The excitation-induced net loss of K⁺ and gain of Na⁺ in rat soleus and EDL. Experimental conditions as described in the legend to Fig. 2. Following equilibration for 30 min in K.R. buffer, contralateral muscles from the same animal were either allowed to rest or stimulated in air at 60 Hz for 60 s (soleus) or 30 s (EDL). For each pair of contralateral muscles, the excitation-induced loss of K⁺ and gain of Na⁺ were calculated from the differences in K⁺ and Na⁺ contents, respectively, and expressed as nmol/pulse/g wet wt (Δ Na⁺ and Δ K⁺). Each column represents the mean of observations on 12 muscles with bars denoting SEM, The difference between the excitation-induced Na⁺ uptake in soleus and EDL muscles was significant (P = 0.0l4). Likewise, the difference between excitation-induced K⁺ release of soleus and EDL muscles was significant (P = 0.011).