Preliminary studies on the characteristics of phenylalanine and β-methyl-glucoside transport in the tench intestine in vitro

Abstract

1. 1. The unidirectional transepithelial fluxes of L-phenylalanine, β-methyl-D-glucoside and sodium ions across emusculated sheets of tench mid-intestine were determined in flux chambers. 2. 2. No net sodium flux was detectable, but phenylalanine was preferentially transferred from the mucosal to the serosal fluid. 3. 3. There was also a net movement of β-methyl-glucoside towards the serosal medium, but it was much smaller than that of phenylalanine. 4. 4. This transport was accompanied by an accumulation of each substrate from the mucosal medium into the tissue to a similar level and against a concentration gradient. 5. 5. The poor transfer of the monosaccharide into the serosal medium could therefore be attributed to a low permeability of the baso-lateral membrane of the enterocyte for this substance. 6. 6. The influx of L-phenylalanine and of β-methyl- d-glucoside into the epithelial cells of tench midintestine was examined by incubating slices of emusculated intestine in radioactively-labelled solutions of the substrate for 2 min. 7. 7. The steady-state uptake was assessed after similar incubations lasting 45 min. 8. 8. Phenylalanine influx obeys the Michaelis-Menten equation with a K m of 2.9 mM and is dependent on the presence of sodium ions in the incubation medium. 9. 9. β-Methyl-glucoside influx reveals the same characteristics with a K m of 2.0 mM but a considerably lower V max; in addition, it is inhibited by galactose. 10. 10. The influx of both substrates is reduced by harmaline, which also inhibits the uptake of radioactive sodium by this preparation. 11. 11. The steady-state uptake of β-methyl-glucoside is also inhibited by ouabain and by 2.4-dinitrophenol. 12. 12. These results suggest that the mechanisms for sodium-dependent influx of monosaccharides and neutral amino-acids in the tench intestine are similar to those found in mammalian tissues. 13. 13. The principal difference appears to involve the release of monosaccharides across the baso-lateral membrane of the enterocyte.