Inhibition of angiogenesis by high salt diet is associated with impaired muscle performance following chronic muscle stimulation

Abstract

Objective: High dietary salt has been demonstrated to inhibit angiogenesis in skeletal muscle. The purpose of this study was to determine whether high salt impairs steady state muscle performance following a chronic stimulation protocol.

Methods: Sprague-Dawley rats were placed on a control diet (CD, 0.4% NaCl) or high salt diet (HSD, 4.0% NaCl) prior to implantation of an electrical muscle stimulator. In chronically stimulated animals, hind limb muscles were stimulated to contract eight hours daily for seven days. Sham animals received a stimulator that was never activated.

Results: Following chronic stimulation, tibialis anterior (TA) muscles of animals on CD demonstrated an 84.6% increase in force of contraction at the end of an acute stimulation bout relative to sham animals fed CD. Decreased muscle fatigue was associated with an increase in capillaries per TA fiber (C:F). Chronic stimulation in HSD rats induced a smaller improvement (52.2%) in final force compared to HSD sham rats. This impairment of muscle performance in high salt-fed rats correlated with inhibited angiogenesis. Infusion of angiotensin II in HSD animals restored angiogenesis and muscle fatigue to CD levels.

Conclusions: This study suggests that angiogenic inhibition by high salt is associated with impaired skeletal muscle performance following chronic stimulation.

Figure 1

Force generated by one hour of acute stimulation of the TA muscle. ◇ Control diet (CD) Sham (n = 7) □ CD Stim (n = 10) ▲ High salt diet (HSD) Sham (n = 7) • HSD Stim (n = 9). (*) Significant difference vs. CD Stim, p < 0.05. (#) Significant difference at all points of stimulation except peak vs. corresponding sham group, p < 0.05.

Figure 2

Capillary: fiber ratio in all fiber types of the TA muscle in CD and HSD Sham and Stim groups (n = 5/group) following the seven day stimulation period. (*) Significant difference vs. corresponding sham group, p < 0.05. (#) Significant difference vs. HSD Stim group, p < 0.05.

Figure 3

Force generated by one hour of acute stimulation of the TA muscle expressed as a percent of peak force. □ CD Stim (n = 10) • HSD Stim (n = 9) ▲ HSD+ANGII Stim (n = 6) × Combined Sham groups (n = 21).

Figure 4

Capillary: fiber ratio in all fiber types of the TA muscle in HSD+ANGII Sham (n = 4) and Stim (n = 5) groups following the seven day stimulation period. (*) Significant difference vs. HSD+ANGII sham group, p < 0.05.

Figure 5

Representative micrographs of muscle cross sections stained with MHC-specific antibodies (green) and GS-I lectin (red) to identify capillaries. Images a and b display MHC type I specific staining in the core and cortex, respectively. Images c and d display MHC type IIa staining, and images e and f demonstrate MHC type IIb staining.

Figure 6

Iliac arterial blood flow in the stimulated hind limb during one hour of acute stimulation. ◇ Control diet (CD) Sham (n = 9) □ CD Stim (n = 5) ▲ High salt diet (HSD) Sham (n = 11) • HSD Stim (n = 7). (*) Significant difference vs. CD Stim, p < 0.05.

Figure 7

Iliac arterial blood flow in the stimulated hind limb during one hour of acute stimulation, expressed as a percent of baseline flow. ◇ Control diet (CD) Sham (n = 9) □ CD Stim (n = 5) ▴ High salt diet (HSD) Sham (n = 11) • HSD Stim (n = 7).