Low-intensity exercise under ischemic conditions enhances metabolic stress in patients with heart failure

Angiotensin-converting-enzyme inhibitor prevents skeletal muscle fibrosis in myocardial infarction mice

BACKGROUND

Transforming growth factor beta (TGF-β)-Smad2/3 is the major signaling pathway of fibrosis, which is characterized by the excessive production and accumulation of extracellular matrix (ECM) components, including collagen. Although the ECM is an essential component of skeletal muscle, fibrosis may be harmful to muscle function. On the other hand, our previous studies have shown that levels of angiotensin II, which acts upstream of TGF-β-Smad2/3 signaling, is increased in mice with myocardial infarction (MI). In this study, we found higher skeletal muscle fibrosis in MI mice compared with control mice, and we investigated the mechanisms involved therein. Moreover, we administered an inhibitor based on the above mechanism and investigated its preventive effects on skeletal muscle fibrosis.

METHODS

Male C57BL/6 J mice with MI were created, and sham-operated mice were used as controls. The time course of skeletal muscle fibrosis post-MI was analyzed by picrosirius-red staining (days 1, 3, 7, and 14). Mice were then divided into 3 groups: sham + vehicle (Sham + Veh), MI + Veh, and MI + lisinopril (an angiotensin-converting enzyme [ACE] inhibitor, 20 mg/kg body weight/day in drinking water; MI + Lis). Lis or Veh was administered from immediately after the surgery to 14 days postsurgery.

RESULTS

Skeletal muscle fibrosis was significantly increased in MI mice compared with sham mice from 3 to 14 days postsurgery. Although mortality was lower in the MI + Lis mice than the MI + Veh mice, there was no difference in cardiac function between the 2 groups at 14 days. Skeletal muscle fibrosis and hydroxyproline (a key marker of collagen content) were significantly increased in MI + Veh mice compared with the Sham + Veh mice. Consistent with these results, protein expression of TGF-β and phosphorylated Smad2/3 in the skeletal muscle during the early time points after surgery (days 1-7 postsurgery) and blood angiotensin II at 14 days postsurgery was increased in MI mice compared with sham mice. These impairments were improved in MI + Lis mice, without any effects on spontaneous physical activity, muscle strength, muscle weight, and blood pressure.

CONCLUSIONS

ACE inhibitor administration prevents increased skeletal muscle fibrosis during the early phase after MI. Our findings indicate a new therapeutic target for ameliorating skeletal muscle abnormalities in heart diseases.

Resistance training with interval blood flow restriction effectively enhances intramuscular metabolic stress with less ischemic duration and discomfort

Increases in muscle size and strength similar to those obtained with high resistance load can be achieved by combining lower loads with continuous blood flow restriction (BFR). However, high ratings for distress have been reported for continuous BFR. Therefore, we investigated the efficacy (metabolic stress) of BFR applied only during intervals in resistance exercise. Seven healthy men performed three 1-min sets of plantar flexion (30 reps/min) with 1-min rest intervals under 4 conditions: low-load resistance exercise (L, 20% 1-repetition maximum (1RM)) without BFR (L-noBFR), L with BFR during exercise sets (L-exBFR), L with BFR during rest intervals (L-intBFR), and L with continuous BFR during both exercise and rest intervals (L-conBFR). Based on the results of the first experiment, we performed additional protocols using a moderate load (M, 40% 1RM) with intermittent (exercise or rest intervals) BFR (M-exBFR and M-intBFR). Intramuscular metabolic stress, defined as decreases in phosphocreatine and intramuscular pH, was evaluated by ³¹P magnetic resonance spectroscopy. Rated perceived exertion (RPE) was also assessed. At the end of exercise, total decreases in phosphocreatine and intramuscular pH were similar among L-noBFR, L-intBFR, and L-exBFR and significantly less than those in L-conBFR (p < 0.05). In contrast, changes in these variables in M-intBFR but not in M-exBFR were similar to those in L-conBFR. Nevertheless, RPE was lower in M-intBFR than in both M-exBFR and L-conBFR (p < 0.05). The effect of intermittent BFR during exercise might be insufficient to induce metabolic stress when using a low load. However, effective metabolic stress for muscle adaptation could be obtained by moderate-load resistance exercise with BFR during intervals with less ischemic duration and discomfort.