Skeletal muscle response to short endurance training in heart transplant recipients

We sought to examine the effects of endurance training on the ultrastructural characteristics of skeletal muscle in heart transplant recipients (HTRs) and age-matched control subjects (C).

Deconditioning is one of the factors involved in the peripheral limitation of exercise capacity of HTRs, and training has proven to be beneficial.

Biopsies of the vastus lateralis muscle, analyzed by ultrastructural morphometry, and quadriceps muscle cross-sectional area, assessed by computed tomography (CT), were performed in 12 HTRs and 7 age-matched C before and 6 weeks after an endurance training program. Maximal oxygen uptake (peak VO2) was determined by an incremental exercise test. Additionally muscle biopsies were performed before and after a 6-week control period in four HTRs to check for spontaneous improvement.

Training resulted in similar increases in peak VO2 (11% in HTRs, 8.5% in C), ventilatory threshold (23% in HTRs, 32% in C) and total endurance work (54% in HTRs, 31% in C). Volume density of total mitochondria increased significantly (26% in HTRs, 33% in C) with a predominant increase of subsarcolemmal mitochondrial volume density (74% in HTRs, 70% in C). The capillary/fiber ratio increased by 19% in C only. In the nontrained group, none of the structural markers was spontaneously modified.

Six weeks of endurance training in HTRs and C led to similar improvements of aerobic work capacity. However, the decreased muscular capillary network in HTRs remained unchanged with training. Immunosuppressive therapy might be responsible for the discrepancy between the normal mitochondrial content and the reduced capillary supply of these patients.