The deformability of red blood cells is important in the microcirculat
ion where capillary diameters are often smaller than those of the red
blood cells. In the present study, ektacytometry was used to examine t
he effect of hypoxia on the deformability of red blood cells from five
mammalian species: Human, cat, rat, rabbit, and dog. Deformability wa
s 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; secon
d, we used the Elongation Constant (EC), which quantitates the exponen
tial dependence of the fraction of maximal elongation on the varying s
hear stress. The EI was measured at high shear stresses (150-250 dyn/c
m(2)), as well as at lower shear stresses (15, 32 and 64 dyn/cm(2)) th
at 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 un
altered in response to hypoxia. These ektacytometry experiments indica
te that (1) the elongation constant is a new and useful parameter for
characterizing the deformability of red blood cells and (2), the defor
mability of human, cat, dog, and rabbit red blood cells is unaltered b
y hypoxia. results constrain the possible mechanisms that could accoun
t for observation that hypoxia decreases the filterability of certain
species of red blood cells, which was reported previously.