Role of the renal nerves and angiotensin II in the renal function curve.

The relationship between renal perfusion pressure and urinary sodium is involved in arterial pressure regulation. The aim of this study was to investigate the role of renal nerves and angiotensin II in the pressure-natriuresis relationship. Experiments were performed in anaesthetised cats in which one kidney was surgically denervated. Renal perfusion pressure (RPP), renal blood flow (RBF) glomerular filtration rate (GFR, creatinine clearance), urinary volume (V) and sodium excretion (Una + V) were separately measured from both kidneys. RPP was progressively reduced in two consecutive steps by a suprarenal aortic snare. Two groups of animals were studied: the first without any pharmacological treatment (Untreated), the second during treatment with an angiotensin converting enzyme inhibitor (Captopril, 0.4 mg/Kg intravenously followed by an infusion of 0.4 mg/Kg/h). In the Untreated group RPP was reduced from 152.4 +/- 7.3 to 113.6 +/- 5.8 and 83.0 +/- 4.4 mmHg during the first and second step respectively. RBF and GFR were only slightly reduced during the second step of reduced RPP. In control conditions V and UNa + V were greater in the denervated compared to the innervated kidney. The graded decrease in RPP reduced both V and UNa + V in the innervated as well as in the denervated kidney. In the Captopril group V and UNa + V were larger than in the Untreated group in both the innervated and the denervated kidney. A decrease of RPP similar to that observed in the Untreated group, produced similar haemodynamic changes. Also in the Captopril group the graded decrease in RPP reduced both V and UNa + V in the innervated as well as in the denervated kidney. Matching UNa + V against RPP values significant correlations were found in the innervated and denervated kidneys of both groups. Both renal denervation and ACE inhibition were accompanied by an increased gain of the pressure-natriuresis curve, but only renal denervation shifted the crossing of the pressure axis to the left. In the ACE inhibited animals renal denervation only shifted the curve to the left. In conclusion our data suggest that i) at each level of RPP renal nerves and angiotensin II decrease renal sodium excretion, ii) renal nerves and angiotensin II increase the slope of the renal function curve, iii) renal nerves shift to the right the renal function curve.     

肾脏血管紧张素Ⅱ受体及其在肾脏病中的改变

血管紧张素II(AII)对肾脏有多种生理调节功能,在许多肾脏疾病中也起着重要作用。本文对AII受体在肾内的分布、生理作用和生化特性,以及在肾脏疾病中的变化作一介绍。     

Ang Ⅱ拮抗剂对慢性肾衰大鼠肾血流量和肾内氧耗的影响

目的观察血管紧张素II(AngⅡ)拮抗剂对5/6(ablation/infarction,A/I)肾切除诱导慢性肾衰竭(CRF)大鼠肾功能、肾血流量及肾内氧耗的影响。方法制备5/6(A/I)肾切除诱导慢性肾衰大鼠模型,设正常组(A组,n=14只),模型组(B组,n=14只),AngⅡ拮抗剂治疗组(氯沙坦钾联合福辛普利钠)(C组,n=14只)。给予相应干预,疗程60 d。分别测量尾动脉收缩压(SBP)、舒张压(DBP),检测大鼠尾静脉血清肌酐(Scr)、尿素氮(BUN)、血红蛋白(Hb),计算内生肌酐清除率(Ccr)。干预60 d后,检测肾血流量(RBF)、腹主动脉和肾静脉血气(AABG and RVBG),左肾静脉压(RVpO2),计算残余肾内氧耗(QO2/TNa)及观察残肾组织病理变化。结果 (1)造模后与A组比较,B、C两组的Scr、BUN和尾动脉SBP、DBP显著增加(P0.01),Ccr、Hb显著降低(P0.01),提示造模成功。(2)干预后与B组比较,C组的Scr、尾动脉SBP、DBP、QO2/TNa明显下降(P0.01),BUN降低(P0.05),Hb、Ccr、RVpO2显著升高(P0.01),RBF升高(P0.05)。(3)残肾组织病理形态学变化显示,C组的肾组织病理变化明显减轻,优于B组。结论 AngⅡ拮抗剂可以增加慢性肾衰大鼠肾血流量,降低肾内氧耗,改善肾功能及减轻肾组织病理变化,其肾脏保护作用机制可能与其调节细胞能量代谢,改善肾内氧耗有关。     

