Impaired renal blood flow and cortical pressure autoregulation in contralateral kidneys of Goldblatt hypertensive rats

Renal sympathetic denervation improves pressure-natriuresis relationship in cardiorenal syndrome: insight from studies with Ren-2 transgenic hypertensive rats with volume overload induced using aorto-caval fistula

The aim was to evaluate the effects of renal denervation (RDN) on autoregulation of renal hemodynamics and the pressure-natriuresis relationship in Ren-2 transgenic rats (TGR) with aorto-caval fistula (ACF)-induced heart failure (HF). RDN was performed one week after creation of ACF or sham-operation. Animals were prepared for evaluation of autoregulatory capacity of renal blood flow (RBF) and glomerular filtration rate (GFR), and of the pressure-natriuresis characteristics after stepwise changes in renal arterial pressure (RAP) induced by aortic clamping. Their basal values of blood pressure and renal function were significantly lower than with innervated sham-operated TGR (p < 0.05 in all cases): mean arterial pressure (MAP) (115 ± 2 vs. 160 ± 3 mmHg), RBF (6.91 ± 0.33 vs. 10.87 ± 0.38 ml.min-1.g-1), urine flow (UF) (11.3 ± 1.79 vs. 43.17 ± 3.24 µl.min-1.g-1) and absolute sodium excretion (UNaV) (1.08 ± 0.27 vs, 6.38 ± 0.76 µmol.min-1.g-1). After denervation ACF TGR showed improved autoregulation of RBF: at lowest RAP level (80 mmHg) the value was higher than in innervated ACF TGR (6.92 ± 0.26 vs. 4.54 ± 0.22 ml.min-1.g-1, p < 0.05). Also, the pressure-natriuresis relationship was markedly improved after RDN: at the RAP of 80 mmHg UF equaled 4.31 ± 0.99 vs. 0.26 ± 0.09 µl.min-1.g-1 recorded in innervated ACF TGR, UNaV was 0.31 ± 0.05 vs. 0.04 ± 0.01 µmol min-1.g-1 (p < 0.05 in all cases). In conclusion, in our model of hypertensive rat with ACF-induced HF, RDN improved autoregulatory capacity of RBF and the pressure-natriuresis relationship when measured at the stage of HF decompensation.

Impaired renal autoregulation and pressure-natriuresis: any role in the development of heart failure in normotensive and angiotensin II-dependent hypertensive rats?

The aim of the present study was to assess the autoregulatory capacity of renal blood flow (RBF) and of the pressure-natriuresis characteristics in the early phase of heart failure (HF) in rats, normotensive and with angiotensin II (ANG II)-dependent hypertension. Ren-2 transgenic rats (TGR) were employed as a model of ANG II-dependent hypertension. HF was induced by creating the aorto-caval fistula (ACF). One week after ACF creation or sham-operation, the animals were prepared for studies evaluating in vivo RBF autoregulatory capacity and the pressure-natriuresis characteristics after stepwise changes in renal arterial pressure (RAP) induced by aortic clamping. In ACF TGR the basal mean arterial pressure, RBF, urine flow (UF), and absolute sodium excretion (UNaV) were all significantly lower tha n in sham-operated TGR. In the latter, reductions in renal arterial pressure (RAP) significantly decreased RBF whereas in ACF TGR they did not change. Stepwise reductions in RAP resulted in marked decreases in UF and UNaV in sham-operated as well as in ACF TGR, however, these decreases were significantly greater in the former. Our data show that compared with sham-operated TGR, ACF TGR displayed well-maintained RBF autoregulatory capacity and improved slope of the pressure-natriuresis relationship. Thus, even though in the very early HF stage renal dysfunction was demonstrable, in the HF model of ANG II-dependent hypertensive rat such dysfunction and the subsequent HF decompensation cannot be simply ascribed to impaired renal autoregulation and pressure-natriuresis relationship.

Chronic administration of catalase inhibitor attenuates hypertension in renovascular hypertensive rats

AIMS

Reactive oxygen species like hydrogen peroxide (H₂O₂) are produced endogenously and may participate in intra- and extracellular signaling, including modulation of angiotensin II responses. In the present study, we investigated the effects of chronic subcutaneous (sc) administration of the catalase inhibitor 3-amino-1,2,4-triazole (ATZ) on arterial pressure, autonomic modulation of arterial pressure, hypothalamic expression of AT1 receptors and neuroinflammatory markers and fluid balance in 2-kidney, 1clip (2K1C) renovascular hypertensive rats.

MATERIALS AND METHODS

Male Holtzman rats with a clip occluding partially the left renal artery and chronic sc injections of ATZ were used.

KEY FINDINGS

Subcutaneous injections of ATZ (600 mg/kg of body weight/day) for 9 days in 2K1C rats reduced arterial pressure (137 ± 8, vs. saline: 182 ± 8 mmHg). ATZ also reduced the sympathetic modulation and enhanced the parasympathetic modulation of pulse interval, reducing the sympatho-vagal balance. Additionally, ATZ reduced mRNA expression for interleukins 6 and IL-1β, tumor necrosis factor-α, AT1 receptor (0.77 ± 0.06, vs. saline: 1.47 ± 0.26 fold change), NOX 2 (0.85 ± 0.13, vs. saline: 1.75 ± 0.15 fold change) and the marker of microglial activation, CD 11 (0.47 ± 0.07, vs. saline, 1.34 ± 0.15 fold change) in the hypothalamus of 2K1C rats. Daily water and food intake and renal excretion were only slightly modified by ATZ.

SIGNIFICANCE

The results suggest that the increase of endogenous H₂O₂ availability with chronic treatment with ATZ had an anti-hypertensive effect in 2K1C hypertensive rats. This effect depends on decreased activity of sympathetic pressor mechanisms and mRNA expression of AT1 receptors and neuroinflammatory markers possibly due to reduced angiotensin II action.

Prerenal acute kidney injury—still a relevant term in modern clinical practice?

The traditional taxonomy of acute kidney injury (AKI) has remained pervasive in clinical nephrology. While the terms ‘prerenal’, ‘intrarenal’ and ‘postrenal’ highlight the diverse pathophysiology underlying AKI, they also imply discrete disease pathways and de-emphasize the nature of AKI as an evolving clinical syndrome with multiple, often simultaneous and overlapping, causes. In a similar vein, prerenal AKI comprises a diverse spectrum of kidney disorders, albeit one that is often managed by using a standardized clinical algorithm. We contend that the term ‘prerenal’ is too vague to adequately convey our current understanding of hypoperfusion-related AKI and that it should thus be avoided in the clinical setting. Practice patterns among nephrologists indicate that AKI-related terminology plays a significant role in the approaches that clinicians take to patients that have this complex disease. Thus, it appears that precise terminology does impact the treatment that patients receive. We will outline differences in the diagnosis and management of clinical conditions lying on the so-called prerenal disease spectrum to advocate caution when administering intravenous fluids to these clinically decompensated patients. An understanding of the underlying pathophysiology may, thus, avert clinical missteps such as fluid and vasopressor mismanagement in tenuous or critically ill patients.

Purinoceptors, renal microvascular function and hypertension

Proper renal blood flow (RBF) and glomerular filtration rate (GFR) are critical for maintaining normal blood pressure, kidney function and water and electrolyte homeostasis. The renal microvasculature expresses a multitude of receptors mediating vasodilation and vasoconstriction, which can influence glomerular blood flow and capillary pressure. Despite this, RBF and GFR remain quite stable when arterial pressure fluctuates because of the autoregulatory mechanism. ATP and adenosine participate in autoregulatory control of RBF and GFR via activation of two different purinoceptor families (P1 and P2). Purinoceptors are widely expressed in renal microvasculature and tubules. Emerging data show altered purinoceptor signaling in hypertension-associated kidney injury, diabetic nephropathy, sepsis, ischemia-reperfusion induced acute kidney injury and polycystic kidney disease. In this brief review, we highlight recent studies and new insights on purinoceptors regulating renal microvascular function and renal hemodynamics. We also address the mechanisms underlying renal microvascular injury and impaired renal autoregulation, focusing on purinoceptor signaling and hypertension-induced renal microvascular dysfunction. Interested readers are directed to several excellent and comprehensive reviews that recently covered the topics of renal autoregulation, and nucleotides in kidney function under physiological and pathophysiological conditions (Inscho 2009, Navar et al. 2008, Carlstrom et al. 2015, Vallon et al. 2020).

Treatment with Mesenchymal Stem Cells Improves Renovascular Hypertension and Preserves the Ability of the Contralateral Kidney to Excrete Sodium

BACKGROUND

Mesenchymal stem cells (MSC) improve renal function and renovascular hypertension in the 2-kidney 1-clip model (2K-1C). While MSC play an immunomodulatory role, induce neoangiogenesis, and reduce fibrosis, they do not correct sodium loss by the contra-lateral kidney.

OBJECTIVES

We investigated the tubular function of both stenotic and contralateral kidneys and the effect of MSC treatment by evaluating diuresis, natriuresis, and the expression of the main water and sodium transporters.

