AJP - Gastrointestinal and Liver Physiology, Vol 247, Issue 5 463-G467, Copyright © 1984 by American Physiological Society
A contribution to analysis of three-compartment models for intestinal weak electrolyte absorption
M. L. Lucas
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.
