Roles of ANG II and bradykinin in the renal regional blood flow responses to ACE inhibition in sodium-depleted dogs

Chronic Overexpression of Bradykinin in Kidney Causes Polyuria and Cardiac Hypertrophy

Acute intra-renal infusion of bradykinin increases diuresis and natriuresis via inhibition of vasopressin activity. However, the consequences of chronically increased bradykinin in the kidneys have not yet been studied. A new transgenic animal model producing an excess of bradykinin by proximal tubular cells (KapBK rats) was generated and submitted to different salt containing diets to analyze changes in blood pressure and other cardiovascular parameters, urine excretion, and composition, as well as levels and expression of renin-angiotensin system components. Despite that KapBK rats excrete more urine and sodium, they have similar blood pressure as controls with the exception of a small increase in systolic blood pressure (SBP). However, they present decreased renal artery blood flow, increased intrarenal expression of angiotensinogen, and decreased mRNA expression of vasopressin V1A receptor (AVPR1A), suggesting a mechanism for the previously described reduction of renal vasopressin sensitivity by bradykinin. Additionally, reduced heart rate variability (HRV), increased cardiac output and frequency, and the development of cardiac hypertrophy are the main chronic effects observed in the cardiovascular system. In conclusion: (1) the transgenic KapBK rat is a useful model for studying chronic effects of bradykinin in kidney; (2) increased renal bradykinin causes changes in renin angiotensin system regulation; (3) decreased renal vasopressin sensitivity in KapBK rats is related to decreased V1A receptor expression; (4) although increased renal levels of bradykinin causes no changes in mean arterial pressure (MAP), it causes reduction in HRV, augmentation in cardiac frequency and output and consequently cardiac hypertrophy in rats after 6 months of age.

Preoperative angiotensin-converting enzyme inhibitors and angiotensin receptor blocker use and acute kidney injury in patients undergoing cardiac surgery

BACKGROUND

Using either an angiotensin-converting enzyme inhibitor (ACEi) or an angiotensin receptor blocker (ARB) the morning of surgery may lead to 'functional' postoperative acute kidney injury (AKI), measured by an abrupt increase in serum creatinine. Whether the same is true for 'structural' AKI, measured with new urinary biomarkers, is unknown.

METHODS

The TRIBE-AKI study was a prospective cohort study of 1594 adults undergoing cardiac surgery at six hospitals between July 2007 and December 2010. We classified the degree of exposure to ACEi/ARB into three categories: 'none' (no exposure prior to surgery), 'held' (on chronic ACEi/ARB but held on the morning of surgery) or 'continued' (on chronic ACEi/ARB and taken the morning of surgery). The co-primary outcomes were 'functional' AKI based upon changes in pre- to postoperative serum creatinine, and 'structural AKI', based upon peak postoperative levels of four urinary biomarkers of kidney injury.

RESULTS

Across the three levels (none, held and continued) of ACEi/ARB exposure there was a graded increase in functional AKI, as defined by AKI stage 1 or worse; (31, 34 and 42%, P for trend 0.03) and by percentage change in serum creatinine from pre- to postoperative (25, 26 and 30%, P for trend 0.03). In contrast, there were no differences in structural AKI across the strata of ACEi/ARB exposure, as assessed by four structural AKI biomarkers (neutrophil gelatinase-associated lipocalin, kidney injury molecule-1, interleukin-18 or liver-fatty acid-binding protein).

CONCLUSIONS

Preoperative ACEi/ARB usage was associated with functional but not structural acute kidney injury. As AKI from ACEi/ARB in this setting is unclear, interventional studies testing different strategies of perioperative ACEi/ARB use are warranted.

Contribution of bradykinin B2 receptors to the inhibition by valsartan of systemic and renal effects of exogenous angiotensin II in salt-repleted humans

