Mechanisms of Human Erythrocytes Volume Stabilization
Functional completeness of erythrocytes depends on high deformability of these cells, that allows them to pass through narrow tissue capillaries. The erythrocytes high deformability is provided due to maintenance of discoid shape with an optimal cell surface area to volume ratio. In its turn this ratio is maintained due to cell volume stabilization at a given cell surface area. In this work, using mathematical simulation, we studied role of Na/K-ATPase, calcium activated potassium channels and adenylate metabolism in human erythrocyte volume stabilization at increase in cell membrane permeability to cations. The simulation took into account a contribution of glycolytic metabolites and adenylates to cytoplasm osmotic pressure. It was shown that the presence in the cell of Na/K-ATPase and two opposite transmembrane gradients of Na+ and K+ ions provide a significantly improved cell volume stabilization at the increase in cell membrane permeability, compared with hypothetical cells, in which the osmotic balance between cell and extracellular compartment is provided due to a gradient of only one ion (Na+). In this case the erythrocyte volume deviates from the optimal value by less than 10% at change in cell membrane permeability from 50 to 200% of its normal value. In this case, however, the intracellular ion concentrations may change significantly (by several times). The adenylate metabolism system can provide an additional regulation of transport ATPases due to regulation of intracellular ATP levels. Under these conditions stabilization of steady-state values of intracellular ion concentrations (ion homeostasis) and of cell volume in the range of cell membrane permeability changes from 50 to 1500% of the normal value. In this case, however, the cell volume and intracellular ion concentrations may significantly deviate from the stabilized values during transitional processes. Simultaneous function of both, ion transport systems and adenylate metabolism allows to provide ion homeostasis and efficient erythrocyte volume stabilization (within 5% deviation from the optimal value) both in steady-state conditions and during transitional processes at increase in cell membrane permeability up to 10-15 times compared with the normal value.