EFFECTS OF AGING AND OBESITY ON RESPIRATORY MUSCLE PHENOTYPE IN ZUCKER RATS

Because obesity results in an increased work of breathing, we tested t he hypothesis that the oxidative properties and myosin heavy chain (MH C) isoform profiles in respiratory muscles would differ between lean a nd obese animals. Furthermore, we postulated that obesity-related chan ges in respiratory muscles would be independent of age. To test these hypotheses, samples of the costal diaphragm, crural diaphragm, and par asternal intercostal muscles were removed from three age groups (young , adult, and old) of obese and lean Zucker rats. Citrate synthase (CS) activity was measured as a marker of oxidative capacity, and MHC isof orms were identified with gel electrophoresis. Analysis revealed that CS activity was significantly higher in the crural and costal diaphrag ms and parasternal intercostals of obese animals compared with lean an imals (P < 0.05); this obesity related increase in CS activity was rel atively independent of age. Furthermore, respiratory muscle percent ty pe IIb MHC was lower and percent type I MHC isoforms were higher in ob ese animals compared with lean animals. These data support the notion that obesity results in a fast-to-slow shift in MHC phenotype and an i ncrease in oxidative capacity in major inspiratory muscles. The shift in MHC isoforms in obese animals is also age related, whereas the obes ity-mediated increase in oxidative capacity is relatively independent of age.