MUSCLE ADAPTATIONS TO HINDLIMB SUSPENSION IN MATURE AND OLD FISCHER-344 RATS

We examined skeletal and cardiac muscle responses of mature (8 mo) and old (23 mo) male Fischer 344 rats to 14 days of hindlimb suspension. Hexokinase (HK) and citrate synthase (CS) activities and GLUT-4 glucos e transporter protein level, which are coregulated in many instances o f altered neuromuscular activity, were analyzed in soleus (Sol), plant aris (Pi), tibialis anterior (TA), extensor digitorum longus (EDL), an d left ventricle. Protein content was significantly (P < 0.05) lower i n all four hindlimb muscles after suspension compared with controls in both mature (21-44%) and old (17-43%) rats. Old rats exhibited signif icantly lower CS activities than mature rats for the Sol, P1, and TA. HK activities were significantly lower in the old rats for the P1 (19% ) and TA(33%), and GLUT-4 levels were lower in the old rats for the TA (38%) and EDL (24%) compared with the mature rats. Old age was also a ssociated with a decrease in CS activity (12%) and an increase in HK a ctivity (14%) in cardiac muscle. CS activities were lower in the Sol ( 20%) and EDL (18%) muscles from mature suspended rats and in the Sol ( 25%), P1 (27%), and EDL (25%) muscles from old suspended rats compared with corresponding controls. However, suspension was associated with significantly higher HK activities for all four hindlimb muscles exami ned, in both old (16-57%) and mature (10-43%) rats, and higher GLUT-4 concentrations in the TA muscles of the old rats (68%) but not the mat ure rats. These results indicate that old age is associated with decre ased CS and HK activities and GLUT-4 protein concentration for several rat hindlimb muscles, and these variables are not coregulated during suspension. Finally, old rat skeletal muscle appears to respond to sus pension to a similar or greater degree than mature rat muscle responds .