Pyruvate Dehydrogenase Activity in Red and Superficial White Gastrocnemius Muscles of the Rat during Contraction

Abstract

The increase in glucose oxidation during exercise is dependent upon the mitochondrial enzyme pyruvate dehydrogenase (PDH) which is allosterically regulated by reversible phosphorylation through a family of PDH kinases and phosphatases. However, the relationship between PDH activation and muscle fiber type during exercise has not been well described because fiber specific muscles have not been reported, and, because voluntary exercise involves motor unit recruitment which further complicates interpretation. To determine whether PDH activation differs in red versus superficial white muscle in vivo, gastrocnemius muscle contractile activity was controlled by electrical pacing via surgically implanted electrodes on the sciatic nerve of anesthetized male Wistar rats. Muscle force production of the triceps surae muscle group was recorded from the Achilles tendon and the knee pinned in place with a tungsten rod through the head of the femur. Gastrocnemius muscle samples were collected from the superficial white (predominantly Type IIb fibers) and deep red (predominantly Type IIb fibers) portions and freeze clamped at rest and during 5 min of twitch stimulation at 0.5Hz and 1.0Hz intensities. PDH activity was assayed in vitro using a radioisotopic tracer assay with [2‐14C]‐pyruvate as the substrate. Total PDH activity in the deep red gastrocnemius was almost two‐fold greater than that of the superficial white (1.9±0.1 vs 1.1±0.1 U/gWW, n=15). Fractional PDH activity (% total activity) was significantly increased in muscle samples collected from superficial white relative to deep red gastrocnemius at rest (15.0±2.0 vs 9.5±0.7%), and during 0.5Hz (60.9±8.1 vs 37.4±3.9%), and 1.0Hz stimulation frequencies (68.3±2.8 vs 40.5±2.8%) (n=4–5 for all groups). However, the active fraction of PDH activity was not different when comparing superficial white to deep red gastrocnemius at rest (0.12±0.02 vs 0.15±0.02 U/gWW) or during stimulation at 0.5Hz (0.67±0.08 vs 0.71±0.04 U/gWW), or 1.0Hz intensities (3±0.08 vs 0.82±0.02 U/gWW). These data suggest that the active fraction of PDH is dependent on the contraction intensity and therefore muscle fiber type differences do not appear to play a role in the number of PDH active sites during contractile activity. Therefore future designs of therapeutic exercise to optimize glucose disposal should be chosen based on the maximal intensity that can be maintained without fatigue to maximize the working muscle mass and PDH activity.Support or Funding InformationSupported by NIH DK095210