Intestinal kinetic parameters: effects of unstirred layers and transport preparation

Three in vitro tissue preparations were used to derive kinetic parameters for the transport of D-glucose in rabbit jejunum, and the resistance of the unstirred water layer was varied by altering the rate of stirring of the bulk phase. The apparent permeability coefficient (Pd*) of the rabbit jejunum for D-glucose was much higher from everted sacs and full-thickness biopsies than from intestinal discs. Failure to adjust the experimentally determined flux for the contribution of the passive component led to errors in the estimation of the maximal transport rate (Jdm) and in the apparent Michaelis constant (Km*). Jdm was higher in biopsies than everted sacs or discs, Km was also higher in biopsies. With each tissue preparation Km* and Pd* were markedly influenced by stirring the bulk phase, whereas Jdm was unchanged. The results indicate that failure to account for the effect of the passive component and the unstirred layer leads to major errors in the estimation of Km*, Pd*, and Jdm. Furthermore, the magnitude of these kinetic constants is influenced by the type of in vitro system used to derive the constants, and it is therefore invalid to extrapolate the results obtained using one preparation to those utilizing another preparation, or to the in vivo situation.