IMMOBILIZATION EFFECTS ON CONTRACTILE PROPERTIES OF AGING RAT SKELETAL-MUSCLE

The effect of four weeks of ankle immobilization on muscle mass and in situ contractile properties of soleus (SOL), plantaris (PLA), and ext ensor digitorum longus (EDL) of 31- and 37-month-old (mo) Fisher 344/B rown Norway rats was examined. Following contractile tests, muscles we re harvested, weighed, and analyzed for fiber type and fiber cross-sec tional area. Body weights (g) were similar in both control (CON) group s (31 mo, 578+/-20; 37 mo, 553+/-26; mean+/-SE) and both immobilized ( IM) groups (31 mo, 496+/-6; 37 mo, 461+/-51. Immobilization-related di fferences in peak tetanic tension (g) were less in 37 mo than 31 mo ra ts (age x treatment, p<0.05) for SOL (31 mo, CON 156+/-11, IM 63+/-12; 37 mo, CON 70+/-16, IM 46+/-8), PLA (31 mo, CON 435+/-13, IM 239+/-40 ; 37 mo, CON 155+/-14, IM 152+/-20) and EDL (31 mo, CON 227+/-13, IM 1 39+/-17; 37 mo, CON 117+/-16, IM 108+/-4). Immobilization-related diff erences in muscle mass (mg) were smaller in 37 mo rats compared to 31 mo animals for SOL (31 mo, 206+/-14 vs 129+/-8, 37 mo, 148+/-5 vs 114/-2, age x treatment p<0.06) and PLA (31 mo, 409+/-14 us 257+/-22 37 m o, 234+/-17 vs 181+/-18, age x treatment p<0.05), but immobilization-r elated muscle mass differences were similar in both age groups for EDL (31 mo, 178+/-7 vs 134+/-9; 37 mo, 157+/-10 vs 112+/-7). There were n o immobilization-related changes in fiber type distribution in any of the three muscles studied in either age group. The results suggest tha t disuse-related change is diminished when superimposed on muscles tha t have already undergone marked age-related decline.