Reduced rat renal vascular response to angiotensin II after chronic inhibition of nNOS

Endogenous angiotensin II modulates nNOS expression in renovascular hypertension

Nitric oxide (NO) influences renal blood flow mainly as a result of neuronal nitric oxide synthase (nNOS). Nevertheless, it is unclear how nNOS expression is modulated by endogenous angiotensin II, an inhibitor of NO function. We tested the hypothesis that the angiotensin II AT1 receptor and oxidative stress mediated by NADPH oxidase contribute to the modulation of renal nNOS expression in two-kidney, one-clip (2K1C) hypertensive rats. Experiments were performed on male Wistar rats (150 to 170 g body weight) divided into 2K1C (N = 19) and sham-operated (N = 19) groups. nNOS expression in kidneys of 2K1C hypertensive rats (N = 9) was compared by Western blotting to that of 2K1C rats treated with low doses of the AT1 antagonist losartan [10 mg x kg(-1) x day(-1); N = 5] or the superoxide scavenger tempol [0.2 mmol x kg(-1) x day(-1); N = 5], which still remain hypertensive. After 28 days, nNOS expression was significantly increased by 1.7-fold in the clipped kidneys of 2K1C rats and by 3-fold in the non-clipped kidneys of 2K1C rats compared with sham rats, but was normalized by losartan. With tempol treatment, nNOS expression increased 2-fold in the clipped kidneys and 1.4-fold in the non-clipped kidneys compared with sham rats. The changes in nNOS expression were not followed by changes in the enzyme activity, as measured indirectly by the cGMP method. In conclusion, AT1 receptors and oxidative stress seem to be primary stimuli for increased nNOS expression, but this up-regulation does not result in higher enzyme activity.

Interaction of Endothelial Nitric Oxide and Angiotensin in the Circulation

Discovery of the unexpected intercellular messenger and transmitter nitric oxide (NO) was the highlight of highly competitive investigations to identify the nature of endothelium-derived relaxing factor. This labile, gaseous molecule plays obligatory roles as one of the most promising physiological regulators in cardiovascular function. Its biological effects include vasodilatation, increased regional blood perfusion, lowering of systemic blood pressure, and antithrombosis and anti-atherosclerosis effects, which counteract the vascular actions of endogenous angiotensin (ANG) II. Interactions of these vasodilator and vasoconstrictor substances in the circulation have been a topic that has drawn the special interest of both cardiovascular researchers and clinicians. Therapeutic agents that inhibit the synthesis and action of ANG II are widely accepted to be essential in treating circulatory and metabolic dysfunctions, including hypertension and diabetes mellitus, and increased availability of NO is one of the most important pharmacological mechanisms underlying their beneficial actions. ANG II provokes vascular actions through various receptor subtypes (AT1, AT2, and AT4), which are differently involved in NO synthesis and actions. ANG II and its derivatives, ANG III, ANG IV, and ANG-(1-7), alter vascular contractility with different mechanisms of action in relation to NO. This review article summarizes information concerning advances in research on interactions between NO and ANG in reference to ANG receptor subtypes, radical oxygen species, particularly superoxide anions, ANG-converting enzyme inhibitors, and ANG receptor blockers in patients with cardiovascular disease, healthy individuals, and experimental animals. Interactions of ANG and endothelium-derived relaxing factor other than NO, such as prostaglandin I2 and endothelium-derived hyperpolarizing factor, are also described.

Uric acid and the state of the intrarenal renin-angiotensin system in humans

BACKGROUND

Experimental hyperuricemia is marked by an activated intrarenal renin-angiotensin system (RAS). The renal vascular response to exogenous angiotensin II (Ang II) provides an indirect measure of intrarenal RAS activity. We tested the hypothesis that the serum uric acid concentration predicts the renal vascular response to Ang II.

METHODS

A total of 249 subjects in high sodium balance had the renal plasma flow (RPF) response to Ang II measured. Para-aminohippuric acid (PAH) clearance was used to estimate RPF. Multivariable regression analysis determined if the serum uric acid concentration independently predicts the RPF response to Ang II. Variables considered included age, gender, race, body mass index (BMI), hypertension status, blood pressure, basal RPF, creatinine clearance, serum insulin, serum glucose, serum high-density lipoprotein (HDL), serum triglycerides, and plasma renin activity (PRA).

RESULTS

Uric acid concentration negatively correlated with the RPF response to Ang II (r=-0.37, P < 0.001). In univariate analysis, age, BMI, hypertension, triglycerides, and blood pressure were negatively associated, and basal RPF, HDL, and female gender were positively associated with the RPF response to Ang II. In multivariable analysis, serum uric acid concentration independently predicted the RPF response to Ang II (beta=-5.3, P < 0.001).

