Adaptations of skeletal muscle pyruvate dehydrogenase kinase in response to food-restriction in mitochondrial subpopulations

University, 2006 Dr. Sandra J. Peters Pyruvate dehydrogenase (PDH) catalyses the decarboxylation of pyruvate, to form acetyl-CoA. PDH activity is down-regulated by intrinsic PDH kinases (predominantly PDK2 and PDK4 isoforms), but the understanding of the PDK isoform distribution and adaptation to nutritional stresses has been restricted to mixed mitochondrial populations, and not delineated between subsarcolemmal (SS) and intermyofibrillar (IMF) subpopulations. SS and IMF mitochondria exhibit distinct morphological and biochemical properties; however the functional differences are not well understood. This study investigated the effect of fed (FED) versus 48 h total foodrestriction (FR) on rat red gastrocnemius muscle PDK2 and 4 isoform content in SS and IMF mitochondria. PDK4 content was ~3-5 fold higher in SS mitochondria compared to IMF (p=0.001), and increased with FR -3-4- fold in both subpopulations (p<0.001). PDK2 was -2.5-4 fold higher in SS mitochondria compared to IMF (p=0.001), but PDK2 was unaltered with FR. Citrate synthase activity (|imol/min/mg mitochondrial protein) was not different between either subpopulation. As well there were no significant differences between mitochondrial subpopulations in PDH complex components in both fed and FR states. These results demonstrate that there is a markedly higher content of both PDK isofonns in SS compared to IMF mitochondria. Although PDK2 does not increase in either subpopulation in response to FR, PDK4 increases to a similar extent in both SS and IMF after 48 h food-restriction.

Decreased motor unit firing rate in the potentiated tibialis anterior in humans

With repeated activity, force production, rate of force production, and relaxation time are impaired. These are characteristics ofa fatigued muscle (Vandenboom, 2004). However, brief bouts of near maximal to maximal activity results in the increased ability of the muscle to generate force, termed post activation potentiation (P AP)(V andervoort et aI., 1983). The purpose of the present study was to characterize motor unit firing rate (MUFR) in the unfatigued, potentiated tibialis anterior (TA). Using a quadrifilar needle electrode, MUFR was measured during a 5s 50% MVC in which the TA was either potentiated or unpotentiated; monopolar electrodes measured surface parameters. A lOs MVC was used to potentiate the muscle. Firing rate decreased significantly from 20.15±2.9Opps to 18.27±2.99pps, while mean power frequency decreased significantly from 60. 13±7.75 Hz to 53.62±8.56 Hz. No change in root mean square (RMS) was observed. Therefore, in the present study, MUFR decreases in response to a potentiated TA.