Mechanism of specific force deficit in the senescent rat diaphragm

Aging is associated with a decline in the maximal in vitro specific force in the rat costal diaphragm. The purpose of this study was to determine if this force deficit is associated with a decrease in the concentration of myofibrillar protein in diaphragm fibers of senescent rats. Isometric twitch and tetanic contractile properties were measured on diaphragm strips from young adult (9-month-old; n=12) and senescent (26-month-old; n=13) male specific pathogen free-barrier protected Fischer 344 rats. Maximal tetanic force (P-o) normalized to the cross-sectional area (CSA) of the in vitro diaphragm strips was 16.4% lower in the senescent diaphragms (21.03 +/- 0.4 N/cm(2)) compared to the young adult (25.16 +/- 0.5 N/cm(2)) (p < 0.001). Diaphragm water content was significantly higher in the senescent group (75.9% of total wet mass) compared to the young adult (72.1% of total wet mass, p < 0.05) Subtracting the contribution of water from the CSA of the diaphragm strips significantly reduced (p < 0.05) the senescent specific P-o deficit (from - 16.4 to - 6.4%). Further, correcting P-o for the contribution of myofibrillar protein to CSA resulted in no age group differences in specific force. These data indicate that the age-related decline in diaphragm in vitro maximal specific P-o can be explained by an age-related increase in the water content of the diaphragm muscle. Future experiments are necessary to determine the mechanism(s) responsible for this observation. (C) 1997 Elsevier Science B.V.