Age-related increases in diaphragmatic maximal shortening velocity

Recent evidence demonstrates that aging results in an increase in fast (type IIB) myosin heavy chain (MHC) in the rat diaphragm. It is unknown whether this age-related change in fast MHC influences the diaphragmatic maximal shortening velocity (V-max). Therefore, we tested the hypothesis that aging is associated with an increase in the diaphragmatic V-max and that the increase in the V-max is highly correlated with the percentage of type IIb MHC. In vitro contractile properties were measured with costal diaphragm strips obtained from young (4 mo old; n = 8) and old (24 mo old; n = 8) male Fischer-344 rats. Diaphragmatic maximal tetanic specific force production was 14.5% lower in the old compared with the young animals (23.0 +/- 0.4 vs. 19.7 +/- 0.8 N/cm(2); P < 0.05). In contrast, the diaphragmatic V,, was significantly higher in the old compared with the young animals (5.5 +/- 0.1 vs. 4.4 +/- 0.3 lengths/s; P < 0.05). Although the percent type IIb MHC was significantly higher (approximately +14%; P < 0.05) in the old compared with the young animals, the correlation between V-max and percent type IIb MHC was relatively low (r = 0.50; P > 0.05). These data support the hypothesis that an age-related increase in diaphragmatic V-max occurs; however, factors in addition to type IIb MHC are involved in regulating diaphragmatic V-max. Interestingly, although aging resulted in a decrease in diaphragmatic maximal specific force production, power output at all muscle loads was maintained in the old animals due to the increase in diaphragmatic shortening velocity.