Activity-dependent excitability changes suggest Na+/K+ pump dysfunction in diabetic neuropathy

The present study was undertaken to evaluate the role of Na(+)/K(+) pump dysfunction in the development of diabetic neuropathy (DN). Nerve excitability techniques, which provide information about membrane potential and axonal ion channel function, were undertaken in 15 patients with established DN and in 10 patients with diabetes who had no evidence of neuropathy (DWN). Excitability parameters were recorded at baseline, and then before and after 1 min of maximal voluntary contraction (MVC) of abductor pollicis brevis. Compared to controls, CMAP amplitude was significantly decreased in DN patients with associated reductions in strength-duration time constant and refractoriness, consistent with a reduction in nodal Na(+) conductances. Following MVC for 1 min, there was an increase in normalized threshold in all diabetic patients and controls, consistent with axonal hyperpolarization. When compared to control values, the increase in threshold following MVC was significantly less in DN patients (DN group 13.1 +/- 2.2%; controls 20.4 +/- 1.9%; P < 0.05) and the rate of recovery was slower (P < 0.01). In DWN patients, CMAP amplitude was preserved, and excitability values following MVC were not significantly different to control values. The reduced threshold change and slower recovery in DN patients following MVC are likely to be secondary to Na(+)/K(+) pump dysfunction. Alteration in Na(+)/K(+) pump function, coupled with reductions in nodal Na(+) currents, may be sufficient to trigger conduction failure in DN patients and are likely to contribute to the clinical symptoms of weakness and fatigue.