To investigate whether bradykinin (BK) participates in the inhibition of renal effects of exogenous angiotensin II (AngII) by AngII type 1 receptor (AT1R) blockade, eight salt-repleted volunteers underwent four p-aminohippurate- and inulin-based renal studies of AngII infusion at increasing rates of 0.625, 1.25, and 2.5 ng.kg.min(-1) for 30 min. Studies 1 and 2 were preceded by 3 days of placebo, whereas studies 3 and 4 used 240 to 320 mg.day(-1) valsartan. Bradykinin B2-type receptor (BKB2R) antagonist icatibant (50 mug.kg(-1)) was coinfused in studies 2 and 4. Mean blood pressure (MBP), glomerular filtration rate (GFR), renal blood flow (RBF), and renal sodium excretion (UNaV) were measured. In study 1, MBP rose by 12.8%, UNaV decreased by 68%, and GFR and RBF also fell (p < 0.001 for all). In study 2, GFR and RBF fell as in study 1, but the rise in MBP and the fall in UNaV were accentuated [+20.0%, analysis of variance (ANOVA), p < 0.02 versus study 1 and -80.0%, p < 0.05, respectively]. In study 3, AngII had no effects, and in study 4, renal hemodynamics remained unaffected, but MBP still rose and UNaV fell (ANOVA, p < 0.02 and 0.005 versus study 3, respectively). Icatibant accentuated AngII-induced changes in MBP and UNaV. Previous AT1R blockade prevented any systemic and renal effects of AngII, but significant changes in MBP and UNaV still followed AngII plus icatibant even after AT1R blockade. BK, through BKB2Rs, participates in the inhibitory action of AT1R blockers toward actions of exogenous AngII on MBP and UNaV in healthy humans.

Losartan chemistry and its effects via AT1 mechanisms in the kidney

Besides the importance of the renin-angiotensin system (RAS) in the circulation and other organs, the local RAS in the kidney has attracted a great attention in research in last decades. The renal RAS plays an important role in the body fluid homeostasis and long-term cardiovascular regulation. All major components and key enzymes for the establishment of a local RAS as well as two important angiotensin II (Ang II) receptor subtypes, AT1 and AT2 receptors, have been confirmed in the kidney. In additional to renal contribution to the systemic RAS, the intrarenal RAS plays a critical role in the regulation of renal function as well as in the development of kidney disease. Notably, kidney AT1 receptors locating at different cells and compartments inside the kidney are important for normal renal physiological functions and abnormal pathophysiological processes. This mini-review focuses on: 1) the local renal RAS and its receptors, particularly the AT1 receptor and its mechanisms in physiological and pathophysiological processes; and 2) the chemistry of the selective AT1 receptor blocker, losartan, and the potential mechanisms for its actions in the renal RAS-mediated disease.

Distal tubular epithelial cells of the kidney: Potential support for proximal tubular cell survival after renal injury

The tubular epithelium of the kidney is susceptible to injury from many causes, such as ischemia-reperfusion and the associated oxidative stress, nephrotoxins, inflammatory and immune disorders and many others. The outcome is often acute kidney injury, which may progress to chronic kidney disease and fibrosis. Acute kidney injury involves not only direct injury to the distal tubular (DT) and proximal tubular (PT) epithelium during and immediately following the injurious event, but the closely-associated and sometimes dysfunctional renal vascular endothelium also plays an important part in modulating the tubular epithelial injury. In comparison with the PT, the DT epithelium is less sensitive to cell death, especially after ischemic injury. It is more prone to apoptosis than necrosis when it dies, and has key paracrine and autocrine functions in secreting an array of inflammatory, reparative, and survival cytokines that include chemotactic cytokines, polypeptide growth factors, and vasoactive peptides. In a neighborly way, the cytokines and growth factors secreted by the DT epithelium may then act positively on the ischemia-sensitive PT that has receptors to many of these proteins, but may not be able to synthesize them. A more complete understanding of these cellular events will allow protection against nephron destruction, regeneration leading to re-epithelialization of the injured tubules, or prevention of progression to chronic kidney disease. This review looks at these functions in the DT epithelial cells, specifically the cells in the medullary thick ascending limb of the loop of Henle, in contrast with those of the straight segment of the PT.

Angiotensin II Exerts Positive Feedback on the Intrarenal Renin-Angiotensin System by an Angiotensin Converting Enzyme-Dependent Mechanism1

BACKGROUND

Plasma angiotensin II (ANG II) is not increased significantly in renovascular hypertension (RVH), but tissue ANG II levels are elevated in both kidneys of renovascular rats. Because the contralateral, non-ischemic kidney is critical for maintenance of hypertension in RVH, this study sought to understand the mechanism by which intrarenal ANG II levels are augmented in the non-ischemic kidney. This study tested the hypothesis that an incremental increase in plasma ANG II induces the intrarenal renin-angiotensin system (RAS) in the non-ischemic kidney by an angiotensin converting enzyme (ACE) dependent mechanism.

METHODS

To simulate the incremental increase in plasma ANG II induced by the ischemic kidney in RVH, an ANG II infusion model was used. This model used a chronic infusion of ANG II (40 ng/min) or vehicle by osmotic minipump into uninephrectomized rats. Parallel groups were treated with the ACE inhibitor Enalaprilat (200 mg/kg/day). Intrarenal ACE activity was measured by radioenzymatic assay. ANG II levels were quantified by radioimmunoassay.

