Low force contractions induce fatigue consistent with muscle mRNA expression in people with spinal cord injury

Abstract

Spinal cord injury (SCI) is associated with muscle atrophy, transformation of muscle fibers to a fast fatigable phenotype, metabolic inflexibility (diabetes), and neurogenic osteoporosis. Electrical stimulation of paralyzed muscle may mitigate muscle metabolic abnormalities after SCI, but there is a risk for a fracture to the osteoporotic skeletal system. The goal of this study was to determine if low force stimulation (3 Hz) causes fatigue of chronically paralyzed muscle consistent with selected muscle gene expression profiles. We tested 29 subjects, nine with a SCI and 20 without and SCI, during low force fatigue protocol. Three SCI and three non-SCI subjects were muscle biopsied for gene and protein expression analysis. The fatigue index (FI) was 0.21 ± 0.27 and 0.91 ± 0.01 for the SCI and non-SCI groups, respectively, supporting that the low force protocol physiologically fatigued the chronically paralyzed muscle. The post fatigue potentiation index (PI) for the SCI group was increased to 1.60 ± 0.06 (P <0.001), while the non-SCI group was 1.26 ± 0.02 supporting that calcium handling was compromised with the low force stimulation. The mRNA expression from genes that regulate atrophy and fast properties (MSTN, ANKRD1, MYH8, and MYCBP2) was up regulated, while genes that regulate oxidative and slow muscle properties (MYL3, SDHB, PDK2, and RyR1) were repressed in the chronic SCI muscle. MSTN, ANKRD1, MYH8, MYCBP2 gene expression was also repressed 3 h after the low force stimulation protocol. Taken together, these findings support that a low force single twitch activation protocol induces paralyzed muscle fatigue and subsequent gene regulation. These findings suggest that training with a low force protocol may elicit skeletal muscle adaptations in people with SCI.

Keywords: Electrical stimulation; genotype; paralysis; phenotype; potentiation.

Figures

Figure 1.

Torque measurement apparatus, with adjustable load cell and stabilization cuff (bottom). Representative twitches at the start, the peak, and the end of the first bout of the 3 Hz stimulation protocol in paralyzed muscle (top). The initial and peak twitches show little difference in amplitude, but the later twitches show a decrease in amplitude.

Figure 2.

(A)The group means and standard errors for the fatigue index (FI), as a function of the maximum twitch, for bout 1 and bout 2. The twitches after the maximum twitch within each bout were grouped in bins of 10% of the remaining twitches. (B) The mean fatigue index (FI) for each group for bout 1 and bout 2. *Significant difference between SCI and non‐SCI for bout 1; **Significant difference between SCI and non‐SCI for bout 2; ***Significant difference between bout 1 and 2 for the SCI group.

Figure 3.

(A) The group means and standard errors for the potentiation index (PI), as a function of the maximum twitch, for bout 1 and bout 2. The twitches before the maximum twitch within each bout were grouped in bins of 10% of the preceding twitches. (B) The potentiation index (PI) for each group for bout 1 and 2. *Significant difference between SCI and non‐SCI for bout 1; **Significant difference between SCI and non‐SCI for bout 2; ***Significant difference between bout 1 and 2 for the SCI.

Figure 4.

(A) The relative gene expression intensity for MSTN, ANKRD1, MYH8, MYCBP2 between a chronic SCI and non‐SCI muscle phenotype. (B) A bidirectional standard error plot of the fatigue index versus the relative gene expression intensity (MSTN, ANKRD1, MYH8, and MYCBP2) for the chronic SCI phenotype (closed circle) and non‐SCI phenotype (open circle). The relative expression intensity for all genes were significant at the P <0.05 level (see text for details).

Figure 5.

(A) The relative gene expression intensity for MSTN, ANKRD1, MYH8, MYCBP2 between a chronic SCI and non‐SCI muscle phenotype. (B) A bidirectional standard error plot of the fatigue index versus the relative gene expression intensity (MYL3, SDHB, PDK2, and RYR1) for the chronic SCI phenotype (closed circle) and non‐SCI phenotype (open circle). The relative expression intensity for all genes was significant at the P <0.05 level (see text for details).

Figure 6.

The relative protein expression level of SDH, an oxidative enzyme in the citric acid cycle which is partially translated by SDHB, was significantly depressed in chronic SCI compared to non‐SCI at the P <0.05 level(A). The relative protein expression level of SERCA2, a calcium transport protein associated with slow‐twitch muscle and the associated RyR1, was significantly depressed in chronic SCI compared to non‐SCI at the P <0.10 level (B).

Figure 7.

The gene expression changes from a single subject (subject 6) 3 h after a low force stimulation protocol. The expression of MSTN, ANKRD1, MYH8, MYCBP2, MYL3, SDHB, PDK2, and RYR1 is shown as the fold change difference between the stimulated muscle relative to the non stimulated muscle.