Fig. 3. Force sensitivity is modulated by the external calcium concentration. (A,B) Ax2 cells were allowed to adhere to glass at a low (5-10 µM) calcium concentration in MES-Na buffer, after which the calcium concentration was raised to 1 mM at low shear stress. The flow was then stopped and video recording started (t=0 minutes). After 2 minutes, a constant shear stress was applied (arrow):, =0.1 Pa; , =0.5 Pa; circles, =1 Pa. The instant cell velocity modulus (A) and directionality (B) are plotted as a function of time. (C,D) Average cell speed (C) and directionality (D) are plotted as a function of the applied shear stress at a 1 mM () or 5 µM () calcium concentration. The average cell speed and directionality are determined from the steady-state portion of the motility response to shear stress (from 7 to 15 minutes in Fig. 3A,B). Data are collected from several independent experiments performed in Sörensen or MES-Na buffer.