The deformability of red blood cells is important in the microcirculation where capillary diameters are often smaller than those of the red blood cells. In the present study, ektacytometry was used to examine the effect of hypoxia on the deformability of red blood cells from five mammalian species: Human, cat, rat, rabbit, and dog. Deformability was characterized in both normoxic (PO2 = 129 +/- 6 mm Hg) and hypoxic (PO2 = 47 +/- 6 mm Hg) conditions in two different ways. First, we used the Elongation Index (EI) which quantitates the extent of elongation of red blood cells in response to increasing fluid shear stress; second, we used the Elongation Constant (EC), which quantitates the exponential dependence of the fraction of maximal elongation on the varying shear stress. The EI was measured at high shear stresses (150-250 dyn/cm(2)), as well as at lower shear stresses (15, 32 and 64 dyn/cm(2)) that occur in the microcirculation. In response to hypoxia at high shear stresses, the EI of the rat red blood cells decreased by 9.3% (P < 0.05), but was not altered in the other four species studied. Moreover, in all five species, the EC and EI at the lower shear stresses were unaltered in response to hypoxia. These ektacytometry experiments indicate that (1) the elongation constant is a new and useful parameter for characterizing the deformability of red blood cells and (2), the deformability of human, cat, dog, and rabbit red blood cells is unaltered by hypoxia. results constrain the possible mechanisms that could account for observation that hypoxia decreases the filterability of certain species of red blood cells, which was reported previously.
