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

Glycolytic fibres in rat extensor digitorum longus (EDL) and tibialis anter ior (TA) were selectively activated, as demonstrated by glycogen depletion, by indirect electrical stimulation via electrodes implanted in the vicinit y 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 app lied 10 times per day, each bout being of 15 min duration with 60 min recov ery, for 2 weeks. Cryostat sections of muscles were stained for alkaline ph osphatase to depict capillaries, succinate dehydrogenase (SDH) to demonstra te oxidative fibres, and periodic acid-Schiff reagent (PAS) to verify glyco gen depletion. Specific activity of hexokinase (HK), 6-phosphofructokinase, pyruvate kinase, glycogen phosphorylase and cytochrome c oxidase (COX) wer e estimated separately in homogenates of the EDL and the predominantly glyc olytic cortex and oxidative core of the TA. Stimulation increased the activ ity of NK 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 mu M mi n(-1) (g wet wt)(-1) in stimulated muscles compared with 3.7 +/- 0.52 mu M min(-1) (g wet wt)(-1) in contralateral muscles (difference not significant ), while the percentage of oxidative fibres (those positively stained for S DH) was also similar in stimulated (14.0 +/- 2.8%) and contralateral (12.2 +/- 1.9%) muscles. In contrast, the capillary to fibre ratio was significan tly 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, pos sibly due to haemodynamic factors, and serves metabolite removal to a great er extent than substrate delivery.