Aim: We examined the time course of capillary structure changes in rat skeletal muscle at 1, 3 and 7 days after strenuous eccentric exercise.

Methods: The right gastrocnemius muscles of anaesthetized male Wistar rats were subjected to 300 controlled eccentric contractions using electrical stimulation. The contralateral gastrocnemius muscle was used as control. All morphometric parameters were determined in in situ perfused gastrocnemius muscles in red (Gr, predominantly slow-twitch fibre) and white (Gw, predominantly fast-twitch fibre) portions.

Results: Muscle fibre damage was evident on days 1, 3 and 7 in Gr (29.3-53.9% damaged fibres) and Gw (58.9-86.8% damaged fibres) of exercised legs. Electron micrographs of transverse sections did not display collapsed or obstructed capillaries in exercised legs, and capillary endothelial cells retained their normal structures. However, capillary luminal shapes and area were altered in exercised legs on days 1 and 3. The ratio between minimal and maximal capillary diameter in a transverse section (i.e. luminal ellipticity) significantly differed when comparing control (Gr, 0.75 +/- 0.02; Gw, 0.79 +/- 0.03) and exercised legs (Gr, 0.65 +/- 0.03; Gw, 0.66 +/- 0.04) at 1 day after exercise. The mean capillary luminal area was significantly increased in exercised legs after 1 day (Gw, +24.3%) and 3 days (Gr, +31.9%; Gw, +62.2%) compared with control.

Conclusion: We conclude that (1) capillary endothelial cell structure was maintained in damaged muscles, (2) changes in capillary lumen shapes and distensibility occur in the degenerated muscle up to 3 days after the eccentric contraction period.