Fig. 5. (A) Bar chart showing the mean (±s.e.m.) frequency (%) of the fast-linear sperm population present in control, solubilised apical plasma membrane protein fraction (sAPM) and 100 µmol l^–1 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) treatments. Both treatments caused reduction of bicarbonate-induced activation (P<0.05). (B,C) In comparison to the control treatment, both sAPM and 100 µM DIDS significantly reduced beat cross frequency (Hz; BCF) in the fast-linear sperm group if bicarbonate/CO₂ was absent. In the presence of bicarbonate/CO₂, there was no inhibitory effect of sAPM on BCF, but DIDS still induced significant reduction of flagellar beat frequency (F_1,24536=18.9; P<0.001). Data are mean ± s.e.m; N=5 replicates). (D) Three representative trajectories of spermatozoa exposed to DIDS show high linearity because there is relatively little deviation from the average path. Black dots represent x-y coordinates measured at 20 ms intervals; grey lines are fitted spline curves.