Fig. 2. Calcium requirements for cell motility and cell adhesion to glass. (A) The average cell speed is plotted as a function of the free calcium concentration in the bathing fluid, for cells submitted to shear stress (2.4 Pa, ) or not (). In the first case, the average cell speed is determined in the steady-state portion of the motility response (from 8 to 15 minutes after onset of the flow, see Fig. 1), except when EGTA is added (1 µM), where it is determined 30 seconds to 3 minutes after EGTA addition (see Fig. 1C). In the second case, the average cell speed is determined from the entire recording. The solid and the dashed lines correspond to a fit with an apparent calcium affinity of 22 µM and a maximum speed of 25 and 10 µm minute^-1, respectively. Standard deviation is 2 µm minute^-1 (data not shown). (B) The critical shear stress for cell detachment is plotted as a function of the calcium concentration in the flowing fluid. Ax2 cell adhesion was probed with the radial flow detachment assay. The solid line corresponds to a fit with an apparent calcium affinity of 2.5 µM and a maximum detachment shear stress of 7.5 Pa. Error bars, 0.5 Pa (data not shown). Free calcium concentrations above or below 5 µM were obtained by adding CaCl₂ or EGTA, respectively to MES-Na buffer (A) or SB (B).