Concentration-dependent effects of NFA on R190A and R190E Slo2.1 channels. (A) I-V relationships for oocytes expressing R190A Slo2.1 channels and bathed in K104 solution. Oocytes were studied 2 d after injection of 0.84 ng cRNA (n = 9). (B) I-V relationships for oocytes expressing R190A Slo2.1 channels and bathed in K104 solution. Oocytes were studied 2 d after injection of 0.84 ng cRNA (n = 6). (C) Concentration–response relationships for oocytes expressing WT, R190A, or R190E Slo2.1 channels and bathed in K104 extracellular solution. Currents were measured at a V_t of +80 mV. The data were fitted with a modified Hill equation (smooth curves) to estimate EC₅₀ and n_H values as follows: WT: EC₅₀ = 2.24 ± 0.02 mM and n_H = 1.73 ± 0.02; R190E: EC₅₀ = 1.28 ± 0.09 and n_H = 0.83 ± 0.06; R190A: EC₅₀ = 0.97 ± 0.02 and n_H = 1.28 ± 0.04. (D) Concentration–response relationships for oocytes expressing WT, R190A, or R190E Slo2.1 channels and bathed in KCM211 extracellular solution. Currents were measured at a V_t of 0 mV. Hill equation parameters were as follows: WT: EC₅₀ = 2.1 ± 0.87 mM and n_H = 1.74 ± 0.50; R190E: EC₅₀ = 1.98 ± 0.43 and n_H = 2.15 ± 0.80; R190A: EC₅₀ = 0.71 ± 0.13 and n_H = 1.34 ± 0.24. For C and D, currents were normalized to the extrapolated maximum response for each cell (I/I_max).