Stimulation of renal gluconeogenesis by angiotensin II

Renal gluconeogenesis was studied in suspended tubule fragments isolated by collagenase treatment of rat kidney cortices. Angiotensin II increased glucose formation from pyruvate, lactate, and to a lesser extent from oxoglutarate and glutamine, but not from other substrates such as malate, succinate, dihydroxy-acetone of fructose. Stimulation was significant with peptide concentration exceeding 1 · 10^?8 M and was also shown with an 8-Sar derivative. Other peptides such as 4-Ala-8-Ile-angiotensin II, hexapeptide and bradykinin had no effect. The stimulatory action of angiotensin II was additive to that of l-lysine, and 3′,5′-adenosine cyclic monophosphate, suggesting a different mechanism of action. In the presence of maximally stimulatory concentrations of oleate, phenylephrine and 3′,5′-guanosine cyclic monophosphate, however, the stimulatory effect of angiotensin II was absent. Cyclic GMP levels, however, did not increase in tubules after angiotensin II and phenylephrine addition, making a messenger function of this nucleotide unlikely. Omission of Ca²⁺ from the medium markedly reduced basal gluconeogenesis but did not result in a complete loss of angiotensin II effect. Reduction of medium potassium to 2 mM, however, increased basal gluconeogenesis and blunted the peptide effect. 1 mM ouabain was also able to inhibit the stimulatory effect of angiotensin II. Therefore changes in intracellular potassium levels are discussed as a possible mechanism of angiontensin action, whereas calcium seems not to be specifically linked to this metabolic action of angiotensin on the proximal tubule.     

Contribution of renal purinergic receptors to renal vasoconstriction in angiotensin II-induced hypertensive rats

To investigate the participation of purinergic P2 receptors in the regulation of renal function in ANG II-dependent hypertension, renal and glomerular hemodynamics were evaluated in chronic ANG II-infused (14 days) and Sham rats during acute blockade of P2 receptors with PPADS. In addition, P2X1 and P2Y1 protein and mRNA expression were compared in ANG II-infused and Sham rats. Chronic ANG II-infused rats exhibited increased afferent and efferent arteriolar resistances and reductions in glomerular blood flow, glomerular filtration rate (GFR), single-nephron GFR (SNGFR), and glomerular ultrafiltration coefficient. PPADS restored afferent and efferent resistances as well as glomerular blood flow and SNGFR, but did not ameliorate the elevated arterial blood pressure. In Sham rats, PPADS increased afferent and efferent arteriolar resistances and reduced GFR and SNGFR. Since purinergic blockade may influence nitric oxide (NO) release, we evaluated the role of NO in the response to PPADS. Acute blockade with N(ω)-nitro-l-arginine methyl ester (l-NAME) reversed the vasodilatory effects of PPADS and reduced urinary nitrate excretion (NO(2)(-)/NO(3)(-)) in ANG II-infused rats, indicating a NO-mediated vasodilation during PPADS treatment. In Sham rats, PPADS induced renal vasoconstriction which was not modified by l-NAME, suggesting blockade of a P2X receptor subtype linked to the NO pathway; the response was similar to that obtained with l-NAME alone. P2X1 receptor expression in the renal cortex was increased by chronic ANG II infusion, but there were no changes in P2Y1 receptor abundance. These findings indicate that there is an enhanced P2 receptor-mediated vasoconstriction of afferent and efferent arterioles in chronic ANG II-infused rats, which contributes to the increased renal vascular resistance observed in ANG II-dependent hypertension.     

The role of angiotensin II in renal growth.

