CAPILLARY-TO-FIBER SURFACE RATIO IN RAT FAST-TWITCH HINDLIMB MUSCLES AFTER CHRONIC ELECTRICAL-STIMULATION

We examined the relative plasticity of capillaries and fiber mitochond ria in rat fast-twitch hindlimb muscles in response to chronic electri cal stimulation. Specifically we addressed whether the size of the cap illary-fiber interface increases in proportion to fiber mitochondrial volume, inasmuch as fiber aerobic capacity increases severalfold with chronic stimulation. Tibialis anterior and extensor digitorum longus m uscles of six rats [367 +/- 17 (SD) g body wt] were stimulated (10 H, 8 h/day, 7 days/yk) for 28 consecutive days. Subsequently they were pe rfusion fixed in situ and stimulated, and contralateral control sample s from the midbelly were processed for electron microscopy and morphom etry. Capillary length density, capillary-to-fiber ratio, and fiber mi tochondrial volume density increased two- to threefold in stimulated m uscles, with no change in fiber or capillary diameter. Capillary-to-fi ber surface area ratio per fiber unit mitochondrial volume was unchang ed in stimulated muscles compared with contralateral controls, indicat ing a proportional increase in the size of the capillary-fiber interfa ce and fiber mitochondrial volume in the muscles after chronic electri cal stimulation.