MECHANISM OF SPECIFIC FORCE DEFICIT IN THE SENESCENT RAT DIAPHRAGM

Aging is associated with a decline in the maximal in vitro specific fo rce in the rat costal diaphragm. The purpose of this study was to dete rmine if this force deficit is associated with a decrease in the conce ntration 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 Fisc her 344 rats. Maximal tetanic force (P-o) normalized to the cross-sect ional area (CSA) of the in vitro diaphragm strips was 16.4% lower in t he 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) Subtr acting the contribution of water from the CSA of the diaphragm strips significantly reduced (p < 0.05) the senescent specific P-o deficit (f rom - 16.4 to - 6.4%). Further, correcting P-o for the contribution of myofibrillar protein to CSA resulted in no age group differences in s pecific force. These data indicate that the age-related decline in dia phragm in vitro maximal specific P-o can be explained by an age-relate d increase in the water content of the diaphragm muscle. Future experi ments are necessary to determine the mechanism(s) responsible for this observation. (C) 1997 Elsevier Science B.V.