Fig. 5. Potentiating effect of Ca²⁺ on the levamisole concentration—response relationship. (A) Effect of different concentrations of levamisole applied in Ascaris peri-enteric fluid (APF) bathing solution (6 mmol l^-1 Ca²⁺). (B) Effect of different concentrations of levamisole in low-Ca²⁺ APF (Ca²⁺ replaced with Mg²⁺). (C) Concentration—response plots of the effect of levamisole peak membrane potential responses in normal APF (Lev+Ca) and low-Ca APF (Lev—Ca). Best fit to response plot in the presence of Ca²⁺ in normal APF had an EC₅₀ of 1.2 µmol l^-1, a Hill slope of 1.21 and a maximum response of 7.1 mV. In low-Ca²⁺, the best-fit EC₅₀ was 3.1 µmol l^-1, the Hill slope was 3.9 and the maximum response was 6.8 mV. Note that the presence of Ca²⁺ is associated with an increase in the size of the response. The effect of Ca²⁺ on the response was statistically significant (P=0.045, analysis of variance).