pH sensitivity of Ca²⁺ current. Quantitative relationship between I_Ca,L and pH_o and pH_i in rabbit myocytes without BAPTA dialysis. The clamp step from −40 to −10 mV was chosen because, at this voltage, there were striking differences in the response of I_Ca,L to pH_o and pH_i (Fig. 4, A and B). The magnitude of intracellular acidosis (pH_o 7.4) was varied by applying different concentrations of sodium acetate while keeping extracellular Ca²⁺ activity constant. The pH_i measurements were done in separate cells, with n values ranging from 5 to 13. The n values for I_Ca,L measurements ranged from 5 to 14 for intracellular acidosis and from 7 to 16 for extracellular acidosis. I_Ca,L was stimulated much more by intracellular acidosis (red) than it was suppressed by extracellular acidosis (black). Results for both types of acidosis fit with the equation: −I_Ca,L = a + b/{1 + [10^(pK−pH)]n} (Sabirov et al., 1997). The apparent pK values for these effects were comparable, 6.92 and 6.99, respectively, showing that the pH sensitivity of I_Ca,L in both the extracellular and intracellular domains is strongly expressed over the physiological pH range.