Exchange of multiple indicators across renal-like epithelia: a modeling study of six physiological regimes

Multiple indicator dilution (MID) provides a means of quantifying the unidirectional steady-state fluxes of in vivo tracer movement within organs. In this report a MID model of tracer exchange across renal-like epithelia is developed in mathematical form. Six regimes of epithelial transport function are defined on the basis of the relative sizes of the tracer fluxes. Evidence for the existence of each regime has previously been obtained empirically. These are net secretion, net reabsorption, kinetic sink, antiluminal drive, antiluminal equilibration, and antiluminal trapping. By means of the mathematical model, the indicator dilution properties of the six regimes are compared. The principal similarities and differences among the regimes are documented and related to the physical basis of the transport processes. The findings suggest that over a wide range of physiologically meaningful flux magnitudes the functional regimes can be distinguished via existing empirical methods of MID. The model provides an improved basis for carrying out these distinctions in specific applications.