METHOD

Adult Wistar rats were allocated into four groups: control (CT), CT+MSC, 2K-1C, and 2K-1C+MSC. MSC (2 × 105) were infused through the tail vein 3 and 5 weeks after clipping. Systolic blood pressure (SBP) was monitored weekly by plethysmography. Six weeks after clipping, 24-hour urine and blood samples were collected for biochemical analysis. Gene expression of the Na/H exchanger-3, epithelial sodium channel, Na/K-ATPase, Na/K/2Cl cotransporter, and aquaporins 1 and 2 (AQP1 and AQP2) were analyzed by RT-PCR. Intrarenal distribution of AQP1 and AQP2 was analyzed by immunohistochemistry.

RESULTS

In hypertensive 2K-1C animals, MSC prevented additional increases in BP. AQP1, but not AQP2, was suppressed in the contralateral kidney, resulting in significant increase in urinary flow rate and sodium excretion. Gene expressions of sodium transporters were similar in both kidneys, suggesting that the high perfusing pressure in the contralateral kidney was responsible for increased natriuresis. Contralateral hypertensive kidney showed signs of renal deterioration with lower GFR in spite of normal RPF levels.

CONCLUSIONS

MSC treatment improved renal function and enhanced the ability of the contralateral kidney to excrete sodium through a tubular independent mechanism contributing to reduce SBP.

Oleanolic acid modulates the renin-angiotensin system and cardiac natriuretic hormone concomitantly with volume and pressure balance in rats

Oleanolic acid is known to possess beneficial effects on the regulation of the cardiovascular homeostasis. However, the exact nature of the role of oleanolic acid on the regulation of body fluid balance and blood pressure homeostasis and its mechanisms involved are not well defined. Experiments were performed to identify the effects of oleanolic acid on the renin-angiotensin system and cardiac natriuretic hormone (ANP) system, and also renal function and blood pressure in normotensive and renovascular hypertensive rats. The change in the plasma levels of hormones and the expressions of renin, angiotensin II receptors, ANP, natriuretic peptide receptor-C, M₂ muscarinic receptor and GIRK4 were determined in the kidney, heart and aorta. Oleanolic acid was administered orally for 1 or 3 weeks. Here, we found that oleanolic acid suppressed plasma levels of renin activity and aldosterone and intrarenal levels of renin and angiotensin II type 1 receptor expression and increased angiotensin II type 2 receptor in normotensive and hypertensive rats. Also, oleanolic acid increased plasma levels of ANP. Further, oleanolic acid suppressed angiotensin II type 1 receptor and natriuretic peptide receptor-C expression and increased angiotensin II type 2 receptor and ANP expression in the heart and aorta. Along with these changes, oleanolic acid accentuated urinary volume, electrolyte excretion and glomerular filtration rate in normotensive rats and suppressed arterial blood pressure in hypertensive rats. These findings suggest that beneficial effects of oleanolic acid on the cardiorenal system are closely associated with its roles on the renin-angiotensin system and cardiac natriuretic hormone system.

Renovascular hypertension: Effects of mesenchymal stem cells in the contralateral hypertensive kidney in rats

Mesenchymal stem cells (MSC) induced neovascularization and improved renal morphology of the stenotic kidney in 2 kidneys-1 clip (2K-1C) model of renovascular hypertension. The present study evaluated the effects of MSC in the contralateral hypertensive kidney. Three weeks after left renal artery occlusion, MSC were injected into the tail vein of the 2K-1C rats. Renal function and morphology were analyzed in both kidneys. Labeled MSC were found in stenotic and contralateral kidneys. Hypertensive 2K-1C animals presented increased circulating levels of Angiotensin II (Ang II) and renin. MSC prevented the progressive increase of blood pressure and reduced circulating Ang II and renin levels. Stenotic kidney showed reduced renal plasma flow (RPF) and glomerular filtration rate (GFR), whereas the contralateral kidney had a tendency (p > 0.5) of reduction in GFR in spite of unchanged RPF. MSC treatment caused an improvement in GFR with no effect of on RPF in the stenotic kidney. Contralateral kidney showed increased diuresis and natriuresis that were even higher in MSC-treated animals, indicating that cell treatment improved the capacity of the contralateral kidney to excrete sodium. Contralateral kidney expressed higher levels of inflammatory cytokines (IL-6, TNF-α) and signs of fibrosis, which were attenuated by MSC treatment. MSC treatment improved the stenotic kidney function, and it was also beneficial to the contralateral hypertensive kidney because it improved the morphology and preserved its capacity to excrete sodium.

Hypercholesterolemia Impairs Nonstenotic Kidney Outcomes After Reversal of Experimental Renovascular Hypertension

BACKGROUND

Revascularization of a stenotic renal artery improves kidney function only in select patients with renovascular hypertension (HT) secondary to atherosclerosis. However, the effects of reversal of renovascular HT (RRHT) on the nonstenotic kidney are unclear. We hypothesized that concurrent hypercholesterolemia (HC) attenuates nonstenotic kidney recovery.

METHODS

Female domestic pigs were randomized as Normal, renovascular HT, HT+RRHT, HTC (renovascular HT and HC), and HTC+RHT (n = 7 each). RRHT or sham was performed after 6 weeks of HT. Nonstenotic renal blood flow, glomerular filtration rate, and injurious pathways were studied 4 weeks later.

RESULTS

Mean arterial pressure increased similarly in HT and HTC and decreased after RRHT. Oxidative stress increased in HT and HTC kidneys, and decreased in HT+RRHT, but remained elevated in HTC+RRHT. Renal interstitial fibrosis, glomerulosclerosis, and tubular injury were all attenuated in HT+RRHT, but not HTC+RRHT. Endothelin-1 signaling and PGF2α isoprostane levels were elevated in both HTC and HTC+RRHT pigs.

CONCLUSIONS

RRHT reverses nonstenotic kidney injury in experimental renovascular HT, but concurrent HC blunts regression of kidney injury, possibly due to predominant vasoconstrictors and oxidative stress. These findings reinforce the contribution of the nonstenotic kidney and of prevailing cardiovascular risk factors to irreversibility of kidney dysfunction after revascularization.

Endothelin contributes to blunted renal autoregulation observed with a high-salt diet

Autoregulation of renal blood flow (RBF) is an essential function of the renal microcirculation that has been previously shown to be blunted by excessive dietary salt. Endogenous endothelin 1 (ET-1) is increased following a high-salt (HS) diet and contributes to the control of RBF but the differential effects of ET-1 on renal microvessel autoregulation in response to HS remain to be established. We hypothesized that a HS diet increases endothelin receptor activation in normal Sprague-Dawley rats and blunts autoregulation of RBF. The role of ET-1 in the blunted autoregulation produced by a HS diet was assessed in vitro and in vivo using the blood-perfused juxtamedullary nephron preparation and anesthetized rats, respectively. Using highly selective antagonists, we observed that blockade of either ETA or ETB receptors was sufficient to restore normal autoregulatory behavior in afferent arterioles from HS-fed rats. Additionally, normal autoregulatory behavior was restored in vivo in HS-fed rats by simultaneous ETA and ETB receptor blockade, whereas blockade of ETB receptors alone showed significant improvement of normal autoregulation of RBF. Consistent with this observation, autoregulation of RBF in ETB receptor-deficient rats fed HS was similar to both ETB-deficient rats and transgenic control rats on normal-salt diets. These data support the hypothesis that endogenous ET-1, working through ETB and possibly ETA receptors, contributes to the blunted renal autoregulatory behavior in rats fed a HS diet.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Role of cytochrome P-450 metabolites in the regulation of renal function and blood pressure in 2-kidney 1-clip hypertensive rats

Alterations in renal function contribute to Goldblatt two-kidney, one-clip (2K1C) hypertension. A previous study indicated that bioavailability of cytochrome P-450 metabolites epoxyeicosatrienoic acids (EETs) is decreased while that of 20-hydroxyeicosatetraenoic acids (20-HETE) is increased in this model. We utilized the inhibitor of soluble epoxide hydrolase cis-4-[4-(3-adamantan-1-yl-ureido)-cyclohexyloxy]-benzoic acid (c-AUCB) and HET-0016, the inhibitor of 20-HETE production, to study the role of EETs and 20-HETE in the regulation of renal function. Chronic c-AUCB treatment significantly decreased systolic blood pressure (SBP) (133 ± 1 vs. 163 ± 3 mmHg) and increased sodium excretion (1.23 ± 0.10 vs. 0.59 ± 0.03 mmol/day) in 2K1C rats. HET-0016 did not affect SBP and sodium excretion. In acute experiments, renal blood flow (RBF) was decreased in 2K1C rats (5.0 ± 0.2 vs. 6.9 ± 0.2 ml·min(-1)·g(-1)). c-AUCB normalized RBF in 2K1C rats (6.5 ± 0.6 ml·min(-1)·g(-1)). HET-0016 also increased RBF in 2K1C rats (5.8 ± 0.2 ml·min(-1)·g(-1)). Although RBF and glomerular filtration rate (GFR) remained stable in normotensive rats during renal arterial pressure (RAP) reductions, both were significantly reduced at 100 mmHg RAP in 2K1C rats. c-AUCB did not improve autoregulation but increased RBF at all RAPs and shifted the pressure-natriuresis curve to the left. HET-0016-treated 2K1C rats exhibited impaired autoregulation of RBF and GFR. Our data indicate that c-AUCB displays antihypertensive properties in 2K1C hypertension that are mediated by an improvement of RBF and pressure natriuresis. While HET-0016 enhanced RBF, its anti-natriuretic effect likely prevented it from producing a blood pressure-lowering effect in the 2K1C model.

