Chronic electrical stimulation leads to a fast-to-slow phenotype transformation and reduces muscle mass in rat extensor digitorum longus muscle (EDL). These changes are likely to result from the activity of signal transduction pathways that are activated by contraction and regulate the adaptive process. The aim was to characterise signal transduction pathway activation in rat EDL after 6 weeks of electrical stimulation when signalling maintains a slower, transformed phenotype and lower muscle mass.

We measured the phosphorylation of proteins involved in the regulation of motor proteins (extracellular signal regulated kinase, ERK1/2), mitochondrial biogenesis (AMP-activated kinase, AMPK; p38 kinase, p38), translational signalling (protein kinase B, PKB; glycogen synthase kinase, GSK3beta; 4E-binding protein, 4E-BP1; p70 S6 kinase, p70 S6k), myostatin growth regulation (myostatin; SMAD2/3) and ribosome biogenesis (upstream binding factor, UBF). Miniature stimulators were implanted into the peritoneal cavity under isoflurane/nitrous oxide anaesthesia with their integral electrodes placed close to the common peroneal nerve. Intramuscular buprenorphine (0.1-0.5 mg/kg) was used to provide post-operative analgesia. Rat EDL muscles were chronically stimulated at 10 Hz for 6 weeks. Rats were killed humanely and both EDLs were removed and quickly frozen in liquid nitrogen. Protein and phospho-protein levels were measured by densitometry of Western blots.

Electrical stimulation increased the phosphorylation of ERK1 5.80 ± 5.91-fold, p38 4.31 ± 1.82-fold and c-jun kinase (JNK) mitogen-activated protein kinase 1.99 ± 0.71-fold whereas the phosphorylation of ERK2 decreased to 0.60 ± 0.21 of control. It is unclear how ERK1 activation and ERK2 inhibition affect motor protein isoform expression. AMPK and p38 phosphorylation increased 3.78 ± 2.45 and 4.31 ± 1.82-fold, respectively, which is consistent with the proposed involvement of AMPK and p38 in mitochondrial biogenesis. AMPK activation can also explain the decrease in phosphorylation of the translational activator p70 S6k to 0.33 ± 0.08 of control (Bolster et al. 2002). Myostatin increased 2.68 ± 1.32-fold and increased Smad2 and 3 phosphorylation by 2.74 ± 1.63 and 2.24 ± 0.86, respectively, which suggests atrophy signalling. UBF content increased to 2.05 ± 0.09 of control which is consistent with increased (r)RNA synthesis and ribosome biogenesis seen in chronically stimulated muscle (Cummins & Salmons, 1999). However, UBF phosphorylation was only 0.83 ± 0.10 of control.