No abstract

Tempol is an amphipathic radical nitroxide (N) that acutely reduces blood pressure (BP) and heart rate (HR) in the spontaneously hypertensive rat (SHR). We investigated the hypothesis that the response to nitroxides is determined by SOD mimetic activity or lipophilicity. Groups (n ϭ 6 -10) of anesthetized SHRs received graded intravenous doses of Ns: tempol (T), 4-amino-tempo (AT), 4-oxotempo (OT), 4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxyl iodide (CAT-1), 3-carbamoyl-proxyl (3-CP), or 3-carboxyproxyl (3-CTPY). Others received native or liposomal (L) Cu/Zn SOD. T and OT are uncharged, AT is positively charged and cell-permeable, and CAT-1 is positively charged and cell-impermeable. 3-CP and 3-CTPY have five-member pyrrolidine rings, whereas T, AT, OT, and CAT-1 have six-member piperidine rings. T and AT reduced mean arterial pressure (MAP) similarly (Ϫ48 Ϯ 2 mmHg and Ϫ55 Ϯ 8 mmHg) but more (P Ͻ 0.05) than OT and CAT-1. 3-CP and 3-CTPY were ineffective. The group mean change in MAP with piperidine Ns correlated with SOD activity (r ϭ Ϫ0.94), whereas their ED50 correlated with lipophilicity (r ϭ 0.89). SOD and L-SOD did not lower BP acutely but reduced it after 90 min (Ϫ32 Ϯ 5 and Ϫ31 Ϯ 6 mmHg; P Ͻ 0.05 vs. vehicle). Pyrrolidine nitroxides are ineffective antihypertensive agents. The antihypertensive response to piperidine Ns is predicted by SOD mimetic action, and the sensitivity of response is by hydrophilicity. SOD exerts a delayed hypotensive action that is not enhanced by liposome encapsulation, suggesting it must diffuse to an extravascular site. (39), by enhancing the peripheral sympathetic nervous system (37, 38), or by enhancing renal tubular NaCl reabsorption (17,22,23).Oxidative stress accompanies hypertension in many models of hypertension, including the spontaneously hypertensive rat (SHR) (25). Mitchell, Krishna, and colleagues (15,18,24) have shown that tempol (T) is a permeant amphipathic radical nitroxide (N) that detoxifies oxygen metabolites by redox cycling through one-electron transfer reactions. The nitroxide/ oxoammonium cation pair form an efficient redox coupling that mimics the enzymic action of SOD and confers catalaselike action to heme proteins (14, 15). Although T lowers blood pressure (BP) in many animal models of hypertension accompanied by oxidative stress, including the SHR (9, 21, 26, 27, 29, 36 -38), the mechanisms of its in vivo action are not clearly established.Fink and colleagues (38) have shown that T given to deoxycorticosterone acetate-salt rats reduces BP before it has dissipated O 2 Ϫ ⅐ in the aorta. This acute antihypertensive response is accompanied by reduced renal sympathetic nervous system activity. It is unclear whether these neural actions of T depend on SOD mimetic action. Nevertheless, intravenous injection of liposomal (L), polyethylene-glycol, or heparin-bonded SOD lowers BP in SHR (20) or ANG-II-infused hypertensive rats (19) or restores ACh-induced relaxation in blood vessels from atherosclerotic rabbits (34).We investigated the hypothesis t...

