Spermine block of homotetrameric S1-WT, S1-ND, and S2-ND channels characterized by global fitting. (A, C, and E) Families of normalized ramp I/V curves obtained in patches containing S1-WT (A), S1-ND (C), and S2-ND (E) channels; in the absence of (black lines), or in the presence of 5 (red lines), 20 (cyan lines), 100 (blue lines), and 500 μM (green lines) spermine. Except for S2-ND, the ramp I/V curves were leak subtracted. (B, D, and F) Families of g_rel/V curves obtained from the I/V curves shown in (A, C, and E) (solid symbols, color coding as in A, C, and E). Families of solid black lines represent global fits to the ensembles of four g_rel/V curves. Obtained fit parameters were K₀ = 3.19 mM, ρ = 0.67, n_H = 0.86, M_leak = 0.67, K_leak = 121 μM for S1-WT (B); K₀ = 2.31 μM, ρ = 3.34, n_H = 1.13 for S1-ND (D); and K₀ = 4.69 μM, ρ = 3.05, n_H = 1.26 for S2-ND (F).