Regulation of Cell Volume by Active Cation Transport in High and Low Potassium Sheep Red Cells

Abstract

A B S T R A C T A model cell which controls its cation composition and volume by the action of a K-Na exchange pump and leaks for both ions working in parallel is presented. Equations are formulated which describe the behavior of this model in terms of three membrane parameters. From these equations and the steady state concentrations of Na, K, and C1, values for these parameters in high potassium (HK) and low potassium (LK) sheep red cells are calculated. Kinetic experiments designed to measure the membrane parameters directly in the two types of sheep red cells are also reported. The values of the parameters obtained in these experiments agreed well with those calculated from the steady state concentrations of ions and the theoretical equations. It is concluded that both H K and L K sheep red cells control their cation composition and volume in a manner consistent with the model cell. Both have a cation pump which exchanges one sodium ion from inside the cell with one potassium ion from outside the cell but the pump is working approximately four times faster in the H K cell. The characteristics of the cation leak in the two cell types are also very different since the H K cells are relatively more leaky to sodium as compared with potassium than is the case in the L K cells. Both cell types show appreciable sodium exchange diffusion but this process is more rapid in the L K than in the H K cells. I N T R O D U C T I O NO n e of the r e m a r k a b l e properties of living cells is their capacity to m a i n t a i n a relatively constant v o l u m e t h r o u g h o u t life. Since cells generally contain a large n u m b e r of charged macromolecules which c a n n o t pass t h r o u g h the plasma m e m b r a n e , osmotic forces p r o d u c e a constant t e n d e n c y to swell. In general, such systems can avoid swelling only if the hydrostatic pressure is sufficiently higher inside t h a n outside the cell, or if the cell surface is imThis work has been presented in part before

Internal magnesium, 2,3-diphosphoglycerate, and the regulation of the steady-state volume of human red blood cells by the Na/K/2Cl cotransport system.

“…This of course also applies to cells treated with nystatin and/or A23187. In addition, there were no measurable changes in cell volume that took place during the time course of the flux determinations, even in the presence of ouabain, presumably because the outward leakage of Ki would act to offset any inward movement of Na given that the ratio Pz/PNa is > 1 (see Tosteson and Hoffman, 1960). It should also be noted that, since the cotransport rates are sensitive to the cellular concentrations of 2,3-DPG as well as Mg~ °t, these, together with ATP, were not altered by application of the nystatin technique (data not shown).…”

“…We assume, of course, that the sum Nai + Ki, in defining the relative content of cellular water, also defines the mean cell volume of a population of cells (Fig. 1 A; Tosteson and Hoffman, 1960;Funder and Wieth, 1966b;Lee et al, 1984). Therefore, for cells in the steady state, where the sum Nai + Ki is constant with time, it is possible to calculate the net driving force, A~net , for cotransport from the chemical potential gradients of Na,K, and C1 that exist across the cell membrane, using the formula: as previously proposed (Schmidt and McManus, 1977; see also Haas et al, 1982, andG6bel, 1984).…”

“…It would therefore seem that a more critical evaluation of whether or not the cotransport system was a contributor to steady-state, in vivo cell volume would best be studied in the bloods of individuals that expressed high and low cotransport activities at their in vivo MCVs, where the magnitude of their Na/K pumps and electrodiffusional leaks was comparable. The idea of having comparable pumps and leaks would act to normalize their input as primary determinants of cell volume (Tosteson and Hoffman, 1960;Milanick and Hoffman, 1986) and perhaps thereby amplify the relative effects of cotransport. These considerations thus represent the basis for the approach taken and the selection of the individuals whose red cell populations were analyzed in this study.…”

The Formation and Properties of Thin Lipid Membranes from HK and LK Sheep Red Cell Lipids

“…In addition, "leakage" pathways ordinarily had very low capacitances, regardless of surface area. 2 Of primary interest in the study was the development of a simple method for measuring the relative ionic permeabilities of artificial lipid membranes whose thickness is of the same order of magnitude as cellular membranes,…”

Measurements of intracellular volumes by59Co and2H/1H NMR and their physiological applications

“…18,40,42,43 Swelling is partially due to import of sodium through sodium channels, cotransport (Na/K/2Cl) and antiport (Na/H) systems, and not due to inhibition of the energy dependent Na/K ATPase pumps. 48 These findings argue against the pump-leak hypothesis of cell volume regulation, 49 which is based on the impermeability of the membranes to extracellular sodium as a compensatory mechanism for the osmotic pressure of intracellular proteins. 1 Intracellular calcium accumulation induced by ischemia contributes to swelling of myocytes with intact membranes, whereas toxicity of calcium overload is associated to cellular shrinkage.…”