Properties of some model systems for transcellular active transport

Abstract

The properties of some model systems for transcellular active transport (translocation) are described. The functioning of the models depends upon, firstly the occurrence of solute (substrate) pumping across the opposing bordering membranes of a unit cell, the membranes also being leaky to the substrate and secondly, an intrinsic asymmetry of the opposing membranes. The systems are characterized by the maintenance in the steady state of substrate concentration differences between the contents of the unit cell and its environment.

Making assumptions as to the properties of the individual faces of the cell equations have been written describing the rate of translocation in terms of the parameters of substrate transport at the faces of the cell and the extracellular concentrations. Using assumed values for these parameters and concentrations, translocation rates have been calculated for various conditions. From study of the models the conditions under which translocation will occur are demonstrated and the kinetic properties of the model systems compared with experimental data.

It is also shown that changes in the rates of leaking and of pumping of a substrate across the opposing faces of the unit cell can, under appropriate conditions, change the direction as well as the rate of substrate translocation. The induction of a net counterflow of one substrate through the cell (countertranslocation) can be a consequence of the presence of a second substrate which is also capable of being pumped across the bordering membranes of the cell in the same direction as the first.