Decreased Prevalence of Myosin Heavy Chain Hybrid Isoforms in Lifelong Endurance Trained Octogenarians

Abstract

An increased prevalence of myosin heavy chain (MHC) hybrid muscle fibers (MHC I/IIa, IIax, and/or I/IIax) and the MHC IIx isoform is associated with elderly, sedentary, spinal cord injured, and bed ridden populations. Accordingly, highly trained young athletes typically express few MHC hybrid fibers and little MHC IIx isoform. To date there is no data pertaining to lifelong exercisers in their ability to maintain a comparable fiber type distribution as they age. PURPOSE: To examine single muscle fiber MHC composition in lifelong endurance athletes (n=9, 81±1 y, 68±3 kg, BMI=23±1 kg/m2) and compare to age-matched, independent living, healthy men (n=6; 82±1 y, 77±5 kg, BMI=26±1 kg/m2). The endurance athletes were cross-country skiers, including a former Olympic champion and several national/regional champions, with a history of aerobic exercise and frequent participation in an annual 90 km ski race and other related endurance events throughout their entire adult life (∼61 years/subject). METHODS: Muscle biopsies were obtained from the vastus lateralis. A total of 1905 muscle fibers for the athletes and 1246 muscle fibers for the non-athletes were isolated and analyzed electrophoretically (SDS-PAGE) for MHC content (∼209 fibers/subject). RESULTS: Athletes had a greater percentage (P<0.05) of MHC I fibers than non-athletes (54% vs. 33%). The athletes had fewer (P<0.05) MHC IIax fibers (6% versus 16%) and total hybrid fibers (13 ± 2% vs. 24 ± 5%) compared to non-athletes. The athletes had only 7% of fibers with any MHC IIx isoform present (MHC IIax + MHC IIx + MHC I/IIax) while 29% of the fibers from non-athletes contained the MHC IIx isoform (P<0.05). CONCLUSION: Muscle fibers from elite octogenarian athletes showed more pure MHC I and less MHC hybrid isoforms and MHC IIx isoform than non-athletic octogenarians. This pattern is consistent with the phenotype of well-trained young endurance athletes. This may suggest that lack of activity and not aging per se is the primary cause of an increased prevalence of hybrid fibers and MHC IIx expression with aging. The ability of the octogenarian athletes to retain a high percentage of the pure MHC I and IIa isoforms suggests an advantageous skeletal muscle health phenotype with advanced age.