Kidney-induced hypertension depends on superoxide signaling in the rostral ventrolateral medulla

Reactive oxygen species in peripheral cardiovascular tissues are implicated in the pathogenesis of 2 kidney-1 clip hypertension. We recently identified an imbalance between reactive oxygen species generation and oxidant scavenging in the rostral ventrolateral medulla (RVLM) of 2 kidney-1 clip in rats. We tested whether enhanced superoxide signaling in RVLM of 2 kidney-1 clip rats contributes to the chronic hypertension via sympathetic activation in conscious rats. We enhanced superoxide scavenging in RVLM by overexpressing cytoplasmically targeted superoxide dismutase using an adenoviral vector (Ad-CMV-CuZnSOD) in Wistar rats (male, 150 to 180 g) in which the left renal artery was occluded partially 3 weeks earlier. Hypertension was documented using radiotelemetry recording of arterial pressure in conscious rats for 6 weeks. Renovascular hypertension elevated both serine phosphorylation of p47phox subunit of NADPH and superoxide levels in RVLM. The elevated superoxide levels were normalized by expression of CuZnSOD in RVLM. Moreover, the hypertension produced in the 2 kidney-1 clip rats was reversed 1 week after viral-mediated expression of CuZnSOD. This antihypertensive effect was maintained and associated with a decrease in the low-frequency spectra of systolic blood pressure variability, suggesting reduced sympathetic vasomotor tone. The expression of CuZnSOD was localized to RVLM neurons, of which some contained tyrosine hydroxylase. None of the above variables changed in control rats receiving Ad-CMV-eGFP in RVLM. In Goldblatt hypertension, superoxide signaling in the RVLM plays a major role in the generation of sympathetic vasomotor tone and the chronic sustained hypertension in this animal model.

Pentosan polysulfate treatment preserves renal autoregulation in ANG II-infused hypertensive rats via normalization of P2X1 receptor activation

Inflammatory factors are elevated in animal and human subjects with hypertension and renal injury. We hypothesized that inflammation contributes to hypertension-induced renal injury by impairing autoregulation and microvascular reactivity to P2X(1) receptor activation. Studies were conducted in vitro using the blood-perfused juxtamedullary nephron preparation. Rats receiving ANG II (60 ng/min) infusion were treated with the anti-inflammatory agent pentosan polysulfate (PPS) for 14 days. The magnitude and progression of hypertension were similar in ANG II and ANG II+PPS-treated rats (169 ± 5 vs. 172 ± 2 mmHg). Afferent arterioles from control rats exhibited normal autoregulatory behavior with diameter decreasing from 18.4 ± 1.6 to 11.4 ± 1.7 μm when perfusion pressure was increased from 70 to 160 mmHg. In contrast, pressure-mediated vasoconstriction was markedly attenuated in ANG II-treated rats, and diameter remained essentially unchanged over the range of perfusion pressures. However, ANG II-treated rats receiving PPS exhibited normal autoregulatory behavior compared with ANG II alone rats. Arteriolar reactivity to ATP and β,γ-methylene ATP was significantly reduced in ANG II hypertensive rats compared with controls. Interestingly, PPS treatment preserved normal reactivity to P2 and P2X(1) receptor agonists despite the persistent hypertension. The maximal vasoconstriction was 79 ± 3 and 81 ± 2% of the control diameter for ATP and β,γ-methylene ATP, respectively, similar to responses in control rats. PPS treatment significantly reduced α-smooth muscle actin staining in afferent arterioles and plasma transforming growth factor-β1 concentration in ANG II-treated rats. In conclusion, PPS normalizes autoregulation without altering ANG II-induced hypertension, suggesting that inflammatory processes reduce P2X(1) receptor reactivity and thereby impair autoregulatory behavior in ANG II hypertensive rats.

ATP, P2 receptors and the renal microcirculation

Purinoceptors are rapidly becoming recognised as important regulators of tissue and organ function. Renal expression of P2 receptors is broad and diverse, as reflected by the fact that P2 receptors have been identified in virtually every major tubular/vascular element. While P2 receptor expression by these renal structures is recognised, the physiological functions that they serve remains to be clarified. Renal vascular P2 receptor expression is complex and poorly understood. Evidence suggests that different complements of P2 receptors are expressed by individual renal vascular segments. This unique distribution has given rise to the postulate that P2 receptors are important for renal vascular function, including regulation of preglomerular resistance and autoregulatory behaviour. More recent studies have also uncovered evidence that hypertension reduces renal vascular reactivity to P2 receptor stimulation in concert with compromised autoregulatory capability. This review will consolidate findings related to the role of P2 receptors in regulating renal microvascular function and will present areas of controversy related to the respective roles of ATP and adenosine in autoregulatory resistance adjustments.

Impairment of the autoregulation of renal hemodynamics and of the pressure-natriuresis relationship precedes the development of hypertension in Cyp1a1-Ren-2 transgenic rats

OBJECTIVE

The present study was performed to characterize the autoregulatory efficiency of renal blood flow and glomerular filtration rate and the pressure-natriuresis relationship in transgenic rats with inducible angiotensin II (ANG II)-dependent hypertension (Cyp1a1-Ren-2 rats).

METHODS

The renin gene was induced in Cyp1a1-Ren-2 rats through dietary administration of the natural xenobiotic indole-3-carbinol (I3C, 0.3%) for 12 and 24 h, respectively. Noninduced rats served as controls. Anesthetized rats were prepared for renal function studies and an aortic clamp was placed above the junction of the left renal artery to regulate the level of renal arterial pressure. Plasma renin activity, ANG II and aldosterone levels were measured at the end of the experiment by radioimmunoassay.

RESULTS

Administration of I3C resulted in progressive increases in plasma renin activity and plasma and kidney ANG II levels; however, it did not significantly alter aldosterone levels as compared with those in noninduced rats. I3C induction for 12 h did not cause significant changes in blood pressure as compared with those in noninduced rats. I3C induction for 24 h elicited a significant rise in blood pressure. Twelve-hour I3C induction caused an impairment of the autoregulatory efficiency of renal blood flow and glomerular filtration rate and of the pressure-natriuresis relationship as compared with that in noninduced rats. In addition, 24 h I3C induction of the renin gene resulted in a marked reduction in renal blood flow and glomerular filtration rate and a further impairment of the pressure-natriuresis mechanism as compared with that in noninduced rats.

CONCLUSION

Our findings indicate that an impairment of the pressure-natriuresis mechanism precedes the development of ANG II-dependent hypertension in Cyp1a1-Ren-2 transgenic rats.

Increase in heme oxygenase-1 levels ameliorates renovascular hypertension

BACKGROUND

The heme oxygenase system (HO-1 and HO-2) catalyzes the conversion of heme to free iron, carbon monoxide (CO), a vasodepressor, and biliverdin, which is further converted to bilirubin, an antioxidant. HO-1 induction has been shown to suppress arachidonic acid metabolism by cytochrome P450 (CYP450) monooxygenases and cyclooxygenases (COX), and to decrease blood pressure in spontaneously hypertensive rats (SHR). The Goldblatt 2K1C model is a model of renovascular hypertension in which there is increased expression of COX-2 in the macula densa and increased renin release from the juxtaglomerular apparatus of the clipped kidney. We examined whether HO-1 overexpression, as a prophylactic approach, would attenuate renovascular hypertension and evaluated potential mechanisms that may account for its effect.

METHODS

2K1C rats were treated with cobalt protoporphyrin (CoPP) or tin mesoporphyrin (SnMP) one day before surgery and weekly for three weeks thereafter. We measured systolic blood pressure, HO activity, HO-1, HO-2, COX-1 and COX-2 protein expression, heme content, and nitrotyrosine levels as indices of oxidative stress. Urinary prostaglandin excretion (PGE2), plasma renin activity (PRA), and plasma aldosterone levels were also measured.

RESULTS

CoPP administration induced renal HO-1 expression by 20-fold and HO activity by 6-fold. This was associated with a reduction in heme content, nitrotyrosine levels, COX-2 expression and urinary PGE2 excretion, and attenuation of the development of hypertension in the 2K1C rats. There was no decrease in plasma renin activity; however, plasma aldosterone levels were significantly lower. In the 2K1C SnMP-treated rats, blood pressure was significantly higher than that of untreated 2K1C rats throughout the study, and the difference in the size of the smaller left clipped kidney compared to the nonclipped right kidney was significantly increased.

