RESPIRATORY MUSCLE FATIGUE RESISTANCE RELATES TO MYOSIN PHENOTYPE ANDSDH ACTIVITY DURING DEVELOPMENT

We correlated the fatigue resistance (FR) of the costal diaphragm (DIA ) and external abdominal oblique (EAO) of the rat during postnatal dev elopment with their respective 1) myosin heavy chain (MHC) phenotypes and 2) oxidative capacities [indexed by quantitative measurements of s uccinic dehydrogenase (SDH) enzyme activity]. FR was measured in vitro during isometric contractions with the use of the Burke fatigue test. FR of the DIA and EAO was high in newborns and declined during postna tal development. SDH activity was uniformly low in neonatal DIA and EA O and increased during early postnatal development before declining to adult levels. FR did not significantly correlate with SDH activity (r 2 = 0.01) but did relate to the MHC phenotype as indexed by the ratio of adult MHC isoform content (slow + IIa + IIx + IIb) to developmental MHC isoform content (slow + neonatal; r2 = 0.88, P < 0.01). Stepwise regression revealed that neonatal MHC expression alone accounted for 6 0% of the developmental variance in FR. The correlation between FR and MHC phenotype was improved if SDH was also considered, i.e., the rati o of SDH to MHC phenotype (r2 = 0.99, P < 0.01). We conclude that FR o f respiratory muscle during development relates to a balance between t he energetic demands of the muscle contractile proteins as reflected b y MHC isoform composition and its oxidative capacity with MHC phenotyp e alone exerting a strong predictive effect on FR.