Effects of Sarcolipin Ablation on Mitochondrial Enzyme Adaptations to Exercise Training
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Changes in intracellular Ca2+ ([Ca2+]f) and high-energy phosphates are known to induce adaptive changes in skeletal muscle during endurance exercising training, including mitochondrial biogenesis. Levels of [Ca2+]f are regulated by sarco(endo)plasmic reticulum Ca2+-ATPases (SERCAs) which are further regulated by sarcolipin (SLN), through a reduction in the apparent affinity of SERCAs for Ca2+. Furthermore, SLN reduces the efficiency of Ca2+ transport by SERCAs supporting a thermogenic role for SLN in skeletal muscle. Thus, it is possible SLN ablation could reduce Ca2+ and metabolic signaling during exercise training and attenuate increases in mitochondrial content. To investigate the potential role of SLN in the exercise-induced adaptive response of skeletal muscle, mice devoid of SLN (SLNKO) underwent endurance training for 8 weeks and were compared to WT controls. Maximal oxygen uptake (V̇O2 max) was measured with an exercise stress test while mitochondrial content was assessed through measurement of protein expression and maximal enzyme activities of several mitochondrial enzymes in soleus and extensor digitorum longus (EDL) muscles, which express high and low levels of SLN, respectively. All data were analyzed using a two-way analysis of variance (ANOVA) and student t-tests were conducted on enzyme data. V̇O2 max was found to not be significantly altered with exercise training in either genotype. Exercise training significantly increased the contents of adenine nucleotide translocase (ANT), cytochrome-c (cyt-c) and cytochrome-c oxidase subunit IV (COXIV) in soleus independent of genotype. Likewise, exercise training significantly increased cyt-c and COXIV expression (P<0.04), while increases in ANT expression were not significant (P=0.13) in the EDL. Two-way ANOVAs of mitochondrial enzymes in soleus revealed an interaction existed for succinate dehydrogenase (SDH) where its activity was increased only in the SLNKO mice (P<0.02). In comparison, exercise training significantly elevated activities of cytochrome c oxidase (COX) and citrate synthase (CS) activities (P<0.02) but not β-hydroxyacyl-CoA dehydrogenase (β-HAD; P=0.08), independent of genotype. Upon closer examination using student t-tests, it was determined that exercise training induced greater increases in COX and CS activity in SLNKO compared to WT controls (P<0.02), similar to and consistent with SDH data. In EDL, only SDH activity increased following exercise training, an effect that was independent of genotype. In conclusion, these data suggest that SLN ablation does not attenuate exercise-induced mitochondrial adaptations and may increase mitochondrial enzyme adaptations to exercise training in slow-twitch muscle. Further examination of the effects of SLN on Ca2+ and metabolic signaling may provide mechanisms explaining the results of this thesis.