CONCLUSION

These findings define an action of prolonged HO-1 induction to interrupt and counteract the influence of the renin-angiotensin-aldosterone system (RAAS) to increase in blood pressure in the 2K1C model of renovascular hypertension. Multiple mechanisms include a decrease in oxidative stress as indicated by the decrease in cellular heme and nitrotyrosine levels, an anti-inflammatory action as evidenced by a decrease in COX-2 and PGE2, interference with the action of angiontensin II (Ang II) as evidenced by an increase in PRA in the face of a decrease in PGE2 and aldosterone, as well as the inhibition of aldosterone synthesis.

Romancing the macula densa at UAB

The Nephrology Research and Training Center, established in 1977 at the University of Alabama at Birmingham by Thomas E. Andreoli, served as a catalyst to stimulate multiple areas of investigations in renal physiology and nephrology. Individuals with backgrounds in biophysics, membrane transport, renal hemodynamics, structural biology, and nephrology interacted with each other, thus providing an exciting and collegial environment. The laboratory of renal hemodynamics focused on the control of renal blood flow, glomerular filtration rate in normal and hypertensive models, and on the important role of the macula densa in providing communication from the tubules to the vascular elements. Studies initially focused on the role of the macula densa feedback mechanism in mediating renal autoregulatory behavior. Subsequent experiments examined various aspects of the feedback system, including the identification and characterization of membrane transport events that sense changes in tubular fluid concentration and transfer information to intracellular signaling mechanisms. More recent investigations have focused on the capability of the macula densa cells to synthesize and release various vasoactive mediators that can influence vascular tone of the glomerular arterioles. In particular, the ability of the macula densa cells to secrete ATP has stimulated continued interest in the hypothesis that ATP may serve an important role in mediating signals to afferent arteriolar vascular smooth muscle cells.

Elevated arterial pressure impairs autoregulation independently of AT(1) receptor activation

OBJECTIVE

These studies determined the ability of AT1 receptor blockade or 'triple therapy', to reverse angiotensin II-induced hypertension and improve autoregulatory behavior.

DESIGN

Experiments to determine if regulation of systolic blood pressure, in the normotensive range, would improve renal microvascular autoregulatory behavior in angiotensin II-infused rats.

METHODS

Hypertension was induced by chronic angiotensin II infusion (60 ng/min) for 10-14 days. Two groups of angiotensin II-infused rats received either AT1 receptor blockade, with candesartan cilexetil, or triple therapy, with hydralazine, hydrochlorothiazide and reserpine, beginning on day 6 or day 0 of angiotensin II infusion, respectively. Sham animals were studied as normotensive controls. Systolic blood pressure was measured by tail cuff. Autoregulatory behavior was assessed using the juxtamedullary nephron technique in response to step (15 mmHg) increases in perfusion pressure from 65 to 170 mmHg.

RESULTS

Angiotensin II infusion increased systolic blood pressure from a baseline of 125 mmHg to 162 and 182 mmHg after 10 and 14 days, respectively. Candesartan cilexetil and triple therapy normalized the blood pressure to between 119 and 126 mmHg. Increasing perfusion pressure, from 65 to 170 mmHg, reduced afferent arteriolar diameter by 30% in sham-treated kidneys. Autoregulation was significantly blunted in angiotensin II-infused rats, resulting in a pressure-mediated vasoconstriction of only 10%. Candesartan cilexetil, or triple therapy, significantly improved autoregulatory behavior, as indicated by pressure-mediated vasoconstrictor responses of 30 and 40%; respectively, despite continued angiotensin II infusion.

CONCLUSIONS

These data suggest that chronic elevation of arterial blood pressure, rather than chronic AT1 receptor stimulation, is sufficient to induce hypertensive impairment of renal autoregulatory capability.

Effect of essential hypertension on kidney function as measured in rat by dynamic MRI

In this study, dynamic MRI was applied to investigate the effect of genetic hypertension on the renal clearance of Gd(DTPA). Assuming that the paramagnetic agent was essentially cleared through the kidney by glomerular filtration, a first-order kinetic model was used to estimate a rate constant k(cl) describing the transport from the renal cortex to the inner medulla. Experiments were carried out on 12-15-week-old spontaneously hypertensive rats (young-SHR) and age-matched Wistar-Kyoto rats (young-WKY). An additional group of 22-25-week-old SHR (old-SHR) was investigated to assess the long-term effect of chronic hypertension on kidney function. The glomerular filtration rate as measured by the inulin method correlated well with k(cl) results as measured by MRI. A clearance index 30% lower than young-WKY was observed in young-SHR, reflective of a kidney dysfunction. In old-SHR, k(cl) was only 50% of the young-WKY value. As computed from Gd(DTPA) concentration profiles, this translated into delayed time-to-peak (TTP) values, decreased peak Gd(DTPA) concentrations ([Gd](peak)), and a slow elimination of Gd(DTPA) from the blood pool, renal cortex, and inner medulla (e.g., high t(1/2) values). Finally, using the same MRI approach, the data showed that the acute administration of the calcium antagonist verapamil at an antihypertensive dose was followed by a near normalization of SHR renal function. These results indicate that chronic hypertension remains a major pathogenic factor in the progression of glomerular degeneration, as opposed to a primary glomerular defect independent of systemic pressure.

Effects of endothelin-A receptor antagonism on bilateral renal function in renovascular hypertensive rats

Recent studies indicated an enhanced expression of Endothelin (ET) in the kidney contralateral to the vascular clip in two-kidney, one-clip (2K-1C) Goldblatt hypertension. We proposed that the enhanced intrarenal ET production might be responsible for altered haemodynamic and excretory capability of the unclipped kidney (UK) of 2K-1C renovascular hypertensive rats. Therefore, we examined the changes in arterial pressure and split renal function in the clipped (CK) and UK simultaneously, in response to chronic administration of the selective ETA receptor blocker (A-127722), given orally at a dose of 30 mg/kg/day for 3 weeks starting from the beginning of the 4th week of clipping. Systolic pressure averaged 177 +/- 7 mmHg in control rats (n=15) and 164 +/- 9 mmHg in treated rats (n=16) and the difference was not statistically different. Glomerular filtration rate (GFR), renal plasma flow (RPF) and renal vascular resistance (RVR) in the UK were not different between control and treated groups. Data were then analyzed by classifying rats as moderate hypertensives (MAP < 180 mmHg), and severe hypertensives (MAP > 180 mmHg). In the moderately hypertensive group, average MAP was 143 +/- 5 mmHg and 138 +/- 4 mmHg in control (n=9) and treated (n=10) groups, respectively. In the severely hypertensive group, average MAP was 192 +/- 5 mmHg and 188 +/- 5 mmHg in control (n=6) and treated (n=6) groups, respectively. GFR and RPF were significantly improved in the UK of only the severely hypertensives who received the antagonist. However, the ET(A) antagonist blunted the sodium loss in both CK and UK of severely hypertensive rats. We conclude that ET(A) receptors do not play a role in the progression of hypertension in 2K-1C renovascular hypertensive rats. Yet, ET(A) receptors play an important role in altering renal hemodynamics of the unclipped kidneys in severe degrees of renovascular hypertension.

Split Renal Function Outcome after Renal Angioplasty in Patients with Unilateral Renal Artery Stenosis

Abstract. The general use of bilateral rather than separate renal function evaluation has led to the publication of conflicting results concerning the effect of percutaneous transluminal renal angioplasty (PTRA) on renal function, especially in patients with atherosclerotic renal artery stenosis. The aim of this study was to evaluate prospectively, in standardized conditions, split renal function (SRF) and GFR outcome after successful PTRA, by measuring single kidney GFR with synchronous inulin or 51Cr-ethylenediaminetetraacetic acid clearance and 99mTc-diethylenetriamine pentaacetic acid scintigraphy, in a well-defined population of patients with unilateral renal artery stenosis. Thirty-two consecutive hypertensive patients (18 with atherosclerotic and 14 with dysplastic disease) with significant unilateral stenosis of the main native renal artery (≥60%) and normal renal function were included in the study. Renal and angiographic follow-up evaluations were performed 6 mo after PTRA. PTRA alone or combined with stenting (n = 2) was technically successful in all patients. Repeat PTRA was necessary in two patients, evaluated 6 mo after the second PTRA. Six mo after PTRA, total GFR had increased slightly but significantly in the 29 patients with positive lateralization indices. SRF and single-kidney GFR of the stenotic kidney increased significantly, whereas concurrently the GFR and SRF of the nonstenotic kidney decreased significantly. Six mo after successful PTRA reducing renal ischemia, a reversal of both the hypoperfusion of the stenotic side and the hyperperfusion of the nonstenotic side was observed, which was accompanied by a slight increase in total GFR.

