Early changes in performance, blood flow and capillary fine structure in rat fast muscles induced by electrical stimulation

Abstract

1. Muscle blood flow, capillary fine structure and performance were investigated in the early stages of chronic indirect electrical stimulation of ankle flexors in the rat. 2. The fast muscles tibialis anterior (TA), extensor digitorum longus (EDL) and extensor hallucis proprius (EHP) were unilaterally stimulated via the right common peroneal nerve at 10 Hz and supramaximal voltage for 8 h a day for 2, 3 or 7 days and compared with muscles from control animals. 3. Muscle blood flow (MBF) was estimated at rest and during contractions by radioactive microspheres. It was higher at rest than in unstimulated controls only in muscles stimulated for 2 days; during contractions it was higher in some muscles stimulated for 3 days than in controls, and in all muscles by 7 days (192 +/- 17 vs. 149 +/- 12 ml (100 g)-1min-1 in controls). 4. Electron microscopical evaluation of individual capillaries in EHP fixed by superfusion in situ revealed thickening of capillary endothelium and decreased lumen volume in muscles stimulated for 7 (P < 0.005) but not 3 days. Significantly smaller capillary size indicates the presence of newly formed capillaries. 5. Isometric twitch tension, recorded from combined TA and EDL in stimulated and contralateral legs during 5 min contractions at 4 Hz, gradually declined from 175 +/- 9 to 99 +/- 4 kN m-2 after 7 days of stimulation (P < 0.05) while the fatigue index, calculated as (final twitch tension/peak twitch tension) x 100, increased from 69.8 +/- 3.4 to 90 +/- 3.0 % (P < 0. 05). No significant changes in the fatigue index occurred in muscles stimulated for 2 or 3 days. 6. Lower peak tension, but not fatigue index or MBF, was also observed in muscles contralateral to those stimulated for 3 and 7 days, which thus do not represent appropriate controls. 7. We conclude that the high resting blood flow found in muscles stimulated for 2 days may initiate the capillary growth reported previously, while the relatively modest increase in MBF during contractions in muscles that had been stimulated for 7 days may be due to increased capillary supply. Swelling of the capillary endothelium and decreased volume of the capillary lumen may result in an increased proportion of time spent by red blood cells in capillaries, which would improve oxygen extraction.

Figures

Figure 1. Original records of muscle tension by rat TA and EDL in response to indirect electrical stimulation at supramaximal voltage and optimal muscle length

Top, control muscle; bottom, muscle stimulated for 3 days. Vertical bar, 100 g; horizontal bar, 1 min.

Figure 2. Muscle blood flow in rat TA and EDL

□, stimulated muscles; ○, contralateral muscles. Note the transient rise in resting blood flow after 2 days of stimulation (A) that is not reflected by the functional hyperaemia, which shows only slight increases after 3 days and 7 days (B). * Significantly different from controls.

Figure 3. Muscle performance of rat TA and EDL

Symbols as in Fig. 2. Note the progressive decline in twitch tension development in both stimulated and contralateral limbs (A), while the reciprocal rise in endurance is only seen in stimulated muscles (B). *P < 0.05vs. control; +P < 0.05vs. contralateral.

Figure 4. The effect of electrical stimulation on capillary morphology in EHP (from control, 7 day stimulated and their respective contralateral muscles)

□, no evidence of EC swelling; , EC swelling around < 1/3 of capillary circumference; , EC swelling around > 1/3 of circumference. ★P < 0.05vs. control; ★★P < 0.01vs. control; **P < 0.01vs. contralateral. Inset: electron micrographs of capillaries from control (top, no swelling and containing an erythrocyte) and 7d stimulated EHP (bottom, swelling around 1/3 of the circumference with empty lumen). Scale bar, 1 μm.

Figure 5. The effect of electrical stimulation on apparent patency of capillaries from EHP muscles of rat

□, fully patent capillaries; , partially occluded capillaries; , fully occluded capillaries. ★ P < 0.05 vs. control; ★★ P < 0.01 vs. control. Inset: electron micrographs from capillaries of control (top, patent) and 7d stimulated EHP (bottom, partially occluded). In the former the erythrocyte is likely to experience unimpaired transit, while in the latter the lumen cross-section is too small, possibly representing a capillary sprout, and presumably trapped the erythrocyte. Scale bar, 1 μm.

Figure 6. Stereological analysis of changes in the proportion of capillary cross-section occupied by lumen and lumen surface-to-volume ratio in capillaries from EHP following electrical stimulation

The columns, from left to right, represent control, 3d stimulated, 3d sham-operated, 7d stimulated and 7d contralateral. A, proportion of capillary cross-section occupied by lumen (i.e. the volume density, Vv). B, lumen surface-to-volume ratio (S/V). *P < 0.05vs. control.

Figure 7. Frequency distribution of EC thickness, representing the diffusion distance from lumen to ablumen surface for control and 7d stimulated muscles

Normal distributions are shown for comparison. The arithmetic and geometric mean diffusion distances were 0.849 ± 0.001 and 0.733 ± 0.004 μm (control) and 1.407 ± 0.064 and 1.156 ± 0.021 μm (7d stimulated; P < 0.01vs. control, Kolmogorov-Smirnow test), respectively.

Figure 8. Frequency distribution of capillary size for control and 7d stimulated muscles

Normal distributions are shown for comparison. The 7d group is significantly different from control (P < 0.05, Kolmogorov-Smirnow test).