Single muscle fibre contractile properties in young and old men and women

Abstract

The purpose of this study was to determine whether there was an age-related decline in the isometric and isotonic contractile function of permeabilized slow (MHC I) and fast (MHC IIa) single muscle fibres. Vastus lateralis muscle fibres from six young men (YM; 25 +/- 1 years), six young women (YW; 25 +/- 1 years), six old men (OM; 80 +/- 4 years) and six old women (OW; 78 +/- 2 years) were studied at 15 degrees C for in vitro force-velocity properties, peak force and contractile velocity. Peak power was 23-28 % lower (P < 0.05) in MHC I fibres of YW compared to the other three groups. MHC IIa peak power was 25-40 % lower (P < 0.05) in OW compared to the other three groups. No difference was found in MHC I and IIa normalized peak power among any of the groups. Peak force was lower (P < 0.05) in the YW (MHC I fibres) and OW (MHC IIa fibres) compared to the other groups. Differences in peak force with ageing were negated when normalized to cell size. No age-related differences were observed in single fibre contractile velocity of MHC I and IIa fibres. These data show that YW (MHC I) and OW (MHC IIa) have lower single fibre absolute peak power and peak force compared to men; however, these differences are negated when normalized to cell size. General muscle protein concentrations (i.e. total, sarcoplasmic and myofibrillar) from the same biopsies were lower (4-9 %, P < 0.05) in the OM and OW. However, myosin and actin concentrations were not different (P > 0.05) among the four groups. These data suggest that differences in whole muscle strength and function that are often observed with ageing appear to be regulated by quantitative rather than qualitative parameters of single muscle fibre contractile function.

Figure 1. Representative force and length records and the resulting force-velocity relationship of a single muscle fibre from the vastus lateralis of a 93 year old male

Inset, series of 3 submaximal isotonic load clamps (each step lasted 100 ms). Force and shortening velocity were determined over the final one-third of each isotonic load clamp. The filled circles represent the resulting data points from the 3 isotonic load clamps shown in the inset. The open circles represent the resulting data points for the previous isotonic load clamps. Data were fitted using the Hill equation, (P + a)(V + b) = (P_o + a)/b, where P is force, V is velocity, P_o is peak isometric force, and a and b are constants of force and velocity, respectively. For this muscle fibre, the V_max was 0.80 FL s⁻¹, power was 11.2 μN FL s⁻¹, normalized power was 1.25 W l⁻¹, a/P_o was 0.024 and r² was 0.98. Gel electrophoresis identified this fibre as MHC I.

Figure 2. Representative force records of a maximally Ca²⁺-activated single muscle fibre from the vastus lateralis of a 93 year old male

A, original force recordings following slack steps of 150, 200, 250 and 300 μm. Records have been superimposed to illustrate the increasing duration to force redevelopment with increasing slack distance. B, time required to redevelop force plotted vs. imposed slack distance. Slope of this line defines fibre unloaded shortening velocity (V_o). In this example, V_o of the fibre was 0.98 FL s⁻¹ with a y-intercept of 20.5 μm. The fibre segment studied was 2.27 mm, resulting in a compliance of 0.9 %; r² was 0.99. Gel electrophoresis identified this fibre as MHC I.

Figure 3. Representative 5 % SDS-PAGE gel of human single muscle fibres for MHC identification

Each lane represents a single muscle fibre.

Figure 4. Representative silver-stained gel used for MHC (10 %) and actin (6-12 % gradient) protein quantification

Five standards (MHC: 1000, 800, 600, 400, 200 ng; Actin: 500, 400, 300, 200, 100 ng) were loaded in duplicate on each gel. Each gel also contained samples (800 ng myofibrillar protein) in duplicate from young and old men and women.

Figure 5. Normalized power (W l⁻¹) distribution of MHC I single muscle fibres from young men and women, and old men and women

Each point represents normalized power data from a single muscle fibre.

Figure 6. Normalized power (W l⁻¹) distribution of MHC IIa single muscle fibres from young men and women, and old men and women

Each point represents normalized power data from a single muscle fibre.

Figure 7. Scatter plot of fibre diameter and peak tension for the MHC I muscle fibres from young men (YM), old men (OM), young women (YW) and old women (OW)

Figure 8. Scatter plot of fibre diameter and peak tension for the MHC IIa muscle fibres from young men, old men, young women and old women