Combination of hypercholesterolemia and hypertension augments renal function abnormalities

Hypercholesterolemia and hypertension are both risk factors for end-stage renal disease. This study was designed to examine whether their coexistence augmented impairment in renal function and redox status. Regional renal hemodynamics and function in response to vasoactive challenges with acetylcholine or sodium nitroprusside were quantified by using electron-beam computed tomography in pigs after 12 weeks of either a normal (n=10) or hypercholesterolemic (n=10) diet, renovascular hypertension (n=7), or combined hypercholesterolemia+hypertension (n=6). The hypercholesterolemic and hypercholesterolemic+hypertensive groups had significantly increased serum cholesterol levels, whereas in the hypertensive and hypercholesterolemic+hypertensive groups, mean arterial pressure was significantly elevated compared with the group fed a normal diet. Basal regional renal perfusion and glomerular filtration rates were similar among the groups. In response to acetylcholine, cortical perfusion increased in normal animals (15.6+/-4.7%, P=0.002) but not in hypercholesterolemic or hypertensive animals (8.0+/-7.4% and 8.2+/-5.9%, respectively; P>0.05). Moreover, in the hypercholesterolemic+hypertensive group, cortical perfusion response was further attenuated (2.5+/-4.8%, P=0.02) and significantly different from the group fed a normal diet (P<0.05). The response to sodium nitroprusside followed a similar pattern, and the impairment was augmented in the hypercholesterolemic+hypertensive group. The functional abnormalities in hypercholesterolemia or hypertension were associated with a decrease in systemic and/or renal tissue levels of oxygen radical scavengers that was again accentuated in hypercholesterolemia+hypertension. These results demonstrate that concurrent hypercholesterolemia and hypertension have a greater detrimental effect on renal perfusion responses compared with hypercholesterolemia or hypertension alone, associated with a marked pro-oxidant shift in redox status. These effects may potentially augment renal functional impairment and play a role in the initiation and progression of renal injury in hypertension and atherosclerosis.

Impairment of pressure-natriuresis and renal autoregulation in ANG II-infused hypertensive rats

Chronic infusions of initially subpressor doses of angiotensin II (ANG II) lead to progressive hypertension over a 2-wk period and to augmented intrarenal ANG II levels. The present study was performed to investigate total renal blood flow (RBF) and medullary blood flow (MBF) autoregulatory behavior and pressure-natriuresis in ANG II-infused hypertensive rats and how these are modified by concomitant treatment with an ANG II AT(1) receptor antagonist. ANG II-infused rats (n = 27) were prepared by administration of ANG II at 60 ng/min via osmotic minipump for 13 days. Twelve of the ANG II-infused hypertensive rats were treated with losartan in the drinking water (30 mg. kg.(-1) day(-1)). Rats were anesthetized with pentobarbital sodium (50 mg/kg, ip) and prepared for renal function measurements. An aortic clamp was placed above the junction of the left renal artery to reduce renal arterial pressure. Autoregulatory responses for renal plasma flow, overall RBF, and glomerular filtration rate were impaired in ANG II-infused hypertensive rats; however, MBF autoregulation was not disrupted. Most strikingly, pressure-natriuresis was markedly suppressed in ANG II-infused hypertensive rats. Chronic treatment with losartan prevented the impairment of the pressure-natriuresis relationship caused by chronic ANG II infusion. These findings demonstrate that chronic ANG II infusion leads to marked impairment of sodium excretion and suppression of the pressure-natriuresis relationship, which may contribute to the progressive hypertension that occurs in this model. These renal effects are prevented by simultaneous treatment with an AT(1) receptor blocker.

Impaired renal blood flow autoregulation in two-kidney, one-clip hypertensive rats is caused by enhanced activity of nitric oxide

Increases in renal perfusion pressure will induce shear stress-mediated nitric oxide (NO) release, which could oppose autoregulation of renal blood flow (RBF). Although cardiac, cerebral, and mesenteric autoregulation is enhanced during nitric oxide (NO) synthesis inhibition, this has not been reported for renal autoregulation of blood flow. In the present study, the lower limit and efficiency of RBF autoregulation (as assessed by the degree of compensation) were studied before and during NO inhibition in normotensive Sprague Dawley rats (control; n = 9) and in the non-clipped kidney of two-kidney, one-clip Goldblatt hypertensive animals (2K1C; n = 9; 3 wk; 0.25-mm silver clip). In both groups, renal autoregulation curves were obtained before and during infusion of N(G) -nitro-L-arginine (L-NNA) (bolus 1.5 mg/kg intravenously, infusion 10 microg/kg per min intravenously), using a transit-time flow probe around the left renal artery. In control rats, mean arterial pressure (MAP) increased, RBF decreased, and renal vascular resistance (RVR) increased in response to L-NNA infusion. The lower limit of autoregulation in control animals did not significantly change during L-NNA infusion (78 +/- 3 to 70 +/- 2 mmHg). The degree of compensation in these rats slightly increased during L-NNA infusion, however, this was only significant below 90 mmHg. The 2K1C rats had elevated MAP under baseline conditions. L-NNA infusion resulted in a decrease in RBF and an increase in MAP and RVR. During L-NNA infusion, RVR in 2K1C rats greatly exceeded RVR in control rats. A significant decrease was observed in the lower limit of autoregulation from 85 +/- 3 to 72 +/- 5 mmHg (P < 0.05). In the contralateral kidney of 2K1C rats, the degree of compensation was lower than in control rats under baseline conditions. L-NNA infusion resulted in significantly higher degrees of compensation compared to baseline. In conclusion, the contralateral kidney displayed a high NO dependency, as RBF greatly decreased and RVR dramatically increased in response to L-NNA infusion. The contralateral kidney of 2K1C rats exhibited impaired RBF autoregulation, which was improved by NO inhibition, as judged from a decrease in the lower limit of autoregulation and an increase in the degree of compensation. This study indicates that perfusion pressure-dependent NO release can oppose autoregulation in the kidney. However, the enhanced influence of NO on pressure-dependent RBF may facilitate the preservation of renal function in the nonclipped kidney of 2K1C rats.

Nitric oxide modulates the development and surgical reversal of renovascular hypertension in rats

OBJECTIVE

To evaluate the role of nitric oxide (NO) in the development and unclipping-induced reversal of blood pressure and bilateral renal function in two-kidney, one clip (2K1C) Goldblatt hypertensive rats.

METHODS

Goldblatt hypertensive rats were prepared by clipping the left renal artery 4 weeks before unclipping experiments. NG-nitro-L-arginine methyl ester (L-NAME) was administered after clipping and during unclipping to inhibit nitric oxide (NO) synthesis. Blood pressure and bilateral renal responses were measured.

RESULTS

Chronic L-NAME treatment accelerated and aggravated blood pressure elevations and increased plasma nitrite and nitrate levels in 2K1C rats. Surgical removal of the renal artery clip induced profound reductions in blood pressure in rats with and without L-NAME treatment. However, the magnitude of the unclipping-induced depressor response at the first post-unclipping hour was significantly smaller in L-NAME-treated rats compared to those without L-NAME administration (15 +/- 1 versus 22 +/- 1%, P < 0.05). Two hours after unclipping, blood pressure of both groups fell to a comparable, normal level. Acute intravenous infusion of L-NAME in established 2K1C hypertensive rats further increased blood pressure. Subsequent unclipping caused a depressor response similar to that observed in hypertensive rats treated chronically with L-NAME. Despite the marked decreases in blood pressure, unclipping induced striking increases in glomerular filtration rate (GFR), urine flow and sodium and potassium excretion rates in the ipsilateral kidney. However, the magnitudes of increases in GFR and the diuretic and natriuretic responses in rats without L-NAME treatment were significantly greater than in rats with L-NAME administration. In contrast, unclipping reduced these function indices in the contralateral kidney to a similar level in rats with and without L-NAME treatment.

CONCLUSIONS

NO exerts vasodilator action and thereby lessens renal artery clipping-induced blood pressure elevation. Furthermore, unclipping-induced release of NO partially contributes to the early reduction in blood pressure and changes in bilateral renal function but does not directly mediate the normalization of blood pressure after unclipping in this hypertension model.

Nitric oxide-angiotensin II interactions in angiotensin II-dependent hypertension

Many studies indicate that renal haemodynamic function in angiotensin II- (ANG II) dependent hypertension is not reduced as much as would be predicted from the elevated ANG II levels suggesting that counteracting renoprotective mechanisms are activated. One important renoprotective effect is mediated by increased levels of nitric oxide. Recent studies using the ANG II-infused hypertensive rat model have shown that inhibition of nitric oxide synthesis (NOS) causes greater decreases in renal blood flow and glomerular filtration rate in ANG II-infused hypertensive rats than in control rats. This augmented nitric oxide-dependent influence is localized primarily in the cortex and to the preglomerular vasculature. The differential effects on the renal cortex and medulla are also reflected by the differences in NOS activities and protein expression. Ca2+-dependent NOS activity was significantly greater in the cortex but not the medulla of the ANG II-infused hypertensive rats compared with control rats. This was associated with marked activation of endothelial NOS protein levels and smaller increases in neuronal NOS protein levels in the cortex but not in the medulla. In contrast, the Ca2+-independent NOS activity and the inducible NOS protein levels in the cortex were significantly lower in the ANG II-infused hypertensive rats. These data support the hypothesis that cortical Ca2+-dependent NOS, primarily endothelial NOS, is stimulated during the early phases of ANG II-induced hypertension and exerts a renoprotective effect on cortical haemodynamics.