CONCLUSION

Serum uric acid independently predicted blunted renal vascular responsiveness to Ang II, consistent with results from experimental hyperuricemia showing an activated intrarenal RAS. This could be due to a direct effect of uric acid or reflect a more fundamental renal process. These data may have relevance to the association of uric acid with risk for hypertension and nephropathy.

Neurogenic nitric oxide release increases in mesenteric arteries from ouabain hypertensive rats

OBJECTIVES

We investigated whether chronic ouabain treatment changes the vasoconstrictor responses induced by electrical field stimulation (EFS) in endothelium-denuded rat superior mesenteric arteries and a possible role of neuronal nitric oxide (NO).

METHOD

Mesenteric arteries from untreated and ouabain-treated rats (approximately equal to 8.0 microg/kg per day, for 5 weeks) were used in this study. Vascular reactivity was analyzed by isometric tension recording. Expression of the neuronal NO synthase isoform was analyzed by Western blot. Noradrenaline release was evaluated in segments incubated with [H]noradrenaline.

RESULTS

Systolic (SBP) and diastolic (DBP) blood pressure were higher in ouabain-treated rats than in untreated rats (SBP, untreated: 120 +/- 3.5 mmHg versus ouabain-treated: 150 +/- 4.7 mmHg, P < 0.01; DBP, untreated: 87 +/- 3.0 mmHg versus ouabain-treated: 114 +/- 2.6 mmHg, P < 0.001). EFS-induced vasoconstrictions were smaller in arteries from ouabain-treated rats than in those from untreated animals, while the EFS-induced [H]noradrenaline release and the vasoconstriction induced by exogenous noradrenaline (1 nmol/l-10 micromol/l) remained unmodified. The non-selective NO synthase (NOS) inhibitor, N-nitro-L-arginine methyl ester (100 micromol/l), increased the EFS-induced vasoconstriction in mesenteric arteries from both groups, although the effect was more pronounced in segments from ouabain-treated rats. The selective neuronal NOS inhibitor, 7-nitroindazole (7-NI; 100 micromol/l) increased EFS-induced contraction only in segments from ouabain-treated rats. Neuronal NOS expression was greater in the mesenteric arteries from ouabain-treated rats than in those from untreated animals. Sodium nitroprusside (0.1 nmol/l-10 micromol/l) induced a similar vasodilatation in segments from both groups.

CONCLUSIONS

These results suggest that chronic ouabain treatment is accompanied by an increase in neuronal NO release that reduces EFS-induced vasoconstriction.

Effect of tyrosine kinase blockade on norepinephrine-induced cytosolic calcium response in rat afferent arterioles

We used genistein (Gen) and tyrphostin 23 (Tyr-23) to evaluate the importance of tyrosine phosphorylation in norepinephrine (NE)-induced changes in intracellular free calcium concentration ([Ca(2+)](i)) in rat afferent arterioles. [Ca(2+)](i) was measured in microdissected arterioles using ratiometric photometry of fura 2 fluorescence. The control [Ca(2+)](i) response to NE (1 microM) consisted of a rapid initial peak followed by a plateau phase sustained above baseline. Pretreatment with the tyrosine kinase inhibitor Tyr-23 (50 microM, 10 min) caused a slow 40% increase in baseline [Ca(2+)](i). Tyr-23 attenuated peak and plateau responses to NE, both by approximately 70%. In the absence of extracellular Ca(2+) (0 Ca), Tyr-23 reduced the immediate [Ca(2+)](i) response to NE by approximately 60%, indicative of mobilization of internal stores, and abolished the plateau phase. In other arterioles, the [Ca(2+)](i) response to depolarization induced by KCl (50 mM) was not attenuated by Tyr-23, indicating no direct effect on L-type Ca(+) channels activated by depolarization. The Ca(2+) channel blocker nifedipine (1 microM) inhibited the NE response by approximately 50%; the effects of nifedipine and Tyr-23 were not additive. Nifedipine had no inhibitory effect after Tyr-23 pretreatment, indicating Tyr-23 inhibition of Ca(2+) entry. Another tyrosine kinase inhibitor, Gen (5 and 50 microM), did not affect baseline [Ca(2+)](i). High-dose Gen inhibited the peak and plateau response to NE by 87 and 75%, respectively; low-dose Gen attenuated both responses by approximately 20%. In 0 Ca, Gen (50 microM) abolished the immediate [Ca(2+)](i) mobilization response. Combined nifedipine and Gen (50 microM) inhibited the rapid NE response by approximately 90% in the presence of extracellular Ca(2+). Gen (50 microM) also inhibited by 60% the [Ca(2+)](i) response to 50 mM KCl, indicating a direct interaction with voltage-sensitive, L-type Ca(2+) entry channels. These results indicate that tyrosine phosphorylation is an important link in the chain of events leading to alpha-adrenoceptor-induced Ca(2+) recruitment (both entry and release) in afferent arteriolar smooth muscle cells. Furthermore, different blockers of tyrosine kinase appear to have different modes of action in renal microvessels.