Metabolic Cost of Activation and Mechanical Efficiency of Mouse Soleus Muscle Fiber Bundles During Repetitive Concentric and Eccentric Contractions

Abstract

Currently available data on the energetics of isolated muscle preparations are based on bouts of less than 10 muscle contractions, whereas metabolic energy consumption is mostly relevant during steady state tasks such as locomotion. In this study we quantified the energetics of small fiber bundles of mouse soleus muscle during prolonged (2 min) series of contractions. Bundles (N = 9) were subjected to sinusoidal length changes, while measuring force and oxygen consumption. Stimulation (five pulses at 100 Hz) occurred either during shortening or during lengthening. Movement frequency (2-3 Hz) and amplitude (0.25-0.50 mm; corresponding to ± 4-8% muscle fiber strain) were close to that reported for mouse soleus muscle during locomotion. The experiments were performed at 32°C. The contributions of cross-bridge cycling and muscle activation to total metabolic energy expenditure were separated using blebbistatin. The mechanical work per contraction cycle decreased sharply during the first 10 cycles, emphasizing the importance of prolonged series of contractions. The mean ± SD fraction of metabolic energy required for activation was 0.37 ± 0.07 and 0.56 ± 0.17 for concentric and eccentric contractions, respectively (both 0.25 mm, 2 Hz). The mechanical efficiency during concentric contractions increased with contraction velocity from 0.12 ± 0.03 (0.25 mm 2 Hz) to 0.15 ± 0.03 (0.25 mm, 3 Hz) and 0.16 ± 0.02 (0.50 mm, 2 Hz) and was -0.22 ± 0.08 during eccentric contractions (0.25 mm, 2 Hz). The percentage of type I fibers correlated positively with mechanical efficiency during concentric contractions, but did not correlate with the fraction of metabolic energy required for activation.

Keywords: blebbistatin; cross-bridge cycling; mechanical efficiency; mouse soleus muscle; muscle activation; oxygen consumption.

FIGURE 1

Typical examples of tetanic contractions, tension-length and tension-stimulation frequency relation. (A) Tetanic contraction on ascending part of tension length relationship. (B) Tetanic contraction close to optimal fiber bundle length. In panels (A,B), red crosses indicate times at which stimulus pulses were applied (stimulation frequency was 100 Hz). (C) Active and passive tension-length relationship. In the active condition, each tetanus consisted of 20 stimulus pulses delivered at 100 Hz. Vertical dashed lines indicate the range of bundle lengths encountered during the work loops in 0.25 mm amplitude conditions. The datapoints labeled A and B correspond to the data in panels (A,B) of this figure. Multiple data points corresponding to a single length indicate repeated trials. (D) tetanic tension-stimulation frequency relationship. Each tetanus consisted of 20 stimulus pulses.

FIGURE 2

Typical example of histochemically stained cross-sections of a muscle fiber bundle. Type of staining is indicated within each image. The bottom right scale bar serves all panels and represents 200 μm. The numbered fibers 1–5 label Type I, IIA, IIB, IIX, and a fiber devoid of mitochondrial activity, respectively. Note that the fiber without mitochondrial activity (nr 5) is stained positive in the type I staining.

FIGURE 3

Summary data for a typical example of a concentric trial at a movement frequency of 2 Hz and amplitude 0.25 mm, pre-blebbistatin. (A) Bundle length (top inset) and force against time, relative to the onset of the trial. (B) Work loop depiction (force against bundle length) of the data in panel (A), the two cycles are superimposed. Arrows indicate the progression of time. The red crosses in panels (A,B) indicate instances at which stimuli were delivered. (C) Total amount of oxygen present in the chamber (blue, solid line), and the baseline oxygen trace of the chamber + bundle resting consumption (red, dashed line), against time. The black, dashed line sections are linear fits to the first and last 2 min of the oxygen trace, and the red circles are the means of these fits. The baseline is a 2nd degree polynomial of which the slope at the time points given by the red circles is equal to the slope of the respective black dashed lines, and which passes through the rightmost red circle. Vertical dashed lines indicate the start and end of the stimulation period. (D) left (blue) axis: net (baseline subtracted) oxygen consumption of bundle expressed as caloric equivalent, against time. The net amount of energy liberated by the bundle during the trial was computed as the difference between the mean of the first and last 2 min of the trial (horizontal black, dashed lines). Right (black) axis: total mechanical work computed as the integral of force with respect to bundle length change, against time. The net mechanical work associated with the trial was computed as the difference between the values at the beginning and the end of the stimulation period (vertical, dashed lines). Note that the work calculated directly from panel (B) would yield a net negative value per cycle, as per convention the force from the muscle on the environment is depicted positive, whereas in the calculation of mechanical work it was taken to be negative.

FIGURE 4

Summary data for a typical example of an eccentric contraction with amplitude 0.25 mm and movement frequency 2 Hz, pre-blebbistatin. Figure lay-out identical to that of Figure 3; see legend thereof.

FIGURE 5

Summary data for a typical example of a concentric contraction with amplitude 0.25 mm and movement frequency 2 Hz, post-blebbistatin. Figure lay-out identical to that of Figure 3; see legend thereof.

FIGURE 6

Summary data for a typical example of an eccentric contraction with amplitude 0.25 mm and movement frequency 2 Hz, post-blebbistatin. Figure lay-out identical to that of Figure 3; see legend thereof.

FIGURE 7

Scatterplot of the mechanical efficiency (A) and the fraction of metabolic energy spent on the activation process (B) against the percentage of type I fibers, for each contraction condition. The legend in panel (B) indicates the contraction conditions (amplitude and movement frequency), and serves both panels. The downward pointing triangles refer to concentric contraction and the upward pointing triangles refer to eccentric contraction. For each condition and variable, the Spearman correlation with the percentage of type I fibers is listed in Table 5.

FIGURE 8

Typical example of work loop shape and work per cycle as a function of cycle number for a concentric trial with movement amplitude 0.25 mm and 2 Hz movement frequency, pre-blebbistatin. (A) Bundle force against length, for cycle numbers 1, 10, 50, 100, 150, and 240 (top to bottom curves). (B) Net mechanical work per cycle as a function of cycle number. Ticks on the x-axis correspond with lines in panel (A) (the tick at cycle number 1 was left out for readability). The data in this figure corresponds to the data in Figure 3.

APPENDIX A1

Example of a raw oxygen trace containing artifacts encircled. This type of disturbance in the signal was typical for trials which were deemed invalid and was readily observable in the signal. The most likely cause for this artifact is small gas bubbles which may have been trapped in the chamber during the assembly process, and which interact with the oxygen electrode, which protrudes the chamber at the very top.