Different K⁺-release in distal myogenic and neurogenic muscular weakness during non-ischemic exercise

In myotonic dystrophy, an increased potassium release upon ischemic forearm exercise has been previously described. However, it remains unclear whether this is specific for myotonic dystrophies or just due to distal muscular weakness.

Non-ischemic forearm test (NIFET) was performed and venous K⁺ concentration was measured at rest and at three different force levels (20-30%, 50-60%, 70-80%) related to maximal contraction force (MCF) in patients with distal myogenic (n = 7), neurogenic (n = 7) muscular weakness and healthy volunteers (n = 12). The specific K⁺ release was defined as K⁺ increase related to workload as force-time-integral during repetitive contraction.

Workload was lower at all force levels in both disease groups compared to the control group. With increasing workload, the K⁺ concentrations increased in all study groups. Analysing individual force levels related to the maximum contraction force (MCF), a higher specific K⁺ release was measured at low force levels in myopathies (20-30% MCF) in comparison to higher force levels (p = 0.02). At 20-30% MCF, the specific K⁺ release was significantly higher in myogenic compared to neurogenic muscular weakness (p = 0.005). At 50-60% and 70-80% MCF, the specific K⁺ values converged and did not significantly differ between the three groups (p = 0.09 and p = 0.37).

At low force levels, K⁺ efflux related to workload is higher in patients with myogenic in comparison to neurogenic distal paresis. Our results indicate a different regulation of K⁺ balance in neurogenic and myogenic muscular weakness possibly due to a different recruitment behaviour of motor units and the firing rate of motor neurons.