Kinetics of hemoglobin S polymerization and gelation under shear: I. Shape of the viscosity progress curve and dependence of delay time and reaction rate on shear rate and temperature.

Polymerization and gelation of deoxyhemoglobin S makes red blood cells (RBCs) rigid and is the immediate basis of pathogenesis in sickle cell disease. Hence, characterization of hemoglobin S viscosity and its time-dependent development as RBCs pass through the microvasculature is important in understanding pathogenesis. Because RBCs and the intraerythrocytic milieu in vivo are subject to shear, the shear dependence of polymerization kinetics is also important. In steady-state cone-plate viscometry we find: (1) gelation under shear progresses exponentially with time; (2) shear markedly increases exponential rate and (3) shortens delay time independent of when in the delay time it is applied; (4) shear greatly decreases the temperature dependence of the exponential rate and delay time; (5) simultaneous with its acceleratory effect on polymerization, shear breaks down gel structure. We conclude that shear acts to accelerate gelation by breaking fibers and creating new growing ends, a process that occurs in addition to the homogeneous and heterogeneous nucleation of new fibers that occurs in the absence of shear. Fibers that break are part of a gel network rather than in free solution. The shear dependence of gelation rates means that the critical clinical issue, whether the delay time is long enough and gelation slow enough to permit deoxygenated cells to pass through the microvasculature before they rigidify, depends on in vivo shear rates as well as on degree of unsaturation and hemoglobin concentration.