Angiostatin Does Not Contribute to Skeletal Muscle Microvascular Rarefaction with Low Nitric Oxide Bioavailability

Abstract

Ischemic angiogenesis is dependent on vascular nitric oxide (NO) bioavailability, in part by reducing matrix metalloproteinases (MMP)-2 and -9 activity and angiostatin production. In the metabolic syndrome, the authors have demonstrated that reduced skeletal muscle microvessel density (SKMVD) was correlated with chronic reductions in NO bioavailability, but these relationships are complicated by the presence of the multiple pathological states inherent in the metabolic syndrome. Given this, the authors hypothesized that low NO bioavailability could reduce SKMVD in normal rats, independent of any systemic pathologies associated with the metabolic syndrome, and that this would be correlated with increased angiostatin production. Rats were separated into 3 groups: (1) control, (2) chronic NOS inhibition (10-4 M L-NAME; drinking water), and (3) NOS inhibition/normotensive (combined L-NAME/hydralazine treatment; 50 mg/kg/d; drinking water). Vessel structure, reactivity, and NO bioavailability were assessed in isolated vessels using standard techniques. SKMVD was determined using established immunohistochemical methods. Established protein analyses were performed for MMP-2 and MMP-9 expression/activity and for angiostatin expression. Alterations in vascular endothelial growth factor (VEGF) levels under the conditions of the study were assessed using ELISA. After 6 weeks, MAP was elevated in L-NAME rats vs. control, although this difference was abolished in L-NAME/hydralazine rats. NOS inhibition abrogated dilation to acetylcholine and arterial eNOS activity. While NOS inhibition reduced SKMVD vs. control, hydralazine treatment did not improve density, suggesting that rarefaction in NOS-inhibited rats was independent of elevated pressure. Skeletal muscle demonstrated reduced active hyperemia and increased minimum vascular resistance in L-NAME rats, which was also associated with increased arteriolar wall stiffness. L-NAME/hydralazine treatment, while still causing an elevated resistance, prevented arteriolar wall stiffening. Protein analysis demonstrated that neither expressions nor activities of MMP-2 or MMP-9 were altered from control in skeletal muscle of rats treated with L-NAME and angiostatin production was not altered in any group. Chronic NOS inhibition was associated with no consistent change in plasma VEGF levels. These results suggest that a reduced SKMVD develops with low NO bioavailability, although this process was not associated with significant alterations to either VEGF or angiostatin production.