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A5871 - Skeletal Muscle Specific Calpastatin Overexpression Ameliorates Reductions in Diaphragm Strength and Endurance with Aging
Author Block: E. Schroder, L. Wang, G. S. Supinski, L. P. Callahan; Division of Pulmonary, Critical Care and Sleep Medicine, University of Kentucky, Lexington, KY, United States.
Rationale: It is well documented that aging is a risk factor for the morbidity and mortality associated with respiratory diseases (pneumonia and respiratory infections) of those aged > 65, and that diaphragm dysfunction is a significant contributor to the increased susceptibility to respiratory complications. Recent studies suggest that pathologic activation of calpain may play a role in skeletal muscle dysfunction in aging. Objective: We tested the hypothesis that skeletal muscle-specific overexpression of calpastatin, the endogenous inhibitor of calpain, would ameliorate aging-associated diaphragm dysfunction. Methods: Four-6 and 25-27 month old wild type (WT) and muscle-specific calpastatin overexpressing (CalpOX) male mice (n=24) were studied. Diaphragm strips were studied in vitro in organ baths containing Krebs-Henseleit solution at 24°C. We assessed muscle field stimulated force frequency curves and muscle fatigue (40 Hz stimulation with 0.330 sec trains every 1 sec. for 2 minutes). Immunohistochemistry was employed to examine fiber type. Fiber cross-sectional area and fiber type were measured using Myovision software. Results: Aging significantly reduced diaphragm specific force generation (15.8±1.4 N/cm2 vs 28.6±1.9 N/cm2; p=0.0003 for old WT vs young WT, respectively). Importantly, CalpOX mitigated the aging-induced reductions in diaphragm specific force (15.8±1.4 N/cm2 vs. 25.5±2.2 N/cm2; p=0.0034 for old WT vs old CalpOX, respectively). We evaluated muscle fatigue with the force-time integral as this index is reported to provide a good representation of the metabolic state of the muscle. The force time integral was significantly reduced in the old wild type mice (329.7±46.32) when compared to young WT (627.3±29.85, p=0.025), young CalpOX (665.8±81.91, p=0.0068), and old CalpOX (611.9±62.03, p=0.018) mice. No differences in the force time integral were observed between the old CalpOX and young mice from either strain. Overall fiber cross-sectional area in the diaphragm was not significantly different between strains or with age. Aging resulted in an increase in the percentage of slower fiber types (Type 1 and Type 2A) in both strains of mice and, the percentage of fibers identified as hybrid fibers increased in the old CalpOX diaphragm. Conclusions: To our knowledge, this is the first report of the effects of muscle-specific calpastatin overexpression on diaphragm function in aging rodents. These findings support the concept that calpain activation plays a role in aging-induced diaphragm dysfunction. We speculate that therapeutic agents which target the calpain/calpastatin pathway have the potential to elucidate novel therapies to combat muscle weakness in the aging population.