Formation and relaxation of erythrocyte membrane tethers in micropipettes.

The red blood cell (RBC) membrane forms tethers in response to shear forces acting on tiny membrane points. Tether formation depends on viscous and elastic membrane properties and has been used as indicator of membrane fragility. A micropipette technique was used to study time dependent tether formation and tether relaxation of individual RBC. Point attached RBC were aspirated at a negative pressure of -5 mm H2O into a micropipette with an internal diameter of 7.8 microm. If tether formation occurred and the tether reached a length of approximately 16 microm, the pipette was carefully pulled back. The RBC left the orifice of the micropipette and the tether relaxed and pulled the main body of the RBC back to the attachment point. The relaxation of the tethers was exponential. The time constant for tether relaxation was 0.144 s which is similar to the time constant for recovery of entire RBC from extensional elastic deformation. Repeated tether formation and relaxation of the same RBC led to an earlier begin of tether formation and changed the behavior of tether growth, although the relaxation time did not change. We conclude that repeated tether formation decreases the resistance of the RBC membrane to form tethers upon given shear forces. Weakening of the membrane due to repeated plastic deformation may play a role in the membrane loss of circulating RBC during aging.