Evaluation of Na+/K+ pump function following repetitive activity in mouse peripheral nerve

After conduction of prolonged trains of impulses the increased Na+/K+ pump activity leads to hyperpolarization. The aim of this study was to develop a mouse model to investigate the Na+/K+ pump function in peripheral nerve by measuring the decrease in excitability during activity-dependent hyperpolarization. Acute electrophysiological investigations were carried out in seven adult mice. Nerve excitability was evaluated by tracking the change in threshold current after 5 min of 100 Hz stimulation of the tibial nerve at ankle. We developed a threshold tracking system that allowed us to follow several excitability measures simultaneously from the evoked plantar compound muscle action potential (CMAP) and sciatic compound nerve action potential (CNAP). Three minutes after repetitive supramaximal stimulation maximal CMAP and CNAP amplitudes recovered but the threshold was increased ∼40% for motor axons ∼34% for axons generating CNAP. The threshold recovered with a rate of 3.8%/minute that was similar for nerve and motor responses. By tracking the effect of polarizing currents we found evidence of activity dependent hyperpolarization, and our data suggest that the observed threshold change after repetitive stimulation of the mouse tibial nerve is an indicator of the Na+/K+ pump function in vivo. Evaluation of activity-dependent hyperpolarization may be an important indicator of axonal ability to cope with Na+ load.