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

1. Muscle blood flow, capillary fine structure and performance were investi gated in the early stages of chronic indirect electrical stimulation of ank le 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 pest than in unstimulated contr ols only in muscles stimulated for 2 days; during contractions it was highe r in some muscles stimulated fur 3 days than in controls, and in all muscle s by 7 days (192 +/- 17 vs. 149 +/- 12 mi (100 g)(-1) min(-1) in controls). 4. Electron microscopical evaluation of individual capillaries in EHP fixed by superfusion in situ revealed thickening of capillary endothelium and de creased lumen volume in muscles stimulated for 7 (P < 0.005) but not 3 days . Significantly smaller capillary size indicates the presence of newly form ed capillaries. 5. Isometric twitch tension, recorded from combined TA and EDL in stimulate d and contralateral legs during 5 min contractions at 4 Hz, gradually decli ned 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 twitc h 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 fo r 2 or 3 days. 6. Lower peak tension, but not fatigue index or MBF, was also observed in m uscles contralateral to those stimulated for 3 and 7 days, which thus do no t represent appropriate controls. 7. We conclude that the high resting blood flow found in muscles stimulated fur 2 days may initiate the capillary growth reported previously, while th e relatively modest increase in MBF during contractions in muscles that had been stimulated for 7 days may be due to increased capillary supply Swelli ng 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.