A contribution to analysis of three-compartment models for intestinal weak electrolyte absorption.

The flux ratio equation used to describe intestinal weak electrolyte absorption in terms of a three-compartment model, with an intermediate compartment of distinctive pH, was examined in detail. This took the form of fitting the function to experimental data from the literature, using several different parameter sets. Within wide limits, estimation of intermediate compartment pH is seen to be arbitrary due to redundancy in the model. Several pairs of pH values can satisfactorily fit experimental data. Inspection of the function reveals that it contains four basic submodels whose distinct identity depend on whether or not pH2 greater than pH, and which of the two membranes in series has the highest discrimination between ionized and non-ionized forms. Further analysis of the function reveals that it provides an infinite but bounded set of values for intermediate compartment pH. The model is therefore redundant, nonlinear function fitting reveals ill conditioning, and estimates of pH2 from the present model cannot be regarded as definitive.