Diminished satellite cell fusion and S6K1 expression in myotubes derived from skeletal muscle of low birth weight neonatal pigs.

Low birth weight (LBWT) is consistently associated with impaired postnatal muscle growth in mammals. Satellite cell (SC)-mediated myonuclear incorporation precedes protein accumulation in the early stages of postnatal muscle development and growth. The objective of this study was to investigate proliferation and differentiation of SCs and the regulation of protein synthesis signaling in response to insulin-like growth factor (IGF)-I stimulation in SC-derived myotubes of LBWT neonatal pigs. SCs isolated from muscle of LBWT and NBWT pigs (3-d-old=8) were cultured and induced to proliferate and differentiate to myotubes in vitro. On day 3 of differentiation, myotubes were fasted in serum-free media for 3 h and treated with human recombinant R-insulin-like growth factor-I (rh IGF-I) at 0, 25, and 50 ng × mL for 30 min. There was no difference in proliferation rates of SCs from LBWT and NBWT pigs. However, LBWT SC fusion was 15% lower (≤0.05) without a difference in or mRNA expression in comparison with NBWT pigs, suggesting SCs are not intrinsically different between the two groups. IGF-Ι stimulation at physiological concentrations activated downstream effectors of mTOR similarly in myotubes from LBWT and NBWT pigs. However, abundance of ribosomal protein S6 kinase 1(S6K1) was lower in myotubes of LBWT compared to their NBWT siblings (≤0.05). These results indicate that the modest reduction in SC fusion and S6K1 expression are not the major contributors to the impaired postnatal muscle growth of LBWT pigs.