Effect of nitrendipine on renal cortical and papillary autoregulation in hypertensive rats

Renal cortical and papillary perfusions were assessed by laser-Doppler flowmetry and autoregulatory indices. There was cortical autoregulation (autoregulatory index of 0.31 +/- 0.08) in Wistar from 160 to 100 mm Hg, but not in the papilla, which was abolished by nitrendipine, 0.125 and 0.25 micrograms kg-1 min-1. In SHRSP the cortex, but not the papilla, exhibited autoregulation from 180 to 120 mm Hg (autoregulatory index of 0.27 +/- 0.10) but not during low and high doses of nitrendipine. The non-clipped kidney cortex of 2K1C Goldblatt rats autoregulated from 190 to 130 mm Hg (autoregulatory index of 0.20 +/- 0.13), but was inhibited only by the higher dose of nitrendipine. In mature Wistar and SHRSP cortical autoregulation is blocked effectively by nitrendipine whereas the 2K1C Goldblatt hypertensive rats are relatively resistant.

Nitric oxide modulates but does not impair myogenic vasoconstriction of the afferent arteriole in spontaneously hypertensive rats. Studies in the isolated perfused hydronephrotic kidney

Renal autoregulation curves are reset toward higher renal arterial pressure in spontaneously hypertensive rats (SHR) compared with those in Wistar-Kyoto rats (WKY). We previously demonstrated that myogenic afferent arteriolar constriction is shifted to higher renal arterial pressure. To investigate whether nitric oxide participates in the regulation of myogenic tone, we examined the effect of nitro-L-arginine on myogenic afferent arteriolar constriction in kidneys from SHR and WKY, using the isolated perfused hydronephrotic kidney. Elevating pressures from 40 to 80 mm Hg caused increases in afferent arteriolar diameter in WKY (from 18.2 +/- 0.4 to 19.0 +/- 0.3 micron) and SHR (from 17.3 +/- 0.6 to 18.4 +/- 0.6 micron). Further pressure elevation elicited constriction at 100 mm Hg in WKY (17.9 +/- 0.3 micron), but significant constriction was observed at 120 mm Hg in SHR (17.3 +/- 0.6 micron), indicating a resetting in myogenic responses to higher pressures. In WKY, after treatment with 10 mumol/L nitro-L-arginine, afferent arterioles exhibited pressure-dependent constriction, with a threshold pressure for constriction at 80 mm Hg. The addition of 100 mumol/L nitro-L-arginine had no further effect on myogenic responsiveness in WKY. In contrast, in SHR, nitro-L-arginine dose-dependently shifted the myogenic responses toward lower renal arterial pressure, with threshold pressures for constriction observed at 100 mm Hg (10 mumol/L) and 80 mm Hg (100 mumol/L).(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin and angiotensin converting enzyme tissue levels in two-kidney, one clip hypertensive rats

Renal tissue angiotensin I (Ang I) and II (Ang II) content and angiotensin converting enzyme activity were assessed in both kidneys during initial (7 days) and maintenance (25 days) phases of two-kidney, one clip hypertension in rats. At 7 and 25 days, systolic arterial pressure was 146 +/- 2 and 170 +/- 7 mm Hg, respectively. After 7 days, Ang I content of clipped kidneys was 64% and 70% higher (p < 0.001) than in nonclipped and sham-operated kidneys, respectively, when compared with levels in kidneys from sham-operated rats. In kidneys harvested 25 days after clipping one renal artery, Ang I and Ang II contents in clipped kidneys were increased 102% and 24% (p < 0.01), respectively. Ang II content was also 32% higher in nonclipped kidneys. Angiotensin converting enzyme activity in nonclipped kidneys was greater (p < 0.05) than that in either clipped (46% higher) or sham-operated kidneys (57% higher). Plasma Ang I and Ang II levels were elevated at 7 days but were not different at 25 days in clipped rats. These results demonstrate a dissociation between intrarenal and circulating levels of Ang I and Ang II and suggest that qualitatively different mechanisms may be responsible for the elevated intrarenal Ang II levels during the initial and maintenance phases of renal hypertension.

Adverse effect of the calcium channel blocker nitrendipine on nephrosclerosis in rats with renovascular hypertension

The effect of a 6-week treatment with the calcium channel blocker nitrendipine or the angiotensin converting enzyme inhibitor enalapril on blood pressure, albuminuria, renal hemodynamics, and morphology of the nonclipped kidney was studied in rats with two-kidney, one clip renovascular hypertension. Six weeks after clipping of one renal artery, hypertensive rats (178 +/- 4 mm Hg) were randomly assigned to three groups: untreated hypertensive controls (n = 8), enalapril-treated (n = 8), or nitrendipine-treated (n = 10). Sham-operated rats served as normotensive controls (128 +/- 3 mm Hg, n = 8). After 6 weeks of treatment, renal hemodynamics (glomerular filtration rate and renal plasma flow) were measured in the anesthetized rats. Renal tissue was obtained for determination of glomerular size and sclerosis. Enalapril but not nitrendipine reduced blood pressure significantly. After 6 weeks of therapy, glomerular filtration rate was not different among the studied groups. Renal plasma flow increased, but albumin excretion and glomerulosclerosis did not change after enalapril treatment. In contrast, in the nitrendipine-treated group albuminuria increased from 12.8 +/- 2 progressively to 163 +/- 55 compared with 19.2 +/- 9 mg/24 hr in the hypertensive controls. Furthermore, glomerulosclerosis index was significantly increased in the nitrendipine-treated group compared with the hypertensive controls (0.38 +/- 0.1 versus 0.13 +/- 0.04). In addition, glomerular size was higher in the nitrendipine-treated group (14.9 +/- 0.17 10(-3) mm2) but lower in the enalapril-treated group (11.5 +/- 0.15 10(-3) mm2) compared with the hypertensive controls (12.1 +/- 0.17 10(-3) mm2).(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired myogenic responsiveness of renal microvessels in Dahl salt-sensitive rats

The mechanisms mediating abnormal renal autoregulation in Dahl salt-sensitive (DS) rats have not been fully defined. In the present study, we assessed myogenic responsiveness of interlobular arteries (ILAs), afferent arterioles (AAs), and efferent arterioles in isolated perfused hydronephrotic Dahl rat kidneys. Dahl rats were divided into four groups according to strain (Dahl salt-resistant [DR] or DS rats) and dietary sodium manipulation (rats fed low or high salt diets). Systolic blood pressure was elevated only in DS rats fed the high salt diet (202 +/- 4 mm Hg, p less than 0.05). Myogenic responses were obtained by stepwise elevation of renal arterial pressure. Vessel diameters were determined by computer-assisted videomicroscopy. Preglomerular microvessels of DS and DR rats responded differently to changes in renal arterial pressure. AAs and ILAs manifested diminished myogenic responsiveness to increasing renal arterial pressure in DS rats compared with DR rats (p less than 0.05). Both AAs and ILAs in DS rats manifested a higher threshold pressure for eliciting myogenic responses and a decrease in maximal pressure-induced vasoconstriction. The sensitivity of the AA myogenic response to nifedipine was enhanced in DS rats compared with DR rats (p less than 0.05). For rats fed the high salt diet, preglomerular vessels exhibited reduced myogenic responsiveness in both strains. In contrast to preglomerular microvessels, efferent arterioles from all four groups of rats failed to exhibit pressure-induced vasoconstriction. Our data suggest that diminished myogenic responsiveness of AAs and ILAs in DS rats contributes to impaired renal autoregulation in this strain.

Pathophysiology of renovascular hypertension

Renovascular hypertension has its experimental counterpart in the two-kidney, one clip model (Goldblatt hypertension). From the study of this model, a general pathophysiological scheme has evolved suggesting that temporal stages in the development and maintenance of hypertension are regulated by complicated hormonal and neural interrelations. The central roles played by the renin-angiotensin system and the renal nerves is discussed as they relate to other hormones. In addition, the possible contribution of converting enzyme inhibitors to understanding the pathophysiology of this condition is discussed.

