Abstract
PurposeTo examine the effect of α-actinin-3 deficiency due to homozygosity for the ACTN3 577X-allele on contractile and morphological properties of fast muscle fibers in non-athletic young men.MethodsA biopsy was taken from the vastus lateralis of 4 RR and 4 XX individuals to test for differences in morphologic and contractile properties of single muscle fibers. The cross-sectional area of the fiber and muscle fiber composition was determined using standard immunohistochemistry analyses. Skinned single muscle fibers were subjected to active tests to determine peak normalized force (P0), maximal unloading velocity (V0) and peak power. A passive stretch test was performed to calculate Young’s Modulus and hysteresis to assess fiber visco-elasticity.ResultsNo differences were found in muscle fiber composition. The cross-sectional area of type IIa and IIx fibers was larger in RR compared to XX individuals (P<0.001). P0 was similar in both groups over all fiber types. A higher V0 was observed in type IIa fibers of RR genotypes (P<0.001) but not in type I fibers. The visco-elasticity as determined by Young’s Modulus and hysteresis was unaffected by fiber type or genotype.ConclusionThe greater V0 and the larger fast fiber CSA in RR compared to XX genotypes likely contribute to enhanced whole muscle performance during high velocity contractions.
Highlights
The sarcomeric α-actinins are important structural components of the Z-line where they form the cross-link between the thin actin filaments from adjacent sarcomeres [1]
P0 was similar in both groups over all fiber types
The greater V0 and the larger fast fiber crosssectional area (CSA) in RR compared to XX genotypes likely contribute to enhanced whole muscle performance during high velocity contractions
Summary
The sarcomeric α-actinins are important structural components of the Z-line where they form the cross-link between the thin actin filaments from adjacent sarcomeres [1] In addition to their structural function, α-actinins interact with several structural, signaling and metabolic proteins. An estimated 16% of the human population is completely α-actinin-3 protein deficient due to homozygosity for a common stopcodon polymorphism in the ACTN3 gene called R577X (rs1815739) [3]. This deficiency does not result in a disease phenotype or muscular functional impairment, likely through a compensatory upregulation of the homologous αactinin-2 protein. In non-athletic populations, young RR men perform better on tasks with an explosive strength component compared to XX genotypes, including short sprints, squat jump and fast isokinetic movements [6,7,8], not all studies confirm these genotype-dependent differences [9,10,11,12]
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