Influence of perfusion rate on the kinetics of intestinal sugar absorption in rats and hamsters in vivo.

The effect of perfusion rate (PR) on the apparent glucose and galactose-influx kinetics of rat and hamster jejunum in vivo has been studied. Total absorption (V) and absorption in the presence of 0.5 mM phloridzin (VD) were measured in consecutive periods of 1 minute, and their difference (VT = V - VD) was taken as the mediated transport rate. PR values were 2.8, 5,6, 13.5 or 18.5 ml.min-1, and the sugar concentrations in the perfusion solution (So) were 2, 5 and 10 mM. Plots of 1/VT versus 1/So for the different PR intercept the ordinate axis at the same point, yielding a common Vmax for the same animal species and sugar. From the slopes, apparent K'm values are obtained and apparent mass-transfer coefficients (K'D) for the diffusion component are calculated (VD/So) as well. When the PR increases, K'n decreases, while K'D increases. A simplified model based on the assumption of a steady-state in which diffusion across unstirred water layers (UWL) equals the sum of a passive and a carrier mediated non-passive transepithelial transfer, acceptably accounts for the results. When taking a "true" Km values the lowest ones reported in the literature, it is possible to obtain the sugar concentration at the enterocyte membrane (Sm), the effective UWL thickness (delta) and the "true" mass-transfer coefficient (KD). So/Sm ratios and delta values decrease as PR increases, accounting for the biased K'm and K'D values. As it was expected, KD did not change significantly by modifying PR. Depending on sugar concentration, the passive component is almost equal to or much higher than the carrier-mediated transport, in rat as well as in hamster. Diffusion across unstirred water layers seems to be rate-limiting for intestinal sugar absorption.