RESULTS

Hypertension was evident in ANG II-infused rats, compared to control rats (155 +/- 4 versus 112 +/- 1 mmHg; P < 0.001). Concurrent treatment with Enalaprilat reversed the hypertension induced by ANG II infusion (98 +/- 3 versus 155 +/- 4 mmHg; P < 0.001). ANG II up-regulated intrarenal ACE activity in the non-ischemic kidney (59.2 +/- 11.9 versus 25.2 +/- 6.8 units/mg protein; P < 0.01). Enalaprilat significantly decreased renal ACE activity in ANG II-treated rats, compared to ANG II alone (11.4 +/- 1.0 versus 59.2 +/- 11.9 units/mg protein; P < 0.001). Intrarenal ANG II was increased in ANG II-infused rats, compared to control animals (52.9 +/- 7.1 versus 23.0 +/- 3.2 fmol/mg tissue; P < 0.001), and Enalaprilat prevented ANG II-induced increases in intrarenal ANG II (29.9 +/- 2.6 versus 52.9 +/- 7.1 fmol/mg tissue; P < 0.05).

CONCLUSION

Incremental changes in plasma ANG II induce de novo production of ANG II in the non-ischemic kidney to augment intrarenal ANG II content. ACE inhibition blocks this positive feedback loop, suggesting that ANG II activates the intrarenal RAS by an ACE-dependent mechanism. The impact of ACE inhibition on blood pressure suggests that this feedback loop may be an important mechanism for maintenance of hypertension in RVH.

Aortic blood flow subtraction: an alternative method for measuring total renal blood flow in conscious dogs

We have measured total renal blood flow (TRBF) as the difference between signals from ultrasound flow probes implanted around the aorta above and below the renal arteries. The repeatability of the method was investigated by repeated, continuous infusions of angiotensin II and endothelin-1 seven times over 8 wk in the same dog. Angiotensin II decreased TRBF (350 +/- 16 to 299 +/- 15 ml/min), an effect completely blocked by candesartan (TRBF 377 +/- 17 ml/min). Subsequent endothelin-1 infusion reduced TRBF to 268 +/- 20 ml/min. Bilateral carotid occlusion (8 sessions in 3 dogs) increased arterial blood pressure by 49% and decreased TRBF by 12%, providing an increase in renal vascular resistance of 69%. Dynamic analysis showed autoregulation of renal blood flow in the frequency range <0.06-0.07 Hz, with a peak in the transfer function at 0.03 Hz. It is concluded that continuous measurement of TRBF by aortic blood flow subtraction is a practical and reliable method that allows direct comparison of excretory function and renal blood flow from two kidneys. The method also allows direct comparison between TRBF and flow in the caudal aorta.

Differential effect of angiotensin II on blood circulation in the renal medulla and cortex of anaesthetised rats

The renal medulla is sensitive to hypoxia, and a depression of medullary circulation, e.g. in response to angiotensin II (Ang II), could endanger the function of this zone. Earlier data on Ang II effects on medullary vasculature were contradictory. The effects of Ang II on total renal blood flow (RBF), and cortical and medullary blood flow (CBF and MBF: by laser-Doppler flux) were studied in anaesthetised rats. Ang II infusion (30 ng kg(-1) min(-1) i.v.) decreased RBF 27 +/- 2 % (mean +/- S.E.M.), whereas MBF increased 12 +/- 2 % (both P < 0.001). Non-selective blockade of Ang II receptors with saralasin (3 microg kg(-1) min(-1) i.v.) increased RBF 12 +/- 2 % and decreased MBF 8 +/- 2 % (P < 0.001). Blockade of AT(1) receptors with losartan (10 mg kg(-1)) increased CBF 10 +/- 2 % (P < 0.002) and did not change MBF. Losartan given during Ang II infusion significantly increased RBF (53 +/- 7 %) and decreased MBF (27 +/- 7 %). Blockade of AT(2) receptors with PD 123319 (50 microg kg(-1) min(-1) i.v.) did not change CBF or MBF. Intramedullary infusion of PD 123319 (10 microg min(-1)) superimposed on intravenous Ang II infusion did not change RBF, but slightly decreased MBF (4 +/- 2 %, P < 0.05). We conclude that in anaesthetised surgically prepared rats, exogenous or endogenous Ang II may not depress medullary circulation. In contrast to the usual vasoconstriction in the cortex, vasodilatation was observed, possibly related to secondary activation of vasodilator paracrine agents rather than to a direct action via AT(2) receptors.