Figure 5. SOC-associated [Ca²⁺]_i rise is not too small to cause contraction.

A, Ca²⁺ dependence of contraction in β-escin-permeabilised arterioles (n = 28). Mean ± s.e.m. data as described for control arterioles in Fig. 4D and E. The fitted curve is the Hill equation with a mid-point at 0.59 μM and a slope of 1.2. B-D, effects of elevating the K⁺ concentration (mm) of standard bath solution. D600 was excluded from all solutions. Wortmannin was included during [Ca²⁺]_i measurements. B, external diameter measurement. C, [Ca²⁺]_i indicated by fura-PE3. D, plotted against external K⁺ concentration, the mean ± s.e.m. external diameter as a percentage of that in standard bath solution (n = 7), and changes in fura-PE3 ratio (n/N = 29/8). The superimposed curves are fitted Hill equations with mid-points at 37.3 mm K⁺ (diameter) and 33.5 mm K⁺ (Ca²⁺). The dashed straight line is the mean change in fura-PE3 ratio (0.0697) in response to CPA in the presence of ${\text{Ca}}_{\text{o}}^{2+}$ (from Fig. 1C), which is similar in amplitude to the effect of Gd³⁺ shown in Fig. 2D.