Exercise training produces nonuniform increases in arteriolar density of rat soleus and gastrocnemius muscle

Abstract

Objective: Exercise training has been shown to increase regional blood flow capacity to muscle tissue containing fibers that experience increased activity during exercise. The purpose of this study was to test the hypothesis that the increased blood flow capacity is partially the result of increases in arteriolar density (number of arterioles/mm2 of tissue), specifically in skeletal muscle tissue, with the largest relative increase in muscle fiber activity during training bouts.

Methods: This hypothesis was tested by comparing and contrasting the effects of endurance exercise training (ET) and interval sprint training (IST) on arteriolar density in soleus muscle (S) red (Gr) and white (Gw) portions of gastrocnemius muscle of male Sprague Dawley rats. ET rats completed 10 weeks of treadmill training 30 m/min, 15% grade, 60 min/day, 5 days/week, while IST rats completed 10 weeks of IST consisting of six 2.5-min exercise bouts, with 4.5-min rest between bouts (60 m/min, 15% incline), 5 days/week. The hypothesis would be supported if ET increased arteriolar density in S and Gr and if IST increased arteriolar density in Gw.

Results: ET increased arteriolar density above values of sedentary rats (SED) in both the Gw (ET = 0.93 +/- 0.19 arterioles/microm2; SED = 0.44 +/- 0.09 arterioles/microm2) and Gr (ET = 0.97 +/- 0.1 arterioles/microm2; SED = 0.51 +/- 0.06 arterioles/microm2) muscles, but not in S (ET = 1.69 +/- 0.45 arterioles/microm2; SED = 1.51 +/- 0.34 arterioles/microm2) muscle. In contrast, IST did not alter arteriolar density in Gw or Gr muscle tissue. Although arterial wall thickness was greater in S (3.95 +/- 0.40 microm) and Gr (6.24 +/- 0.59 microm) than Gw (2.76 +/- 0.18 microm), neither ET or IST altered mean wall thickness in either muscle.

Conclusion: Increases in blood flow capacity produced in Gr and Gw by ET appear to be due in part to increased arteriolar density. In contrast, increased arteriolar density does not contribute to increased blood flow capacity of Gw in IST rats.

Figure 1

Illustration of approach to data collection. A. Tracing of image B showing relative positions of Medial and Lateral heads of the gastrocnemius muscle (G_M and G_I), Superficial Digital Flexor (SDF), and white and red portions of gastrocnemius (G_W and G_R). B. Slide image showing typical sample area for data collection frm G_R and Gw. C. Data, image of analyzed artery with vein showing difference between wall to lumen relationship of vein and artery.

Figure 2

Effects of endurance training (ET) on arteriolar density of white gastrocnemius muscle. Values are means ± SEM. n = 9 for ET and 10 for E-SED. * = ET value significantly different from E-SED with p < 0.05.

Figure 3

Effects of endurance training (ET) on arteriolar density of red gastrocnemius muscle. Values are means ± SEM. n = 17 for ET and 18 for E-SED. * = ET value significantly different from E-SED with p < 0.05.

Figure 4

Effects of endurance training (ET) on arteriolar density of soleus muscle. Values are means ± SEM. n = 9 for ET and 10 for E-SED. There were no statistically significant differences between ET and E-SED with p < 0.05.

Figure 5

Arteriolar density of arterioles with different diameters (top panel) in the white gastrocnemius muscle (Gw) following endurance exercise training (ET) and percentage of arterioles in given size range (bottom panel). Values are means ± SEM. * = ET value significantly different from E-SED with p < 0.05.

Figure 6

Arteriolar density of arterioles with different diameters (top panel) in the red gastrocnemius muscle (Gr) following endurance exercise training (ET) and percentage of arterioles in given size range (bottom panel). Values are means ± SEM. * = ET value significantly different from E-SED with p < 0.05.

Figure 7

Arteriolar density of arterioles with different diameters (top panel) in the soleus muscle following endurance exercise training (ET) and percentage of arterioles in given size range (bottom panel). Values are means ± SEM. There were no statistically significant differences between ET and E-SED with p < 0.05.

Figure 8

Effects of interval sprint training (IST) on arteriolar density of white gastrocnemius muscle. Values are means ± SEM. n = 12 for IST and 15 for I-SED. There were no statistically significant differences between IST and I-SED for any of these parameters (p < 0.05).

Figure 9

Effects of interval sprint training (IST) on arteriolar density of red gastrocnemius muscle. Values are means ± SEM. n = 13 for IST and 14 for I-SED. There were no statistically significant differences between IST and I-SED for any of these parameters (p < 0.05).