Angiotensin II (AII) has many of the features of the archetypical growth factors and appears to be a growth regulator in the kidney. AII binds to specific cell surface receptors present on a number of different renal cell types including mesangial, vascular smooth muscle, tubular and interstitial cells, and activates many of the intracellular signalling pathways associated with cell growth. In vitro AII can potentiate the mitogenic effect of other growth factors such as EGF. AII induces hypertrophy of vascular smooth muscle cells but the role of AII in the growth of other renal cell types has not been systematically studied.     

Stimulation of renal gluconeogenesis by angiotensin II.

Renal gluconeogenesis was studied in suspended tubule fragments isolated by collagenase treatment of rat kidney cortices. Angiotensin II increased glucose formation from pyruvate, lactate, and to a lesser extent from oxoglutarate and glutamine, but not from other substrates such as malate, succinate, dihydroxyacetone or fructose. Stimulation was significant with peptide concentration exceeding 1 . 10(-8) M and was also shown with an 8-Sar derivative. Other peptides such as 4-Ala-8-Ile-angiotensin II, hexapeptide and bradykinin had no effect. The stimulatory action of angiotensin II was additive to that of L-lysine, and 3',5'-adenosine cyclic monophosphate, suggesting a different mechanism of action. In the presence of maximally stimulatory concentrations of oleate, phenylephrine and 3',5'-guanosine cyclic monophosphate, however, the stimulatory effect of angiotensin II was absent. Cyclic GMP levels, however, did not increase in tubules after angiotensin II and phenylephrine addition, making a messenger function of this nucleotide unlikely. Omission of Ca2+ from the medium markedly reduced basal gluconeogenesis but did not result in a complete loss of angiotensin II effect. Reduction of medium potassium to 2 mM, however, increased basal gluconeogenesis and blunted the peptide effect. 1 mM ouabain was also able to inhibit the stimulatory effect of angiotensin II. Therefore changes in intracellular potassium levels are discussed as a possible mechanism of angiontensin action, whereas calcium seems not to be specifically linked to this metabolic action of angiotensin on the proximal tubule.     

Intrarenal dopamine modulates progressive angiotensin II-mediated renal injury

It is well-recognized that excessive angiotensin II (ANG II) can mediate progressive renal injury. Previous studies by us and others have indicated that dopamine may modulate actions of ANG II in the kidney. The current studies investigated whether altering intrarenal dopamine levels affected ANG II-mediated renal fibrosis. We utilized a model of increased intrarenal dopamine, catechol-O-methyl-transferase knockout (COMT KO) mice, which have increased kidney dopamine levels due to deletion of a major intrarenal dopamine-metabolizing enzyme. In wild-type mice, chronic ANG II infusion increased renal expression of both of the major dopamine-metabolizing enzymes, COMT and monoamine oxidase. After 8 wk of ANG II infusion, there were no significant differences in blood pressure between wild-type and COMT KO mice. Compared with wild-type, COMT KO mice had decreased albuminuria and tubulointerstitial injury. In response to ANG II infusion, there was decreased expression of both glomerular and tubulointerstitial injury markers (fibronectin, connective tissue growth factor, fibroblast-specific protein-1, collagen I, podocyte vascular endothelial growth factor) in COMT KO mice. We recently reported that ANG II-mediated tubulointerstitial fibrosis is mediated by src-dependent epidermal growth factor receptor (EGFR) activation. In aromatic l-amino acid decarboxylase knockout (AADC KO) mice, a model of intrarenal dopamine deficiency due to selective proximal tubule AADC deletion, which inhibits intrarenal dopamine synthesis, ANG II infusion further increased expression of p-src and pTyr845-EGFR. In contrast, their expression was markedly attenuated in COMT KO mice. These results demonstrate a role for intrarenal dopamine to buffer the detrimental effects of ANG II upon the kidney.     

