Permeability of Red Blood Cells to Ions

Abstract

Recent experiments with radioactive isotopes have demonstrated the permeability of red blood cells to cations. The results obtained by different experimenters are, however, not directly comparable with each other or with the permeability of other systems. Permeability and diffusion phenomena ought to be expressed in standard c g s units. With the aid of published measurements on cells of the same species and with certain assumptions, it is possible to evaluate permeabilities in terms of standard units. Following Rashevsky one may define the permeability, h, of a membrane to a given solute (provided the membrane is the limiting factor in the rate of exchange of solute) as where Q is the rate at which the solute crosses the membrane in mols per second, S is the area of the membrane and C1—C2 the difference in concentration of the diffusing substance between the two sides of the membrane in mols per cc. The permeability constant, h, of a membrane is defined on the assumption that the rate of diffusion, Q, is the difference between the rate of exit which is proportional to C1, and the rate of entry, which is proportional to C2 In the red cell, however, Q, the net rate of ion change, is zero. The permeability factor, b, measures the fraction of the ions inside the cell which cross the membrane each minute. We may assume, for example, that loss of potassium from the cell is entirely passive and may be regarded as a leak, while the uptake of potassium takes place by another path motivated by an unspecified mechanism. If so, the rate of loss, b, should be proportional to C1 and not to C1—C2. In some experiments, b is measured by the rate of uptake of radioactive potassium; however, the rate of active uptake equals the rate of passive loss, if there is no net change in total potassium, so conversion of b to permeability units still entails dividing b by C1.