ound to be 4.1 x 10(9) and 1.5 x 10(12) in hamsters and guinea Digs respect ively. The GC activity was found to be lower in STa treated hamster BBM com pared to that of guinea pig. Scatchard analysis of the stoichiometric data showed a linear plot, and STa bound with association constants of 0.31 x 10 (12) M-1 and 1.04 x 10(12) m(-1) in hamsters and guinea pigs respectively. Autoradiographic analysis of the SDS-PAGE, revealed that I-125-STa bound ap parently to a 45 kDa membrane protein in hamster and a 115 kDa membrane pro tein in guinea pig. Interpretation & conclusions: It appears that a lower density of STa recept or exists in hamsters compared to that in guinea pigs. STa binds with a sin gle population of STa receptors in each species with different ligand bindi ng affinities. Also, the molecular weights of the STa binding proteins diff er in these species. Moreover, the GC activity was found to be lower in ham sters than in guinea pigs.