Inhibition of capillary growth in chronically stimulated rat muscles by N-G-nitro-L-arginine, nitric oxide synthase inhibitor

Shear stress causes release of nitric oxide (NO) from microvascular endothe lial cells in vivo and stimulates their growth in vitro. After chronic elec trical stimulation of lower hind limb skeletal muscles in the rat, measurem ents of capillary diameters and red blood cell velocity indicated that shea r stress is increased in these vessels as a potential source of NO. This st udy therefore investigated whether NO is involved in capillary growth in st imulated muscles. Control rats or those stimulated for 2 or 7 days were tre ated 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) adm inistration, extensor digitorum longus muscles were removed and frozen. Cap illary supply was assessed in cryostat sections as capillary:fiber (C:F) ra tio after staining for alkaline phosphatase; proliferation of capillary-lin ked and interstitial nuclei was evaluated by immunostaining for BrdU incorp oration. 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 aboli shed by L-NNA (1.55 +/- 0.04, NS). The labeling index for BrdU-positive nuc lei colocalized with capillaries as a percentage of total interstitial nucl ei increased in muscles stimulated for 2 days (11.3 +/- 2.2%) and 7 days (1 0.6 +/- 0.8%) compared with controls (2.9 +/- 0.5%, P < 0.01) and was elimi nated by L-NNA at both time points (3.1 +/- 0.6 and 1.0 +/- 0.6%, respectiv ely; 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. Thes e results suggest that NO is involved in capillary growth in chronically st imulated muscles possibly sia its shear-stress-induced release from capilla ries or from interstitial fibroblasts. (C) 2000 Academic Press.