Effects of monensin on tetanic force in soleus muscles obtained from 10.5-wk-old mutant mice. At the intervals indicated, the muscles were stimulated using 1-s trains of 120-Hz pulses of 0.2-ms duration and 12 V. When steady state had been reached, voltage was reduced to 7 V, and the duration of the trains increased to 2 s. 90 min after the onset of stimulation, 10⁻⁵ M tubocurarine was added to all muscles. At 120 min, 10⁻⁵ M monensin was added to four muscles (open circles), and the contralateral muscles served as controls (closed circles). Each point represents the mean of observations on four muscles, with errors bars denoting SEM.