Structure of Skeletal Muscle in Heart Transplant Recipients11This work was supported in part by Grant 31-30946.91 from the Swiss National Science Foundation, Bern and by the Institute of Sports Sciences, Magglingen, Switzerland.

AbstractObjectives. This study sought to define the ultrastructural characteristics of skeletal muscle in heart transplant recipients (HTRs) in relation to exercise capacity compared with that in age-matched control subjects.Background. Muscle structural features seem to play an important role in the limitation of exercise capacity of HTRs long after transplantation.Methods. The structure of the vastus lateralis muscle was analyzed by ultrastructural morphometry in 16 HTRs and 20 healthy control subjects. Maximal oxygen consumption (peakV̇O2) was determined by an incremental exercise test.Results. Peak V̇O2 was significantly lower (by 35%) in HTRs. Fiber size, volume density of mitochondria and intramyocellular lipid deposits were not significantly different between HTRs and control subjects. In contrast, the capillary density and the capillary/fiber ratio were both significantly reduced in HTRs (by 24% and 27%, respectively).Conclusions. A normal volume density of mitochondria and a reduced capillary network are the main characteristics of muscle ultrastructure in HTRs by 10 months after transplantation. The muscle structural abnormalities and reduced exercise capacity might be related to immunosuppressive therapy with cyclosporine and corticosteroids as well as deconditioning