Mutual interaction of ion uptake and membrane potential.

The concentration dependence of cation uptake by the cell may be considerably complicated when this uptake is accompanied by a depolarization of the cell membrane. In case of carrier-mediated transport deviations from Michaelis-Menten kinetics may come to the fore comparable to those found in a dual mechanism of cation uptake or when substrate inhibition is involved. This remains true when only the maximum rate of uptake and not the Km is dependent upon the membrane potential. We have proven this by means of computer simulation of cation transport mediated by a non-mobile carrier. Under restricted conditions still apparent Michaelis-Menten kinetics may be found despite the fact that the membrane potential varies with increasing substrate cation concentration. But even then there are still differences with 'normal' transport kinetics. A non-competitive inhibitor does not only affect the maximum rate of uptake but also the apparent Km. Depolarization of the cells by a cation which passes the cell membrane by means of diffusion, affects the uptake of the substrate cation almost in the same way as a non-competitive inhibitor does and causes both a decrease in the maximum rate of uptake and an increase in Km. In the case of competitive inhibition the apparent affinity of the inhibitor for the carrier depends upon the rate of transfer of this inhibitor through the cell membrane. The mutual influence of cation uptake and membrane potential is dealt with for uniport of either monovalent or divalent cations and for cotransport of monovalent cation with protons, as well. Possible effects of the surface potential are accounted for.