Figure 3. Fractional Zn²⁺ current through muscle AChRs.

A, typical fluorescence and current responses evoked by nicotine (thin bar), simultaneously recorded in two cells expressing γ-AChR (top) and ε-AChR (bottom), equilibrated in Ca²⁺-, Mg²⁺-free solution containing Zn²⁺ (0.5 mm, cross-hatched bar). Note that the current response was terminated by nicotine washout, while fluorescence recovered to basal only upon TPEN application (filled bar). Membrane holding potential, -50 mV. B, plot of ΔF/F₀vs. Q in the same cells as in A (□, γ-AChR; •, ε-AChR) and in a third cell used for calibration measurements (▪). Data points were calculated every 50 ms. The slopes of the straight lines, obtained by best fitting the data, represent the F/Q ratios of the responses. C, in cells expressing γ-AChR, P_f,Zn increased with Zn²⁺ concentration in the extracellular medium. Each point represents the mean ±s.e.m. of 6-11 cells. D, plot of ΔF/F₀vs. Q in a cell loaded with Newport Green, with nicotine applied in the presence of ZnCl₂ (0.5 mm, □), or ZnCl₂ plus CaCl₂ and MgCl₂ (2 mm each, ▪). Note that the values virtually overlap. E, plot of ΔF/F₀vs. Q in a cell loaded with Fluo-3, and superfused with nicotine in standard external solution (○) and in the presence of ZnCl₂ (0.5 mm, •). Data points were obtained and fitted as in B.