Afferent arteriolar responsiveness to altered perfusion pressure in renal hypertension

The present study was performed to determine the role of afferent arterioles in the impaired autoregulatory response shown to occur in the contralateral kidney of Goldblatt hypertensive rats. The responsiveness of juxtamedullary afferent arterioles to alterations in perfusion pressure was studied in the nonclipped kidney of two-kidney, one clip hypertensive and sham-operated rats. Systolic pressure, 5-6 weeks after clipping, averaged 184 +/- 6 mm Hg in the hypertensive rats (n = 16) and 121 +/- 3 mm Hg in the sham-operated control rats (n = 7). By using the in vitro blood-perfused juxtamedullary nephron technique, afferent arterioles were directly visualized, and their inside diameters were measured by videomicroscopic methods. In sham-operated kidneys perfused with blood from normotensive rats, afferent arteriolar diameter averaged 22.8 +/- 1.8 microns at a renal arterial perfusion pressure of 151 +/- 1 mm Hg and increased to 24.8 +/- 1.8 microns when perfusion pressure was reduced to 110 +/- 2 mm Hg. Conversely, in hypertensive kidneys perfused with blood from either hypertensive or normotensive rats, the afferent arterioles failed to vasodilate and actually exhibited a slight decrease in diameter from 24.6 +/- 1.3 to 23.0 +/- 2.3 microns in response to the same reduction in perfusion pressure. Vasodilator capability, however, could be demonstrated in response to verapamil and sodium nitroprusside, which increased afferent diameter in both the sham-operated and hypertensive groups of rats. Thus, unlike arterioles from normotensive rats, juxtamedullary afferent arterioles from two-kidney, one clip Goldblatt hypertensive rats fail to vasodilate after a reduction in perfusion pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

[Changes in renal function induced by anesthesia]

The rate of urine formation and its composition are influenced by the different drugs used during surgery. Anaesthetics act on renal function, not only directly, but also by producing changes in cardiovascular function and in neuroendocrine activity. Many factors may be incriminated: lowered blood pressure and cardiac output, increased sympathetic outflow (renal nerve stimulation and increased plasma catecholamines), increased release of renin, angiotensin and vasopressin. The effects of anaesthetics on the kidney go beyond a simple change in basal haemodynamics and include, for some drugs, an alteration in the ability for the kidney to autoregulate its blood flow and glomerular filtration rate. Studies on toad bladders showed a decrease in transport of water, sodium and organic anions. But, in fact, renal effects of anaesthetics in man and animals depend on the species, the anaesthetic and the method used to study the effect. Most barbiturates and inhalational anaesthetics tend to decrease renal blood flow (RBF) and glomerular filtration rate (GFR). These trends are gradually reversed during recovery. The effects of ketamine and diazepam are not clearly defined. Morphine and fentanyl decrease urine flow and GFR, whilst RBF increases or decreases, depending on whether a direct or indirect measurement technique was used. Muscle relaxants have little effect on renal function. Spinal and epidural anaesthesia only slightly decrease GFR and RBF in proportion to the decrease in mean arterial pressure. Obviously, the preexisting intravascular volume and the quantity of intravenous fluids given strongly influence the renal response to spinal and epidural anaesthesia. Some studies have shown that urine flow rate, creatinine clearance, urinary sodium excretion and RBF are reduced during mechanical ventilation with positive end-expiratory pressure. Surgery itself influences renal function by inducing alterations in prerenal haemodynamics. Operative stress leads to an increase in circulating catecholamines and angiotensin. Significant fluid shifts, excessive blood loss and redistribution of a third space may lead to a prerenal oliguric state, increasing secretion of vasopressin. Acute renal failure (ARF) is a frequently lethal complication of critical surgical illness, due to a variety of factors which interfere with glomerular filtration and tubular reabsorption, such as renal hypoperfusion or nephrotoxic insults. In fact, the initiating aggression ultimately culminates in the development of one or more of the maintenance factors (decreased tubular function, tubular obstruction, decreased GFR and RBF) that reduce urine flow and osmolar excretion. Good management during the perioperative period tends to minimize the risk of developing ARF.

Angiotensin influences on tubuloglomerular feedback mechanism in hypertensive rats

The tubuloglomerular feedback (TGF) mechanism was evaluated in the nonclipped kidney of Goldblatt hypertensive rats from both stop flow pressure (SFP) and single nephron glomerular filtration rate (SNGFR) responses to step increases in late proximal perfusion rate from 0 to 40 nl/min. During control conditions, increases in late proximal perfusion rate produced flow dependent decreases in SFP and SNGFR with maximal values of 10.2 +/- 1.0 mm Hg and 12.9 +/- 2.5 nl/min, values similar to those obtained in normal rats. During ACE inhibition (MK 422; 0.6 mg/kg/hr), arterial pressure decreased from 168 +/- 8 to 137 +/- 7 mm Hg and there was a marked attenuation in the magnitude of SFP feedback responses (delta = 2.5 +/- 0.3 mm Hg). SNGFR feedback responses, however, were not significantly impaired. Direct decreases in renal arterial pressure reduced control SFP but SFP feedback responses were maintained, indicating that the attenuated SFP feedback responses during ACE inhibition were not due to decreased arterial pressure. Superimposed infusion of angiotensin II during ACE inhibition partially restored SFP feedback responses. In contrast, norepinephrine infusion did not result in a similar restoration of SFP feedback sensitivity. These results indicate that the nonclipped kidney of Goldblatt hypertensive rats has an intact TGF mechanism as assessed from SFP and SNGFR feedback responses. Furthermore, ACE inhibition attenuates SFP but not SNGFR feedback responses, and systemic angiotensin II infusions can restore SFP feedback responsiveness towards normal.

Glomerular and vascular atrial natriuretic factor receptors in saralasin-sensitive and -resistant two-kidney, one-clip hypertensive rats

We have investigated whether there is a relation between renin dependency of two-kidney, one-clip (2K1C) hypertensive rats and the density of renal glomerular and vascular atrial natriuretic factor (ANF) receptors. Conscious 2K1C rats with blood pressure of 150 mm Hg or higher were classified according to their sensitivity to the blood pressure-lowering effect of the angiotensin II antagonist saralasin. Both hypertension groups had lower body weights and greater relative heart weights than normotensive controls. Hematocrit was lower and plasma volume higher in saralasin-resistant animals than in either saralasin-sensitive or control rats. Plasma renin activity was higher in the saralasin-sensitive group than in the resistant rats. Plasma ANF concentration was greater in saralasin-resistant than in either normotensive or saralasin-sensitive animals. ANF was reduced in both atria of saralasin-resistant 2K1C animals but only in the left atrium of the sensitive group. Both hypertensive groups showed an increased ventricular ANF concentration. The number of glomerular ANF binding sites was significantly lower in the clipped kidney of both hypertensive groups. This lower density of binding sites was accompanied by an increased affinity. In saralasin-sensitive rats, the density of glomerular ANF receptors in the nonclipped kidney was significantly higher than in the controls. Saralasin-resistant rats exhibited a decreased number of vascular ANF binding sites in both mesenteric arteries and aorta. We conclude that through modulation of its glomerular and vascular receptors, ANF may contribute to the differential sodium handling of saralasin-sensitive and -resistant 2K1C hypertensive rats and to the reduced vascular responsiveness to ANF observed in the saralasin-resistant hypertensive rats.

Relation of glomerular injury to preglomerular resistance in experimental hypertension

A semiquantitative glomerular damage index (GDI) was determined for overall (O), superficial (S), and juxtamedullary (JM) glomeruli in four models of experimental hypertension in the rat to assess the severity and distribution of injury in light of present day knowledge of glomerular hemodynamics. After a four week period of similar hypertension, comparison of Group 1 (renal ablation) with Group 2 (aortic ligature) revealed OGDIs of 0.420 +/- 0.064 (SEM) vs. 0.062 +/- 0.019, P less than 0.0001, SGDIs of 0.250 +/- 0.071 vs. 0.035 +/- 0.007, P less than 0.0089, and JGDIs of 0.455 +/- 0.071 vs. 0.155 +/- 0.036, P less than 0.002. Within Group 1 the SGDI and JMGDI were not significantly different but within Group 2 the SGDI was less (P less than 0.005) than the JMGDI. Arterial/arteriolar damage was comparable in both groups. After an eight week period of similar hypertension, comparison of Group 3 (deoxycorticosterone-saline) with Group 4 (stroke-prone spontaneously hypertensive rats) showed OGDIs of 0.301 +/- 0.065 vs. 0.128 +/- 0.023, P less than 0.025, SGDIs of 0.289 +/- 0.096 vs. 0.072 +/- 0.015, P less than 0.044, and JMGDIs of 0.394 +/- 0.083 vs. 0.307 +/- 0.062, NS. Within Group 3 the SGDI and JMGDI were not significantly different, but within Group 4 the SGDI was less (P less than 0.002) than the JMGDI. Vascular damage in the two groups was comparable. Taking into account known physiologic data, the findings are consistent with the idea that increased preglomerular resistance is protective of glomeruli, whereas decreased resistance with increased pressure and/or flow is injurious.(ABSTRACT TRUNCATED AT 250 WORDS)

Tubuloglomerular feedback and glomerular morphology in Goldblatt hypertensive rats on varying protein diets

