Passive material behavior of granulocytes based on large deformation and recovery after deformation tests

In order to better understand the in vivo rheologic behavior of white cells, we have studied the time-dependent deformability, recovery, and mechanical activation of blood granulocytes. We have used micropipette aspiration methods to measure the large deformation response and recovery after deformation characteristics of neutrophils as functions of time, temperature, and collecting media. The cell response in the pipette experiment was characterized by three time domains: the first phase was the passive deformation response to the fixed suction pressure; the second phase was an obvious transition from the passive to active motile cellular state where the cell exhibited erratic length changes in the pipette; and the third phase was the steady recovery after the suction pressure had been zeroed. Tests on white cells were carried out with three different anticoagulants to evaluate the effect of calcium on deformation and recovery behavior; also, cells were separated by centrifugation in high molecular weight dextran to determine whether or not collection and separation procedures affected the cell properties. Our results have shown that the passive deformation of granulocytes into the pipette was a continuous flow process with no approach to a static deformation limit. In addition, there was an obvious threshold pressure below which the cell would not deform and enter the micropipette. For suction pressures significantly above the threshold, granulocytes were continuously deformed with a similar functional dependence on time. The coefficient of proportionality between aspiration length and time, as well as the exponent, depended on suction pressure, pipette dimension, and temperature. It was observed that the granulocytes always recovered to the spherical state after deformation, independent of the extent of deformation or location where the cell was aspirated. Based on the recovery behavior, plus the dependence of the pressure threshold on pipette size, we propose the concept that the granulocyte membrane and cortical shell behave like a "contractile surface carpet" under tension, where the cell interior responds passively like a highly viscous liquid. The membrane cortex appears to be subject to a persistent stress (tension) of about 10(-2) dyne/cm. Our observations of passive to active transition in the pipette suction experiment indicated that granulocytes may be stimulated by deformation at room temperature. This study represents the first detailed investigation of the large deformation behavior of granulocytes, and the results indicate a simple structural model to represent the passive rheologic behavior of the granulocyte.