Adaptation of V_m and Ca²⁺ signals. (A) Adaptation and recovery of the V_m signal after an adapting stimulus. Sperm were mixed with 100 nM caged resact, equivalent to 100 pM resact because of the “residual” activity of caged resact. The final free resact concentration was ∼400 fM. Sperm were probed with a test stimulus (100% flash; releasing ∼30 pM resact) at different Δt (in seconds): 0.4 (black), 1.4 (red), 3.4 (green), 7.4 (blue), and 15.4 (cyan). (B) Recovery kinetics of the V_m signal as shown in A (n = 3; color coding as in A). The data points were fit with an exponential curve (R² = 0.99). (C) Weber–Fechner plot. Sperm were adopted by background concentrations c_b of 0.27, 2.1, 4.1, 10.1, 20.4, and 52.5 pM. Test stimuli c_b of either resact or cGMP were given. 15 pM (black symbols) or 30 pM (red symbols) resact (c_s) was released from caged resact. The voltage response S = ΔV_m was normalized to the value in the absence of background S₀ and plotted versus the concentration ratio R = c_b/c_s. Similarly, test stimuli of cGMP were probed (blue symbols). Solid lines were calculated using either Eqs. 1 or 2, respectively (resact, n = 3 and cGMP, n = 4). (D) Ca²⁺ signals evoked by a test stimulus of cGMP at different Δt of (in seconds): 1.2 (magenta), 3.2 (green), 7.2 (blue), 15.2 (red), and 29.2 (black). Ca²⁺ signal evoked by 4.1 pM background resact alone is shown in orange. (E) Shift of the dynamic range of Ca²⁺ signals. Test stimuli of cGMP (Δt = 15.2 s) were given either in the absence (black) or presence of adapting resact (in nanomolars): 0.25 (red), 2.5 (blue), and 25 (green). Ca²⁺ signal amplitudes produced by a test cGMP stimulus were plotted; in the presence of background resact, the Ca²⁺ signal amplitude was the difference between the value before the test stimulus and the value at the minimum of the Ca²⁺ drop. The data were fitted with the equation $A=AmaxInIn+K1/2n,$ wherein A is the response amplitude, I denotes the flash intensity, and n is the Hill coefficient. (F and G) Apparent K_1/2 (F) and response compression (G) at different background concentrations. Data are obtained from E. Error bars show SDs. Arrows indicate time of flashes.