Relations between excitability and contractility in rat soleus muscle: role of the Na+-K+ pump and Na+/K+ gradients

The effects of reduced Na+/K+ gradients and Na+-K+ pump stimulation on compound action potentials (M waves) and contractile force were examined in isolated rat soleus muscles stimulated through the nerve.Exposure of muscles to buffer containing 85 mM Na+ and 9 mM K+ (85 Na+/9 K+ buffer) produced a 54 % decrease in M wave area and a 50 % decrease in tetanic force compared with control levels in standard buffer containing 147 mM Na+ and 4 mM K+. Subsequent stimulation of active Na+-K+ transport, using the β2-adrenoceptor agonist salbutamol, induced a marked recovery of M wave area and tetanic force (to 98 and 87 % of the control level, respectively). Similarly, stimulation of active Na+-K+ transport with insulin induced a significant recovery of M wave area and tetanic force.During equilibration with 85 Na+/9 K+ buffer and after addition of salbutamol there was a close linear correlation between M wave area and tetanic force (r = 0·92, P< 0·001). Similar correlations were found in muscles where tetrodotoxin was used to reduce excitability and in muscles fatigued by 120 s of continuous stimulation at a frequency of 30 Hz.These results show a close correlation between excitability and tetanic force. Furthermore, in muscles depressed by a reduction in the Na+/K+ gradients, β-adrenergic stimulation of the Na+-K+ pump induces a recovery of excitability which can fully explain the previously demonstrated recovery of tetanic force following Na+-K+ pump stimulation. Moreover, the data indicate that loss of excitability is an important factor in fatigue induced by high-frequency (30 Hz) stimulation