Regional changes in capillary supply in skeletal muscle of interval-sprint and low-intensity, endurance-trained rats

Exercise training produces regional increases in blood flow capacity among muscle fibers that experience increased activity during exercise. We tested the hypothesis that this increase is partially due to capillary angiogenesis among muscle fibers with large increases in activity during exercise training bouts.

Two training programs were evaluated: a program consisting of 10-12 weeks of exposure to low-intensity (30 m/min, 0 incline, 60 min/day) (LET) exercise bouts, 5 days/week, and a second program consisting of 8-10 weeks of exposure to repetitive bouts (6/day) of sprint (60 m/min, 15% incline) exercise, alternating running (2.5 min) and recovery (4.5 min), 5 days/week (IST). Cage-confined rats were utilized (SED) as controls. After training was completed, rat hindquarters were perfusion-fixed with modified Karnovsky's fixative. Transverse sections from soleus (Sol), and red (GR), mixed (GM), and white (GW) portions of gastrocnemius muscle were prepared to evaluate capillarization. Sections were analyzed using the Olympus Cue 2 Image Analyzer to determine capillary/muscle fiber ratio (C/F), number of capillary profiles per square millimeter of muscle area (CND), capillary surface area per volume of tissue, and capillary volume density.

Average area per muscle fiber and sarcomere length did not differ among groups. LET did not affect capillarization of the GW, whereas increasing C/F in GM (2.3 +/- 0.1 versus 2.1 +/- 0.1 for SED) and GR (3.0 +/- 0.1 versus 2.6 +/- 0.1 for SED). IST increased C/F and CND in GW (1.6 +/- 0.1 versus 1.3 +/- 0.0 for SED and 657 +/- 74 versus 418 +/- 53 for SED, respectively) and increased C/F ratio in GM (2.3 +/- 0.1 versus 2.1 +/- 0.1 for SED). IST did not increase capillarization of the GR. The capillarization of the soleus muscle was not affected by either exercise training program.

IST increased capillarization in muscle tissue composed of a high percentage of fast glycolytic fibers (GW and GM) and LET increased capillarization of muscle tissue composed of a high percentage of fast oxidative-glycolytic fibers (GW and GR).