Characterization of the kinetics of the passive and active transport mechanisms for bile acid absorption in the small intestine and colon of the rat

摘要

Bile acid uptake occurs via passive diffusion in all regions of the intestine and via active absorption in the ileum. Determination of the passive permeability coefficient for ionized monomers (^*P^-) demonstrated that permeability decreased by a factor of 3.4, 6.8, and 8.1 for the addition of a hydroxyl, glycine, or taurine group, respectively, to the steroid nucleus. Removal of the negative charge increased permeation by a factor of 4.4; however, permeability coefficients for the protonated monomers showed the same relative decrease with addition of a hydroxyl group. The calculated incremental free energies of solution (δΔFW→1) associated with these additions equaled + 757 (hydroxyl), + 1178 (glycine), and + 1291 (taurine) cal/mole. Passive permeability coefficients for the transverse colon showed the same relative relationships among the various bile acids. After making appropriate corrections for passive permeability across the ileum, apparent values for the maximal transport velocity (^*Vmax) and Michaelis constant (^*Km) of the active transport system were measured. ^*Vmax depended upon the number of hydroxyl groups on the steroid nucleus; values for the trihydroxy bile acids were high (1543-1906 pmoles/min per cm) while those for the dihydroxy (114-512 pmoles/min per cm) and monohydroxy (45-57 pmoles/min per cm) acids were lower. In contrast, ^*Km values were related to whether the bile acid was conjugated; unconjugated bile acids had values ranging from 0.37 to 0.49 mM, while values for the conjugated bile acids were approximately half as high (0.12-0.23 mM).