Endurance training increases arterial wall thickness in rats

We tested whether endurance training altered the morphology of systemic arteries. Female Sprague-Dawley rats (8 wk, 120 g) were either treadmill run for 16 wk (up to 3 h/day, 5-10% grade, 35 m/min) or used as caged controls (n = 7/group). At 24-h post-running bout, rats were anesthetized (ether) and decapitated. Soleus muscles were excised for determination of citrate synthase activity [control, 38.4 +/- 5.6 (SE); trained, 56.3 +/- 5.3 mumol.min-1 x g-1; P < 0.05]. The cervical stump was ligated, and the vascular system was perfused via the left ventricle at systemic pressure of 80 mmHg: first with papaverine (0.3 mM in phosphate-buffered saline, pH 7.4, 37 degrees C) to relax smooth muscle cells (SMC) and then with aldehyde fixative solution. The abdominal aorta and femoral, axillary, superior mesenteric, and coeliac arteries were excised, postfixed in OsO4, and embedded in Spurr resin. Vessel cross sections were stained for histological analysis using light microscopy and videomicroscopy. In trained rats, all vessels (except coeliac) had 12-18% greater (P < 0.05) medial wall thickness; total wall area was elevated (P < 0.05) in the femoral and axillary arteries and the abdominal aorta. Neither lumen diameter nor the number of SMC nuclei per vessel cross section was different between groups for any vessel. These findings indicate growth of the arterial media without an increase of SMC number in response to endurance training. This adaptation will decrease tangential wall stress of arteries at rest and during exercise.