Simulation studies on the kinetics of intestinal absorption.
1. A model has been used to simulate the absorption of solutes from perfused intestines. The model makes possible the numerical solution of the differential equations describing absorption processes along the length of the intestine which cannot be solved analytically. It allows for water absorption and the non-linear fall in solute concentration down the intestine. It can be modified easily to include other features, e.g. a change in V (maximum rate of absorption) or K (solute concentration at V/2) along the intestine. 2. 90 perfect data sets have been simulated using the model. The Michaelis-Menten equation was fitted to a quarter of them using different algebraic expressions for the apparent solute concentration. The fit of the equation was very good in every case and it was not possible to explain the poorness-of-fit encountered during an earlier survey (Atkins, G.L. and Gardner, M.L.G. (1977) Biochim, Biophys. Acta 468, 127--145) in terms of the fall in solute concentration described above. 3. The equation was also fitted to all the data sets in order to compare the use of several algebraic expressions for the apparent solute concentration. It has been shown that the current practice of using either the initial concentration or the effluent concentration can lead to estimates of V and K up to amost twice their true value. It has been shown that in one situation (glucose absorption by perfused rat intestine) it is possible to use an empirical expression that will reduce the errors considerably. 4. It is also possible, and perhaps preferable, to use a computer program to fit the model directly to data from the simulated experiments and obtain precise estimates of V and K. 5. In order to show that the model can be easily modified to incorporate other characteristics of perfused intestines, simulations were performed in which V decreased linearly down the intestine. In this example, it was concluded that an inhomogeneity due to non-constancy of V cannot be detected by single-pass perfusions.