Dissection of the voltage-dependent and voltage-independent components of spermine block for S1-WT channels. (A) Family of normalized, leak-subtracted ramp I/V curves obtained in a patch containing S1-WT channels in the absence of (black line), or in the presence of 5 (red line), 20 (cyan line), 100 (blue line), and 500 μM (green line) spermine. (B) Family of g_rel/V curves obtained from the I/V curves shown in A (solid symbols, color coding as in A). Dotted and solid lines represent fits to the individual g_rel/V curves of simple Boltzmann functions without (dotted lines) or with (solid lines) an added constant “leak.” (C–F) Plots of the fit parameters V_h (C), V_s (D), and leak (F), obtained by fitting individual g_rel/V curves with Eq. 2, and of the ratio −V_h/V_s (E), as a function of spermine concentration. Color coding is the same as in A. The black line in E was obtained by linear regression, that in F by a nonlinear least-squares fit to Eq. 7. Obtained parameters are presented in the panels.