The effects of shear on the delay time for gelation of hemoglobin S

Intraerythrocytic gelation of deoxyhemoglobin S is the immediate cause of red cell deformation and rigidification in sickle cell disease. Hence the rheology of hemoglobin S gels, heretofore little studied, plays a central role in pathogenesis. I have shown previously that unsheared gels are solid-like whereas sheared gels are viscous in character. In the present studies the delay (nucleation) time for gelation (after a temperature jump) was examined in the absence and presence of shear, employing a sharp rise in viscosity as the end point of the delay time. Shearing rates in the range from 1.92 to 384 s^-1 progressively accelerate the delay time. In another series of experiments the temperature jump was carried out in the absence of shear, shear being instituted later in the delay period. There was a sparing effect of the time of incubation in the absence of shear on the subsequent time under shear needed to induce gelation. A linear relation was observed between these two parameters, indicating that shear operates with constant efficacy in accelerating gelation, independent of when in the delay time it is applied. This suggests that the mechanism of assembly is the same throughout the period. Since shear probably operates through the breakage of fibers with the creation of new growth centers, the results also suggest that long, breakable, fibers are present even relatively early in the delay period. Values were obtained for delay time under shear and (by extrapolation) delay time under no shear. The temperature dependence (and hence apparent activation energy) of the latter is larger than that of the former; i.e. the effect of shear in shortening delay time is more marked as temperatures are lowered. With respect to pathogenesis, these results suggest that intraerythrocytic shearing may be detrimental because it accelerates gelation. On the other hand shearing of gels converts them from solid-like to viscous, and hence more deformable, systems; this effect of shear might be expected to ameliorate pathogenesis. Consequently, the pathogenic effects of intraerythrocytic shearing may depend on when the shear is applied.