Acute intravenous Tempol reduces mean arterial pressure (MAP) and heart rate (HR) in spontaneously hypertensive rats. We investigated the hypothesis that the antihypertensive action depends on generation of hydrogen peroxide, activation of heme oxygenase, glutathione peroxidase or potassium conductances, nitric oxide synthase, and/or the peripheral or central sympathetic nervous systems (SNSs). Tempol caused dose-dependent reductions in MAP and HR (at 174 micromol/kg; DeltaMAP, -57+/- 3 mmHg; and DeltaHR, -50 +/- 4 beats/min). The antihypertensive response was unaffected by the infusion of a pegylated catalase or by the inhibition of catalase with 3-aminotriazole, inhibition of glutathione peroxidase with buthionine sulfoximine, inhibition of heme oxygenase with tin mesoporphyrin, or inhibition of large-conductance Ca(2+)-activated potassium channels with iberiotoxin. However, the antihypertensive response was significantly (P < 0.01) blunted by 48% by the activation of adenosine 5'-triphosphate-sensitive potassium (K(ATP)) channels with cromakalim during maintenance of blood pressure with norepinephrine and by 31% by the blockade of these channels with glibenclamide, by 40% by the blockade of nitric oxide synthase with N(omega)-nitro-L-arginine methyl ester (L-NAME), and by 40% by the blockade of ganglionic autonomic neurotransmission with hexamethonium. L-NAME and hexamethonium were additive, but glibenclamide and hexamethonium were less than additive. The central administration of Tempol was ineffective. The acute antihypertensive action of Tempol depends on the independent effects of potentiation of nitric oxide and inhibition of the peripheral SNS that involves the activation of K(ATP) channels.

Abstract-Angiotensin-converting enzyme inhibitors (ACEIs) decrease the glomerular filtration rate and renal blood flow in the clipped kidneys of early 2-kidney, 1-clip Goldblatt hypertensive rats, but the consequences for oxygenation are unclear. We investigated the hypothesis that angiotensin II type 1 or angiotensin II type 2 receptors or NO synthase mediate renal oxygenation responses to ACEI. Three weeks after left renal artery clipping, kidney function, oxygen (O 2 ) use, renal blood flow, renal cortical blood flow, and renal cortical oxygen tension (PO 2 ) were measured after acute administration of an ACEI (enalaprilat) and after acute administration of ACEI following acute administration of an angiotensin II type 1 or angiotensin II type 2 receptor blocker (candesartan or PD-123,319) or an NO synthase blocker (N G -nitro-L-arginine methyl ester with control of renal perfusion pressure) and compared with mechanical reduction in renal perfusion pressure to the levels after ACEI. The basal renal cortical PO 2 of clipped kidneys was significantly lower than contralateral kidneys (35Ϯ1 versus 51Ϯ1 mm Hg; nϭ40 each). ACEI lowered renal venous PO 2 , cortical PO 2 , renal blood flow, glomerular filtration rate, and cortical blood flow and increased the renal vascular resistance in the clipped kidney, whereas mechanical reduction in renal perfusion pressure was ineffective. 319 and N G -nitro-L-arginine methyl ester, but not candesartan, reduced the PO 2 of clipped kidneys and blocked the fall in PO 2 with acute ACEI administration. In conclusion, oxygen availability in the clipped kidney is maintained by angiotensin II generation, angiotensin II type 2 receptors, and NO synthase. This discloses a novel mechanism whereby angiotensin can prevent hypoxia in a kidney challenged with a reduced perfusion pressure. A reduced renal perfusion pressure (RPP) after clipping of a renal artery in the early (2-to 4-week) 2-kidney, 1-clip (2K,1C) rat model of Goldblatt hypertension increases angiotensin II (Ang II) concentrations in both kidneys 1 and causes Ang II-dependent hypertension. 2-4 A reduced renal tissue oxygen tension (PO 2 ) developing during prolonged infusion of Ang II 5-7 has been ascribed to reactive oxygen species and functional NO deficiency. Prolonged administration of the antioxidant drug Tempol, but not the angiotensin receptor blocker (ARB) candesartan, restores renal tissue PO 2 in a rat model of early 2K,1C hypertension. 5 This may be important, because renal hypoxia, and episodes of renal ischemia, may contribute to hypertension 8 and progressive kidney disease. 9 On the other hand, acute infusions of Ang II into rats increase renal NO generation and increase the dependency of renal blood flow (RBF) on NO. 10,11 Moreover, studies in the early 2K,1C rat model [12][13][14] have shown that the acute administration of an angiotensin-converting enzyme inhibitor (ACEI), or nonselective angiotensin receptor blockade with saralasin, reduces the RBF, and thereby the renal oxygen (O 2 ) delivery, and the glom...