RESUMO
This study represents an attempt to achieve a better understanding of the stomatocyte-echinocyte transition in the shape of red blood cells. We determined experimentally the index of cell shape at various ionic strengths and osmolarities for native and trypsin-treated human erythrocytes. For every given composition of the outer phase, we calculated the ionic strength in the cells and the transmembrane electric potential using a known theoretical model. Next, we described theoretically the electric double layers formed on both sides of the cell membrane, and derived expressions for the tensions of the two membrane leaflets. Taking into account that the cell-shape index depends on the tension difference between the two leaflets, we fitted the experimental data with the constructed physicochemical model. The model, which agrees well with the experiment, indicates that the tension difference between the two leaflets is governed by the different adsorptions of counterions at the two membrane surfaces, rather than by the direct contribution of the electric double layers to the membrane tension. Thus, with the rise of the ionic strength, the counterion adsorption increases stronger at the outer leaflet, whose stretching surface pressure becomes greater, and whose area expands relative to that of the inner leaflet. Hence, there is no contradiction between the bilayer-couple hypothesis and the electric double layer theory, if the latter is upgraded to account for the effect of counterion-adsorption on the membrane tension. The developed quantitative model can be applied to predict the shape index of cells upon a stomatocyte-discocyte-echinocyte transformation at varying composition of the outer medium.