The present experiments were performed to examine the effect of variation in protein intake on renal function and morphology in the non-clipped kidneys of Goldblatt hypertensive rats. After renal artery clipping, rats were put on diets containing 5 (LP), 17.5 (NP), or 51% (HP) protein. After 4 to 5 weeks, all rats had developed hypertension. GFR was directly correlated with protein intake (1.47 +/- 0.15 in HP, 1.19 +/- 0.14 in NP, and 0.93 +/- 0.08 ml/min in LP), as was SNGFR (44.2 +/- 1.89, 39.1 +/- 2.23, and 27.9 +/- 0.86 nl/min in HP, NP, and LP rats). The response of SNGFR to changes in loop of Henle flow rate was attenuated in NP and HP rats: the maximum decrease was reduced (30.0 +/- 5.2% in NP, 22.1 +/- 4.2% in HP) and higher tubular flow rates were required to elicit responses (V1/2, the flow rate at which the response is half-maximum, was 28.9 +/- 2.6 nl/min in NP and 28.2 +/- 1.4 nl/min in HP). In LP rats, the maximum response was a decrease of 47.7 +/- 2.5%, and V1/2 was 18.1 +/- 1.2 nl/min, values similar to those found in normal control rats. The weights of the non-clipped kidneys were 0.96 +/- 0.04 g (LP), 1.06 +/- 0.05 g (NP), and 1.36 +/- 0.06 g (HP). In the LP rats there was no difference between the non-clipped and clipped kidneys. Light microscopic evaluation showed a high incidence of focal glomerulosclerosis in non-clipped kidneys of HP rats, but no glomerular lesions in the non-clipped kidneys of LP rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Nephron responses to converting enzyme inhibition in non-clipped kidney of Goldblatt hypertensive rat at normotensive pressures

To evaluate the effects of angiotensin converting enzyme inhibition (SQ 20881, CEI) on superficial nephron function of the non-clipped kidney in Goldblatt hypertensive rats in the absence of alterations in renal arterial pressure, control renal arterial pressure (RAP) was reduced first to the range generally obtained during CEI (124 +/- 4 mm Hg). RAP was maintained during the CEI period by adjustment of a suprarenal aortic clamp. At the reduced RAP, whole kidney and single nephron glomerular filtration rates (GFR) were reduced from the hypertensive levels and were lower than the measurements in normotensive control rats. During CEI, whole kidney GFR and single nephron GFR increased by 55 and 42%, respectively. There were decreases in absolute as well as fractional proximal reabsorption rates. In the intermediate nephron segment, fractional reabsorption was decreased, but absolute fluid reabsorption increased in proportion to the increased delivery rate. Proximal tubule and peritubular capillary hydrostatic pressures increased significantly during CEI also. These results indicate that an increased activity of the renin-angiotensin system occurring in Goldblatt hypertensive rats subjected to aortic constriction exerts effects to lower GFR and increase proximal reabsorption rate. The concomitant superficial nephron and whole kidney GFR responses to CEI when arterial pressure was maintained suggests that the pre-existing levels of angiotensin exerted similar influences on the total nephron population.

Pressure dependence of exaggerated natriuresis in two-kidney, one clip Goldblatt hypertensive rats

This study evaluated the responses of each kidney of two-kidney, one clip Goldblatt hypertensive rats to acute volume loading to delineate the contribution of elevated perfusion pressure to the mechanisms of the exaggerated natriuresis in this model of hypertension. Eleven Goldblatt animals (0.2 mm clip 3 weeks prior to study) were studied at spontaneous blood pressure for each kidney's response to volume loading. In 11 other Goldblatt animals an aortic clamp between the renal arteries allowed reduction of perfusion pressure for the left, nonclipped kidney to normal levels. Ten normal rats served as controls. Renal function was examined during control periods and following the infusion of 3.5% body wt of 154 mM X liter-1 NaCl at 22.5 ml X hr-1. An exaggerated natriuresis was observed for the left, nonclipped kidney of the hypertensive Goldblatt rats, while the clipped kidney exhibited an attenuated natriuresis compared to either kidney of normal control rats. Reduction of perfusion pressure to the nonclipped kidney of Goldblatt animals to normal levels resulted in reduced clearance and excretory function before volume loading and attenuated its natriuretic response to levels less than those for the Goldblatt group at spontaneous, hypertensive blood pressure. The exaggerated natriuresis observed at hypertensive blood pressure was attributable to increases of filtered load of Na+ and reduced fractional absorption. These observations indicate that an exaggerated natriuresis occurs in the two-kidney, one clip Goldblatt hypertensive rat and that this phenomenon depends on the elevated renal perfusion pressure to the nonclipped kidney.

Pathophysiology of experimental renovascular hypertension

The genesis of renovascular hypertension follows a continuum from an acute to a chronic phase. Reduction in renal perfusion initiates renin release and angiotensin-mediated systemic vasoconstriction. Aldosterone secretion, sodium and water retention, and expansion of the extracellular volume ensue. Sustained hypertension is further maintained by interacting physiologic mechanisms including increased angiotensin II sensitivity, vasopressin, ouabain-like substance, the sympathetic nervous system, CNS mechanisms, autoregulation, and structural changes.

Relative contribution of vasopressin and angiotensin II to the altered renal microcirculatory dynamics in two-kidney Goldblatt hypertension

The renal microcirculation was assessed in non-clipped kidneys of 23 Munich-Wistar rats with two-kidney one-clip Goldblatt hypertension. Four weeks after placement of a renal arterial clip, mean systemic arterial pressure averaged 163 +/- 5 mm Hg in hypertensive rats as compared to 108 +/- 2 in sham-operated controls (n = 6 rats). Non-clipped kidneys in hypertensive rats were characterized by higher glomerular capillary hydraulic pressures, single nephron glomerular filtration rate, and afferent arteriolar resistance. The glomerular capillary ultrafiltration coefficient was significantly reduced in hypertensive rats. In 10 of these rats, intravenous infusion of the angiotensin antagonist, saralasin, or the converting enzyme inhibitor, SQ20881, led to significant reductions in systemic arterial pressure and in afferent and efferent arteriolar resistance, on average by 8 +/- 3%, 15 +/- 4%, 28 +/- 5%, respectively. These changes were associated with significant increase in glomerular plasma flow, while ultrafiltration coefficient remained unaffected. In the presence of saralasin or SQ20881, infusion of a specific antagonist of the vascular action of arginine vasopressin led to significant systemic but not renal vasodilation. Thus, whereas systemic arterial pressure fell further, on average by 23 +/- 2%, renal arteriolar resistance remained constant, resulting in marked reduction in glomerular capillary hydraulic pressures (by 18 +/- 2%) and glomerular plasma flow rate (by 28 +/- 10%). Because of these pronounced reductions in glomerular pressures and flows induced by vasopressin antagonist, single nephron glomerular filtration rate fell markedly in hypertensive rats (by 34 +/- 6%) despite normalization of ultrafiltration coefficient. When hypertensive rats (n = 7) were treated with vasopressin antagonist alone, a modest fall in systemic arterial pressure was again observed in the absence of changes in renal arteriolar resistance. Due to this selective extrarenal vasodilatory action of vasopressin antagonist, glomerular capillary hydraulic pressure, plasma flow rate, and single nephron glomerular filtration rate again fell markedly. When these vasopressin antagonist pre-treated hypertensive rats were given saralasin or SQ20881, marked reductions in renal arteriolar resistance were observed in association with a significant increase in glomerular plasma flow rate. These observations made during acute inhibition of angiotensin II and vasopressin indicate that both of these vasopressin hormones may play important roles in maintaining systemic hypertension in hypertensive rat. By virtue of its preferential constrictor effects on extrarenal rather than renal vasculature vasopressin serves to maintain high glomerular pressures and flows in the non-clipped kidney of Goldblatt hypertensive rats.

Glomerular hemodynamics in persistent renovascular hypertension in the rat

We studied the glomerular hemodynamics and activity of the tubuloglomerular feedback system (TGFS) in Wistar rats with persistent hypertension 60 days after removal of the clipped kidney in the Goldblatt (two-kidney, one clip) hypertension model. Ten hypertensive rats (HBP) were compared with 12 normotensive ones (NBP). Micropuncture studies revealed that values for the single nephron glomerular filtration rate (SNGFR), glomerular plasma flow (QA), and afferent oncotic pressure (PAR.A) were similar in both groups, whereas glomerular capillary pressure (PGC) and effective filtration pressure (EFP) were higher in the HBP group (p less than 0.05). A slight but insignificant increase in afferent resistance was present in the HBP group. A positive correlation was found between mean arterial pressure and stop flow pressure (SFP) (r = 0.64, p less than 0.05) but not with SNGFR, suggesting a reduction in the ultrafiltration coefficient in hypertensive rats. This was further supported by studies of the activity of the TGFS, which demonstrated that interrupting flow to the macula densa was followed by a smaller increment in SNGFR in HBP, in spite of a similar rise in SFP. The mechanism responsible for decreasing glomerular permeability is unknown but could be related to structural changes in glomerular capillary or to an increase in intrarenal angiotensin II, as has been demonstrated previously in this model. It is suggested that these adaptations occurring in the kidney exposed to hypertension can contribute to the maintenance of elevated arterial pressure after removing the stenotic kidney.