Transcriptional co-activators have emerged as potent regulators of metabolism. The most characterised co-activator is the peroxisome proliferator activated receptor-γ co-activator 1α (PGC-1α), which has been identified as a master regulator of mitochondrial biogenesis (Spiegelman and Handschin, 2008). Less well characterized is the PGC1-α related co-activator PRC which shares similarities with PGC1-α, including an acidic NH2-terminal region, an LXXLL signature for nuclear co-activators, and a central proline rich region (Scarpulla, 2008). The aim of this study was to elucidate if PRC has a functional role in skeletal muscle. Five days following differentiation, C2C12 myotubes were treated for 72h with Glucose, Lactate, Pyruvate, Sodium Bicarbonate (5-50mM) or Sodium Chloride (50mM) as an osmotic control. For cell contraction experiments, myotubes were subjected to either low (1Hz, 3h), moderate (10Hz 3h) or high (100Hz, 0.5h) frequency contractions using custom-built stimulation units. Over-expression studies were performed using a plasmid containing the full-length cDNA of PRC in the mammalian expression vector pSV-SPORT. Transient transfections were carried out overnight using lipofectamine and 2ug of plasmid DNA. Following treatment, cells were collected in lysis buffer (50mM Tris pH 7.5; 250mM Sucrose; 1mM EDTA; 1mM EGTA; 1% Triton X-100; 1mM NaVO4; 50mM NaF; 0.10% DTT; 0.50% PIC), centrifuged for 5 mins at 8,000 RPM and the supernatant removed. The levels of mitochondrial metabolic proteins were determined by western blotting using commercially available antibodies. Real time quantitative PCR was performed to measure relative mRNA expression using an Eppendorf Light Cycler, SYBR green PCR plus reagents (Sigma Aldrich, Dorset, UK) and custom designed primers. Differences between groups were assessed by students paired t-tests (SPSS version 10); all results are expressed as Mean ± SEM. PRC expression was increased by pyruvate treatment (207%) as was cytochrome-c (206%) and COX-1 (117%), however there was no effect of lactate, glucose or NaHCO3 on PRC expression. Low frequency contraction increased PRC expression by 37% (1h), 147% (3h) and 37% (6h). This response was less pronounced than PGC-1α (457%, 6h), whilst PRC expression did not change at any of the other frequencies examined. Over-expression of PRC increased the protein content of PRC (22%), COX-2 (57%) and COX-5 (55%). This was coupled with an increase in basal cell respiration (48%) and glucose uptake (41%). These data indicate that PRC can increase respiratory chain function in skeletal muscle cells and is sensitive to alterations in muscle activity and nutrient availability, potentially suggestive of a role for PRC in skeletal muscle adaptation to endurance type exercise.