ABSTRACT
Shear stress causes release of nitric oxide (NO) from microvascular endothelial cells in vivo and stimulates their growth in vitro. After chronic electrical stimulation of lower hind limb skeletal muscles in the rat, measurements of capillary diameters and red blood cell velocity indicated that shear stress is increased in these vessels as a potential source of NO. This study therefore investigated whether NO is involved in capillary growth in stimulated muscles. Control rats or those stimulated for 2 or 7 days were treated with the NO synthase inhibitor, N(G)-nitro-l-arginine (l-NNA, 10 mg.day(-1) in drinking water), or water alone. After bromodeoxyuridine (BrdU) administration, extensor digitorum longus muscles were removed and frozen. Capillary supply was assessed in cryostat sections as capillary:fiber (C:F) ratio after staining for alkaline phosphatase; proliferation of capillary-linked and interstitial nuclei was evaluated by immunostaining for BrdU incorporation. C:F was not increased after 2 days of stimulation but the increase after 7 days (1.88 +/- 0.50 vs control 1.45 +/- 0.04, P < 0.001) was abolished by l-NNA (1.55 +/- 0.04, NS). The labeling index for BrdU-positive nuclei colocalized with capillaries as a percentage of total interstitial nuclei increased in muscles stimulated for 2 days (11.3 +/- 2.2%) and 7 days (10.6 +/- 0.8%) compared with controls (2.9 +/- 0.5%, P < 0.01) and was eliminated by l-NNA at both time points (3.1 +/- 0.6 and 1.0 +/- 0.6%, respectively; both P < 0.05 vs stimulated). A transient increase in BrdU labeling of interstitial nuclei not associated with capillaries (possibly fibroblasts) after 2 but not 7 days stimulation was eliminated by l-NNA treatment. These results suggest that NO is involved in capillary growth in chronically stimulated muscles possibly via its shear-stress-induced release from capillaries or from interstitial fibroblasts.