Effect of Cell Geometry, Internal Viscosity, and pH on Erythrocyte Filterability

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Abstract
A simple filtration method was developed for measuring the passage time of dilute erythrocyte suspensions through 2.8 .mu.m cylindrical pores relative to the passage time of the suspending buffer. This method of determining relative filtration time (RFT) is sensitive to the major determinants of overall red cell deformability by demonstration of the effects of variations in internal viscosity (intracellular Hb concentration), cell geometry (volume, surface area-to-volume ratio and membrane properties. These factors were examined by varying the buffer pH at constant osmolality and by varying the buffer osmolality at constant pH. The influence of cell volume alone was investigated by measuring RFT of suspensions of rat, rabbit and human erythrocytes of naturally different volumes but of the same basic biconcave shape. RFT is significantly increased by decreases in the surface area-to-volume ratio produced by osmotically induced cell swelling. RFT increases nearly linearly with increasing intracellular Hb concentration of cells in hypertonic buffers. Reduction of buffer pH below 7.20 causes a significant increase in RFT that is not explained by cell volume or geometric changes and is presumably due to altered membrane properties. The contribution of cell volume alone in normal biconcave erythrocytes over the range of 60-100 .mu.m3 is much less than that of even slight decreases in the surface area-to-volume ratio. These studies illustrate the relative contributions of each major determinant of red cell deformability and the amount of information that can be obtained using a relatively simple filtration method.