Kinetics of the sodium/beta-methyl-D-glucoside co-transport system in the guinea-pig small intestine.

The kinetics of sodium-dependent beta-methyl-D-glucoside transport were examined in guinea-pig intestinal rings. Large-scale experiments were performed in which both sodium and monosaccharide concentrations were varied within the same animal. The results were evaluated by non-linear regression analysis, and an attempt was made to distinguish between the applicability of different models to describe the data set. Only two of the tested models provided a good fit to the data. These both involved the random formation of a ternary complex from either intermediate binary complex; in one, the constraint that only the ternary complex was able to cross the membrane was included, whereas in the other the ratio of the permeability coefficients for the binary complex between sugar and carrier and for the ternary complex was estimated. But this ratio did not differ significantly from zero, so the two acceptable models were equivalent. In addition, it was necessary to introduce into both these models the concept of a sodium reservoir at the surface of the brush-border membrane, such that the local sodium concentration in the vicinity of the carriers could never fall below a certain critical value (which was estimated at 4.8 mM), even in complete absence of this ion from the bulk medium.