The effect of long‐term high‐frequency stimulation on capillary density and fibre types in rabbit fast muscles.

Abstract

SUMMARY1. Rabbit fast muscles (tibialis anterior, t.a.; extensor digitorum longus, e.d.l.; and peroneal muscles) were stimulated for up to 28 days by electrodes implanted in the vicinity of the lateral popliteal (peroneal) nerve for 8 h/day, using either intermittent high-frequency (three trains at 40 Hz/min, each 5 s duration), or continuous stimulation at 10 Hz. This did not result in muscle hypertrophy even after 28 days.2. Capillary density (number of capillaries/mm2) was increased in e.d.l. from 251 ± 3 to 366 + 6 after 14 days of stimulation and from 251 + 3 to 514 + 13 after 28 days of stimulation at 40 Hz. In t.a., capillary density increased from 373 + 5 to 583 + 10 after 14 days of stimulation at 40 Hz. The capillary/fibre ratio increased in e.d.l. from 1-25+0102 to 1-86+0-04 at 14 days and to 2-07+0-06 at 28 days. In t.a., capillary/fibre ratio increased from 1-40 + 0-03 to 1-83 + 0-05 at 14 days. All these changes were significant (P < 010005).3. Analysis of capillary density, capillary/fibre ratio, fibre areas and proportion of different fibre types in muscles stimulated for shorter periods showed no changes in capillary density, capillary/fibre ratio or fibre areas in e.d.l. or t.a. stimulated for 4 days; there was a decrease in the proportion of fast glycolytic fibres from 42 to 32 % (P < 010025) and increase in fast oxidative from 37-6 to 41-2 % in e.d.l. Muscles stimulated for 7 days showed increases in capillary density and capillary/fibre ratio in fast predominantly glycolytic fibres in e.d.l., and a decrease in capillary density in fast and slow oxidative fibres in t.a. This was partly due to the increase in fibre areas in these groups (capillary/fibre ratio in t.a. was not significantly changed). No changes were observed in fibre areas in e.d.l. Stimulation at 10 Hz produced increase in capillary/fibre ratio in the vicinity of glycolytic fibres after only 4 days.4. High-frequency intermittent stimulation leads to a massive capillary growth which starts first in the muscle with a higher proportion of glycolytic fibres (e.d.l.), has a later onset than continuous low-frequency stimulation, and may be due to a combination of high blood flow and metabolic factors.

Selective long-term electrical stimulation of fast glycolytic fibres increases capillary supply but not oxidative enzyme activity in rat skeletal muscles

“…85.5 Although it is not possible to compare directly results from different species and stimulation regimes, Brown et al (1976) also reported the absence of capillary growth in rabbit muscles using 40 Hz for 4 days, but with a different pattern of stimulation (3 ² 5 s bouts min¢ for 8 h per day) and supramaximal voltage (which activates all muscle fibres). However, using the same stimulation pattern Hudlick a & Tyler (1984) found an increased capillary supply after 14 days. Increases in capillary supply preceding the changes in oxidative enzymes were described previously (Pette et al 1973;Brown et al 1976), and were confirmed more recently (Skorjanc et al 1998) using different stimulation regimes than those used in the present study.…”

“…Using different frequencies of stimulation, similar changes in both enzyme pattern and capillary supply were observed after more intensive stimulation for 14 days (Reichmann et al 1985). Increases in resistance to fatigue induced by various patterns of repetitive muscle stimulation were associated with a greater capacity for oxidative metabolism, involving a coordinated increase in the activities of enzymes from â-oxidation of fatty acids, the citric acid cycle, and terminal substrate oxidation in the respiratory chain (Pette et al 1975;Hudlick a et al 1984;Reichmann et al 1985Reichmann et al , 1991. The increase in the activity of oxidative enzymes was accompanied by a marked decrease in the enzyme activities of the glycolytic pathways (Simoneau & Pette, 1988), thus strengthening the argument that the capacity for sustained activity of skeletal muscle correlates directly with aerobic metabolic capacity and indirectly with anaerobic metabolic capacity.…”

“…Serial cryostat sections, 12 ìm thick, were stained for alkaline phosphatase using an indoxyl-tetrazolium method to depict the anatomical capillary supply (Hudlick a & Tyler, 1984). The extent of glycogen depletion within individual fibres was assessed by periodic acid-Schiff reagent (PAS) staining.…”

“…The extent of glycogen depletion within individual fibres was assessed by periodic acid-Schiff reagent (PAS) staining. In serial sections the metabolic characteristics of glycogen-depleted fibres were determined by myosin ATPase activity following pre-incubation with acid (pH 4.6), to identify type I fibres (Egginton, 1990), and the intensity of staining for succinate dehydrogenase (Hudlick a & Tyler, 1984), to subdivide glycolytic and oxidative type II fibres (Fig. 1).…”

“…Similarly, high-intensity endurance training in animals resulted in increased capillary supply accompanied by increased oxidative capacity (Gute et al 1996). In contrast, chronic electrical stimulation of fast skeletal muscles at a frequency resembling that observed during discharges of slow motoneurons increased capillary supply prior to an increase in the proportion of oxidative fibres (Hudlick a et al 1982) and before any increase in activity of oxidative enzymes (Pette et al 1973;Brown et al 1976), although both parameters were significantly higher than in control muscles after 10-14 days of stimulation (Hudlick a & Tyler, 1984).…”