Glycolytic fibres in rat extensor digitorum longus (EDL) and tibialis anterior (TA)
were selectively activated, as demonstrated by glycogen depletion, by indirect
electrical stimulation via electrodes implanted in the vicinity of the peroneal nerve
using high frequency (40 Hz) trains (250 ms at 1 Hz) and low voltage (threshold of
palpable contractions). This regime was applied 10 times per day, each bout being of
15 min duration with 60 min recovery, for 2 weeks. Cryostat sections of muscles were
stained for alkaline phosphatase to depict capillaries, succinate dehydrogenase (SDH)
to demonstrate oxidative fibres, and periodic acid-Schiff reagent (PAS) to verify
glycogen depletion. Specific activity of hexokinase (HK), 6-phosphofructokinase,
pyruvate kinase, glycogen phosphorylase and cytochrome c oxidase (COX) were
estimated separately in homogenates of the EDL and the predominantly glycolytic
cortex and oxidative core of the TA. Stimulation increased the activity of HK but not
that of oxidative enzymes in fast muscles. Comparison of changes in oxidative
capacity and capillary supply showed a dissociation in the predominantly glycolytic
TA cortex. Here, COX was 3.9 ± 0.68 µM min-1 (g wet wt)-1 in stimulated muscles
compared with 3.7 ± 0.52 µM min-1 (g wet wt)-1 in contralateral muscles (difference
not significant), while the percentage of oxidative fibres (those positively stained for
SDH) was also similar in stimulated (14.0 ± 2.8 %) and contralateral (12.2 ± 1.9 %)
muscles. In contrast, the capillary to fibre ratio was significantly increased (2.01 ±
0.12 vs. 1.55 ± 0.04, P < 0.01). We conclude that capillary supply can be increased
independently of oxidative capacity, possibly due to haemodynamic factors, and
serves metabolite removal to a greater extent than substrate delivery. Experimental
Physiology (2000) 85.5, 567-574.