Abstract
What is the central question of this study? Is electrical pulse stimulation (EPS) an in vitro exercise model able to elicit the hypertrophy of human muscle cells? What is the main finding and its importance? The addition of a restitution period of 8 h after EPS induces the enlargement of human muscle cells, a major physiological end-point to resistance exercise. This is supported by downregulation of myostatin, a negative regulator of muscle mass, and increased phosphorylated mTOR and 4E-BP1, key factors in the growth cascade. This proof-of-concept study provides a model of physiologically mediated muscle growth, which will be the basis for future studies aiming to depict molecular events governing the hypertrophy of human muscle cells. Electrical pulse stimulation (EPS) of muscle cells has previously been used as an in vitro exercise model. The present study aimed to establish an EPS protocol promoting the hypertrophy of human muscle cells, which represents a major physiological end-point to resistance exercise in humans. We hypothesized that adding a resting period after EPS would be crucial for the occurrence of the morphological change. Myoblasts obtained from human muscle biopsies (n = 5) were differentiated into multinucleated myotubes and exposed to 8 h of EPS consisting of 2 ms pulses at 12 V, with a frequency of 1 Hz. Myotube size was assessed using immunohistochemistry immediately, 4 and 8 h after completed EPS. Gene expression and phosphorylation status of selected markers of hypertrophy were assessed using RT-PCR and Western blotting, respectively. Release of the myokine interleukin-6 in culture medium was measured using enzyme-linked immunosorbent assay. We demonstrated a significant increase (31 ± 14%; P = 0.03) in the size of myotubes when EPS was followed by an 8 h resting period, but not immediately or 4 h after completion of EPS. The response was supported by downregulation (P = 0.04) of the gene expression of myostatin, a negative regulator of muscle mass, and an increase in phosphorylated mTOR (P = 0.03) and 4E-BP1 (P = 0.01), which are important factors in the cellular growth signalling cascade. The present work demonstrates that EPS is an in vitro exercise model promoting the hypertrophy of human muscle cells, recapitulating a major physiological end-point to resistance exercise in human skeletal muscle.