Data presented here were obtained from 9.8-s test potentials from −80 to −10 and 0 mV with 2 mM Ca²⁺ in the external recording solution. r₉ is ratio of the current remaining at 9 s to the peak current. t_{1/2 decay} at −10 mV was not determined (ND) because the test potential was too short to accurately assess this parameter in three of six cells. Data are given as mean ± SEM, with the numbers in parentheses indicating the number of myotubes tested.