Orthologous myosin isoforms and scaling of shortening velocity with body size in mouse, rat, rabbit and human muscles

Abstract

Maximum shortening velocity (V(0)) was determined in single fibres dissected from hind limb skeletal muscles of rabbit and mouse and classified according to their myosin heavy chain (MHC) isoform composition. The values for rabbit and mouse V(0) were compared with the values previously obtained in man and rat under identical experimental conditions. Significant differences in V(0) were found between fibres containing corresponding myosin isoforms in different species: as a general rule for each isoform V(0) decreased with body mass. Myosin isoform distributions of soleus and tibialis anterior were analysed in mouse, rat, rabbit and man: the proportion of slow myosin generally increased with increasing body size. The diversity between V(0) of corresponding myosin isoforms and the different myosin isoform composition of corresponding muscles determine the scaling of shortening velocity of whole muscles with body size, which is essential for optimisation of locomotion. The speed of actin translocation (V(f)) in in vitro motility assay was determined with myosins extracted from single muscle fibres of all four species: significant differences were found between myosin isoforms in each species and between corresponding myosin isoforms in different species. The values of V(0) and V(f) determined for each myosin isoform were significantly correlated, strongly supporting the view that the myosin isoform expressed is the major determinant of maximum shortening velocity in muscle fibres.

Figure 1. MHC isoform distribution in soleus (A), tibialis anterior (B) and single muscle fibres (C) of mouse, rat, rabbit and man

A, different proportions of two MHC isoforms (MHC-2A and MHC-1 or slow) are expressed in mouse, rat, rabbit and human soleus. B, two MHC isoforms (MHC-2X and MHC-2B) are detectable in mouse tibialis anterior, three isoforms (MHC-2A, MHC-2X, MHC-2B) are expressed in rat and in rabbit tibialis anterior and only two (MHC-2A and MHC-1 or slow) in human tibialis anterior. Note the different migration of fast MHC isoforms in rabbit. C, examples of MHC composition of single fibres: lane 1, rat pure fast 2B fibre; lane 3, rabbit pure slow fibre; lane 4, mouse pure fast 2X fibre; lane 5, mouse pure fast 2A fibre. Lane 2 shows a mixed rat muscle sample used as a reference. The relative percentage (mean ±s.e.m.) of MHC isoforms of soleus and tibialis anterior is shown in the histograms on the right-hand side of the figure.

Figure 3. Relation between maximum shortening velocity (V₀) of muscle fibres containing myosin orthologues and average body mass of mouse, rat, rabbit and man

Mean V₀ values of slow fibres (filled circles), fast 2A fibres (open circles), fast 2X fibres (stars) and fast 2B fibres (squares) are plotted against body mass. Continuous lines represent the results obtained by fitting the scaling equation Y =aX^b to the data points, which appears linear if both V₀ and body size are plotted on log scales. The values of the parameters (±s.e.m.) for slow fibres were:, where b is the allometric coefficient.

Figure 4. Relation between actin translocation velocity (V_f) by myosin orthologues in in vitro motility assay and average body mass of mouse, rat, rabbit and man

Mean V_f values of slow fibres (filled circles), fast 2A fibres (open circles), fast 2X fibres (stars) and fast 2B fibres (squares) are plotted against body mass. Continuous lines represent the results obtained by fitting the scaling equation Y =aX^b to the data points, which appears linear if both V_f and body size are plotted on log scales. The values of the parameters (±s.e.m.) obtained for slow myosin were:, where b is the allometric coefficient.

Figure 6. Relation between peak power output and body mass in mouse, rat, rabbit and man

Peak power output was calculated from the values of V₀ and P₀, under the assumption that peak power is reached at 20 % P₀ and 20 % V₀. Peak power values of slow fibres (filled circles), fast 2A fibres (open circles), fast 2X fibres (stars) and fast 2B fibres (squares) are plotted against body mass. Continuous lines represent the results obtained by fitting the scaling equation Y =aX^b to the data points, which appears linear if both peak power and body size are plotted on log scales. The mean values of the parameters (±s.e.m.) for slow fibres were:, where b is the allometric coefficient.

Figure 2. Comparison between the values of isometric tension (P₀/CSA) of single muscle fibres of mouse, rat, rabbit and man

Variance analysis on P₀/CSA values does not show significant differences between paralogues in mouse, rat, rabbit and man; significant differences between orthologues (mouse vs. man) are present in slow fibres but not in fast fibres.

Figure 5. Relation between V₀ and V_f in in vitro motility assay for all myosin orthologues examined in this study

Linear regression analysis indicates a highly significant correlation (P < 0.0001) between V₀ and V_f, and the regression line has a slope 0.529 and r²= 0.915.