Reactive Oxygen Species Impair Sympathetic Vasoregulation in Skeletal Muscle in Angiotensin II–Dependent Hypertension

Abstract

Abstract-Sympathetic vasoconstriction is attenuated in exercising muscle by locally generated vasodilators, including NO. Skeletal muscle also produces reactive oxygen species (ROS), such as superoxide (O 2 Ϫ ), which inactivates NO. We, therefore, hypothesized that excessive ROS production would result in enhanced sympathetic vasoconstriction in exercising muscle. To increase O 2 Ϫ by activating NADPH oxidase, rats underwent chronic infusion of angiotensin II (Ang II) or unilateral renal artery stenosis (2K1C) to increase endogenous Ang II. At rest, sympathetic nerve stimulation (range: 1 to 5 Hz) evoked similar graded decreases in femoral vascular conductance (range, Ϫ34% to Ϫ66%) in rats infused with vehicle, Ang II, or norepinephrine and in 2K1C or sham-operated rats. These sympathetically mediated decreases in femoral vascular conductance were markedly attenuated during hindlimb contraction in the vehicle, norepinephrine, and sham rats (range, Ϫ3% to Ϫ26%) and to a lesser degree in the Ang II (range, Ϫ16% to Ϫ47%) and 2K1C (range, Ϫ16% to Ϫ45%) rats. In muscles from Ang II and 2K1C rats, ROS were elevated and the NADPH oxidase subunit gp91 phox was upregulated. The O 2 Ϫ scavenger tempol restored the normal attenuation of sympathetic vasoconstriction in the contracting hindlimbs of the Ang II and 2K1C rats, but this effect was prevented by pretreatment with an NO synthase inhibitor. Taken together, these data indicate that chronically elevated Ang II increases muscle ROS, which disrupts the normal NO-dependent attenuation of sympathetic vasoconstriction. These findings may have implications for muscle oxidative stress and sympathetic vasoregulation when the renin-angiotensin system is chronically activated. Key Words: angiotensin Ⅲ free radicals Ⅲ sympathetic nervous system Ⅲ vasoconstriction Ⅲ muscles T he sympathetic nervous system plays an essential role in the cardiovascular response to exercise by increasing cardiac contractility and rate, augmenting venous return, and increasing vascular resistances in the viscera and inactive muscles. These adjustments increase cardiac output and redirect it to the active muscles. At the same time, the vasoconstrictor response to sympathetic nerve discharge in the active muscles is attenuated in part by the metabolic consequences of contraction. 1-9 Such attenuation, termed functional sympatholysis, 5 is more pronounced in the distal than in the proximal segments of the microcirculation, 1,9 which may help to optimize intramuscular blood flow distribution while preserving sympathetic control of systemic vascular resistance and blood pressure.NO is proposed to be one of the factors mediating functional sympatholysis, 2,3,8,10,11 although it may not always play an essential role. 2,4 Skeletal muscle, which expresses high levels of the neuronal isoform of NO synthase (NOS; nNOS) and lower levels of endothelial NOS, 12 produces NO at a low basal rate that increases during contraction. [13][14][15] We and others have shown that ␣-adrenergic vasoconstriction is enhanced in...

Oxidative stress exaggerates skeletal muscle contraction-evoked reflex sympathoexcitation in rats with hypertension induced by angiotensin II

“…While tempol has been widely used as a superoxide scavenger in physiological studies (11,19,48,55), this compound also possesses a property to open ATP-sensitive and Ca 2ϩ -activated K ϩ channels (7,52,53). Recently, tempol has been shown to suppress the exaggeration of the exercise pressor reflex in rats whose femoral artery had been ligated for 72 h independent of its antioxidant effects (32) but through its effect to open ATP-sensitive K ϩ channels (53).…”

“…In other subsets of sham, ANGII150, and ANGII450 rats, in which neither tempol nor tiron was injected into the hindlimb circulation, intracellular superoxide generation in the resting gastrocnemius muscle was evaluated with dihydroethidium (DHE) staining (11,55). In isoflurane (Ͻ4% in oxygen)-anesthetized rats, the middle parts of gastrocnemius muscle were excised, embedded in OCT compound (Sakura Finetek), rapidly frozen in liquid nitrogen, and stored at Ϫ80°C until analysis.…”

“…ANG II, an effector molecule of the RAAS, plays an important role in developing and maintaining hypertension by constricting vessels via its action on smooth muscle cells (5,14). Moreover, this peptide has been known to activate NADPH oxidases, thereby triggering the production of superoxide and other reactive oxygen species (ROS) in various tissues, including skeletal muscle (13,41,51,55,57). The development of oxidative stress has also been implicated in the pathogenesis of hypertension (26,55).…”

“…Moreover, this peptide has been known to activate NADPH oxidases, thereby triggering the production of superoxide and other reactive oxygen species (ROS) in various tissues, including skeletal muscle (13,41,51,55,57). The development of oxidative stress has also been implicated in the pathogenesis of hypertension (26,55). Superoxide functions in the neural process for encoding peripheral mechanical and thermal stimuli (18,25).…”

“…The purpose of the present study was to test this hypothesis. In rats, we chronically stimulated the RAAS by an exogenous infusion of ANG II for 14 days to develop experimental hypertension (16,27,29,55). This model was used to focus on the physiological roles played by increased RAAS activity in the pathological characteristics of hypertension.…”