Kinetics of uptake of glucose in rabbit jejunum: influence of sodium, unstirred layers, and passive permeation

Abstract

Failure to account for the effect of the unstirred water layer and the contribution of passive permeation will lead to errors in the estimation of the kinetic constants of glucose uptake into the intestine. It is widely accepted that variations in the concentration of sodium in the bulk phase profoundly influence the rate of uptake of glucose in the intestine, but the kinetic basis for this effect remains in dispute. Accordingly, a previously validated in vitro technique was used to assess the effect of Na+ on the uptake of glucose into rabbit jejunum under conditions selected to reduce the unstirred layer resistance. Varying Na+ had no effect on the uptake of lauryl alcohol and therefore on unstirred layer resistance. The passive permeability coefficient for glucose uptake was estimated from the uptake of L-glucose, of D-glucose at 4 °C, or in the presence of 1 mM phlorizin or 40 mM galactose. The permeability for glucose increased as Na+ rose. The values of both the maximal transport rate and the Michaelis constant (Km) were influenced by Na+. A linear relationship was noted between Na+ and the maximal transport rate; the value of Km fell as Na+ was increased to 75 mequiv./L, but Km did not decline further with higher values of Na+. These results support the theoretical predictions of the presence of both an affinity and a velocity effect of the sodium gradient on the intestinal transport system for glucose.