Effects of unstirred layers on the kinetics of carrier-mediated solute transport by two systems

Equations describing the effect of unstirred layers on the simultaneous transport of a solute via two mediated systems have been derived. It has been previously established that unstirred layers cause convex curvature of Eadie-Hofstee transformations of kinetic data for single mediated transport systems. On the other hand, multiple transport systems produce concave curvature in this plot. A numerical example is presented which illustrates the fact that under some circumstances the opposing effects can nearly cancel out, producing an erroneous apparently linear relation. Furthermore, statistical analysis of these curves suggests that the normal error encountered in physiological transport measurements can readily obscure evidence of unstirred layers. It is suggested that kinetic analysis of transport, particularly in epithelial tissues, should whenever possible, include independent measurements of unstirred layers, employ a broad range of substrate concentrations, and vary the stirring rate of the medium. Evaluation of the 'true' kinetic constants can then be made with the relations derived here. In addition, several limiting cases have been discussed which reduce to simpler kinetic descriptions published earlier.