We have used a biomembrane force probe decorated with P-selectin to form point attachments with PSGL-1 receptors on a human neutrophil (PMN) in a calcium-containing medium and then to quantify the forces experienced by the attachment during retraction of the PMN at fixed speed. From first touch to final detachment, the typical force history exhibited the following sequence of events: i), an initial linear-elastic displacement of the PMN surface, ii), an abrupt crossover to viscoplastic flow that signaled membrane separation from the interior cytoskeleton and the beginning of a membrane tether, and iii), the final detachment from the probe tip most often by one precipitous step of P-selectin:PSGL-1 dissociation. Analyzing the initial elastic response and membrane unbinding from the cytoskeleton in our companion article I, we focus in this article on the regime of tether extrusion that nearly always occurred before release of the extracellular adhesion bond at pulling speeds ≥1 μm/s. The force during tether growth appeared to approach a plateau at long times. Examined over a large range of pulling speeds up to 150 μs/s, the plateau force exhibited a significant shear thinning as indicated by a weak power-law dependence on pulling speed, f∝ = 60 pN(νpull/ μm/s)0.25. Using this shear-thinning response to describe the viscous element in a nonlinear Maxwell-like fluid model, we show that a weak serial-elastic component with a stiffness of ∼0.07 pN/nm provides good agreement with the time course of the tether force approach to the plateau under constant pulling speed. © 2005 by the Biophysical Society.
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