Aim:  Muscles containing predominantly fast‐twitch (type II) fibres [ext. dig. longus (EDL)] show considerably lower contractile endurance than muscles containing mainly slow‐twitch (type I) fibres (soleus). To assess whether differences in Na⁺‐K⁺ fluxes and excitability might contribute to this phenomenon, we compared excitation‐induced Na⁺‐K⁺ leaks, Na⁺ channels, Na⁺‐K⁺ pump capacity, force and compound action potentials (M‐waves) in rat EDL and soleus muscles.

Methods:  Isolated muscles were mounted for isometric contractions in Krebs–Ringer bicarbonate buffer and exposed to direct or indirect continuous or intermittent electrical stimulation. The time‐course of force decline and concomitant changes in Na⁺‐K⁺ exchange and M‐waves were recorded.

Results:  During continuous stimulation at 60–120 Hz, EDL showed around fivefold faster rate of force decline than soleus. This was associated with a faster loss of excitability as estimated from the area and amplitude of the M‐waves. The net uptake of Na⁺ and the release of K⁺ per action potential were respectively 6.5‐ and 6.6‐fold larger in EDL than in soleus, which may in part be due to the larger content of Na⁺ channels in EDL. During intermittent stimulation with 1 s 60 Hz pulse trains, EDL showed eightfold faster rate of force decline than soleus.

Conclusion:  The considerably lower contractile endurance of fast‐twitch compared with slow‐twitch muscles reflects differences in the rate of excitation‐induced loss of excitability. This is attributed to the much larger excitation‐induced Na⁺ influx and K⁺ efflux, leading to a faster rise in [K⁺]_o in fast‐twitch muscles. This may only be partly compensated by the concomitant activation of the Na⁺‐K⁺ pumps, in particular in fibres showing large passive Na⁺‐K⁺ leaks or reduced content of Na⁺‐K⁺ pumps. Thus, endurance depends on the leak/pump ratio for Na⁺ and K⁺.