Effects of locally formed angiotensin II on renal hemodynamics

The kidney produces angiotensin II (AngII) by conversion of both locally formed and systemically delivered angiotensin I (AngI). The latter may be physiologically significant because the kidney can convert 20-25% of systemically delivered AngI. To determine possible differences between the effects of circulating and locally converted AngII, we compared the renal responses to renal arterial infusions of AngI and AngII in equiconstrictor doses. Both reduced the renal blood flow and increased the filtration fraction; it is important that the AngI infusions consistently reduced glomerular filtration rates (GFR), which indicates effects proximal to or at the glomerulus. Micropuncture experiments revealed that AngI infusions reduced proximal tubular and peritubular capillary pressures and the single-nephron GFR; glomerular capillary pressure was not altered significantly. AngI infusions increased both pre- and postglomerular resistances and reduced the glomerular filtration coefficient. In other studies designed to estimate net intrarenal AngII generation, it was determined that the kidney degrades about 90% of arterially delivered AngII. Thus, most of the AngII in renal venous blood was formed intrarenally. Local production of AngII was enhanced, in association with increased renin release, after reductions in renal arterial pressure. Such increases in intrarenal AngII production may contribute to the AngII-dependent changes in renal vascular resistance that occur in conditions where the renin-angiotensin system is stimulated.     

Acute effects of angiotensin II on renal haemodynamics and excretion in conscious dogs

The effects of a 60-min intravenous infusion of angiotensin II (A II; 4 or 20 ng A II/min/kg body weight) on renal blood flow (RBF; electromagnetic flow transducer, control value 19-25 ml/min/kg), glomerular filtration rate (GFR; control value 4.2-5.0 ml/min/kg), mean arterial blood pressure, sodium excretion, water excretion, and plasma A II and plasma aldosterone concentrations were examined in 6 chronically instrumented female conscious beagle dogs kept on three different dietary sodium intakes (SI): SI 0.5 or SI 2.5 mmol Na/kg/day or SI 4.5 mmol Na/kg/day plus an oral saline load prior to the experiment SI 4.5(+) dogs. Four nanograms A II decreased RBF and GFR in SI 4.5(+) dogs without changing the filtration fraction (FF%); in SI 0.5 dogs the RBF decreased, and the FF% increased. Twenty nanograms A II decreased RBF and increased FF% in all dietary protocols, less in SI 4.5(+) dogs. The mean arterial blood pressure increased in all dietary protocols by 10-15 mm Hg (4 ng A II) and 32-37 mm Hg (20 ng A II). Sodium and water excretions decreased by 32 and 46%, respectively, in SI 4.5(+) dogs at both doses of A II. The plasma aldosterone concentration increased in all but one protocol: 4 ng A II, SI 4.5(+) dogs. It is concluded that when A II plasma concentrations are most likely borderline to pathophysiological conditions (up to an average of 370 pg/ml), the GFR is less decreased than the RBF. This phenomenon also can be observed at lower plasma A II concentrations (average 200 pg/ml), when the renin-angiotensin system had been previously moderately activated.     

PKA-mediated effect of MAS receptor in counteracting angiotensin II-stimulated renal Na-ATPase

We showed previously that angiotensin-(1-7) [Ang-(1-7)] reversed stimulation of proximal tubule Na⁺-ATPase promoted by angiotensin II (Ang II) through a d-ala⁷-Ang-(1-7) (A779)-sensitive receptor. Here we investigated the signaling pathway coupled to this receptor. According to our data, Ang-(1-7) produces a MAS-mediated reversal of Ang II-stimulated Na⁺-ATPase by a Gs/PKA pathway because: (1) the Ang-(1-7) effect is reversed by GDPβS, an inhibitor of trimeric G protein and Gs polyclonal antibody. Cholera toxin, an activator of Gs protein, mimicked it; (2) in the presence of Ang II, Ang-(1-7) increased the PKA activity 10-fold; (3) the peptide inhibitor of PKA blocked the Ang-(1-7) effect on Ang II-stimulated Na⁺-ATPase; (4) Ang-(1-7) reverses the Ang II-stimulated PKC activity; (5) cAMP mimicked the Ang-(1-7) effect on the Ang II-stimulated Na⁺-ATPase. Our results provide new understanding about the signaling mechanisms coupled to MAS receptor-mediated renal Ang-(1-7) effects.