Aaron J. Polichnowski

Renal microvascular dysfunction, hypertension and CKD progression.

Purpose of reviewDespite apparent blood pressure (BP) control and renin–angiotensin system (RAS) blockade, the chronic kidney disease (CKD) outcomes have been suboptimal. Accordingly, this review is addressed to renal microvascular and autoregulatory impairments that underlie the enhanced dynamic glomerular BP transmission in CKD progression. Recent findingsClinical data suggest that failure to achieve adequate 24-h BP control is likely contributing to the suboptimal outcomes in CKD. Whereas evidence continues to accumulate regarding the importance of preglomerular autoregulatory impairment to the dynamic glomerular BP transmission, emerging data indicate that nitric oxide-mediated efferent vasodilation may play an important role in mitigating the consequences of glomerular hypertension. By contrast, the vasoconstrictor effects of angiotensin II are expected to potentially reduce glomerular barotrauma and possibly enhance ischemic injury. When adequate BP measurement methods are used, the evidence for BP-independent injury initiating mechanisms is considerably weaker and the renoprotection by RAS blockade largely parallels its antihypertensive effectiveness. SummaryAdequate 24-h BP control presently offers the most feasible intervention for reducing glomerular BP transmission and improving suboptimal outcomes in CKD. Investigations addressed to improving myogenic autoregulation and/or enhancing nitric oxide-mediated efferent dilation in addition to the more downstream mediators may provide additional future therapeutic targets.

High Perfusion Pressure Accelerates Renal Injury in Salt-Sensitive Hypertension

Renal injury in the Dahl salt-sensitive rat mimics human salt-sensitive forms of hypertension that are particularly prevalent in black individuals, but the mechanisms that lead to the development of this injury are incompletely understood. We studied the impact of renal perfusion pressure (RPP) on the development of renal injury in this model. During the development of salt-induced hypertension over 2 wk, the RPP to the left kidney was maintained at control levels (125 +/- 2 mmHg) by continuous servocontrol inflation of an aortic balloon implanted between the renal arteries; during the same period, the RPP to the right kidney rose to 164 +/- 8 mmHg. After 2 wk of a 4% salt diet, DNA microarray and real-time PCR identified genes related to fibrosis and epithelial-to-mesenchymal transition in the kidneys exposed to hypertension. The increased RPP to the right kidney accounted for differences in renal injury between the two kidneys, measured by percentage of injured cortical and juxtamedullary glomeruli, quantified proteinaceous casts, number of ED-1-positive cells per glomerular tuft area, and interstitial fibrosis. Interlobular arteriolar injury was not increased in the kidney exposed to elevated pressure but was reduced in the control kidney. We conclude that elevations of RPP contribute significantly to the fibrosis and epithelial-to-mesenchymal transition found in the early phases of hypertension in the salt-sensitive rat.

Effects of Renal Perfusion Pressure on Renal Medullary Hydrogen Peroxide and Nitric Oxide Production

Studies were designed to determine the effects of increases of renal perfusion pressure on the production of hydrogen peroxide (H2O2) and NO2−+NO3− within the renal outer medulla. Sprague-Dawley rats were studied with either the renal capsule intact or removed to ascertain the contribution of changes of medullary blood flow and renal interstitial hydrostatic pressure on H2O2 and NO2−+NO3− production. Responses to three 30-minute step changes of renal perfusion pressure (from ≈85 to ≈115 to ≈145 mm Hg) were studied using adjustable aortic occluders proximal and distal to the left renal artery. Medullary interstitial H2O2 determined by microdialysis increased at each level of renal perfusion pressure from 640 to 874 to 1593 nmol/L, as did H2O2 urinary excretion rates, and these responses were significantly attenuated by decapsulation. Medullary interstitial NO2−+NO3− increased from 9.2 to 13.8 to 16.1 &mgr;mol/L, with parallel changes in urine NO2−+NO3−, but decapsulation did not significantly blunt these responses. Over the range of renal perfusion pressure, medullary blood flow (laser-Doppler flowmetry) rose ≈30% and renal interstitial hydrostatic pressure rose from 7.8 to 19.7 cm H2O. Renal interstitial hydrostatic pressure and the natriuretic and diuretic responses were significantly attenuated with decapsulation, but medullary blood flow was not affected. The data indicate that pressure-induced increases of H2O2 emanated largely from increased tubular flow rates to the medullary thick-ascending limbs of Henle and NO largely from increased medullary blood flow to the vasa recta. The parallel pressure–induced increases of H2O2 and NO indicate a participation in shaping the “normal” pressure-natriuresis relationship and explain why an imbalance in either would affect the blood pressure salt sensitivity.

The Effect of High-Load vs High-Repetition Training on Endurance Performance

&NA; Ebben, W.P., A.G. Kindler, K.A. Chirdon, N.C. Jenkins, A.J. Polichnowski, and A.V. Ng. The effect of high‐load vs. high‐repetition training on endurance performance. J. Strength Cond. Res. 18(3):513–517. 2004.—The purpose of this study was to compare the effects of high‐load (H‐load) periodized resistance training and high‐repetition (H‐rep) reverse step loading periodized resistance training on endurance performance. Twenty‐six female university rowers (age = 20 ± 1 year) were randomly assigned to H‐load (5 novice, 8 varsity) or H‐rep (7 novice, 6 varsity) groups. Subjects were pre‐ and posttested using a 2,000‐m rowing ergometer test. Outcome variables included SymbolO2 peak, time to test completion, total power, average power per stroke, total number of strokes, stroke rate, and body mass. Subjects trained for 8 weeks using identical exercises. Varsity rowers who performed H‐load training demonstrated greater improvement compared with those who performed H‐rep training. Novice rowers who performed H‐rep training demonstrated greater improvement compared with those who performed H‐load training. High‐load periodized training appears to be more effective for athletes with advanced training status, and H‐rep reverse step loading periodized training is more effective for those who are relatively untrained. Symbol. No caption available.

Severe Renal Mass Reduction Impairs Recovery and Promotes Fibrosis after AKI

Preexisting CKD may affect the severity of and/or recovery from AKI. We assessed the impact of prior graded normotensive renal mass reduction on ischemia-reperfusion-induced AKI. Rats underwent 40 minutes of ischemia 2 weeks after right uninephrectomy and surgical excision of both poles of the left kidney (75% reduction of renal mass), right uninephrectomy (50% reduction of renal mass), or sham reduction of renal mass. The severity of AKI was comparable among groups, which was reflected by similarly increased serum creatinine (SCr; approximately 4.5 mg/dl) at 2 days, tubule necrosis at 3 days, and vimentin-expressing regenerating tubules at 7 days postischemia-reperfusion. However, SCr remained elevated compared with preischemia-reperfusion values, and more tubules failed to differentiate during late recovery 4 weeks after ischemia-reperfusion in rats with 75% renal mass reduction relative to other groups. Tubules that failed to differentiate continued to produce vimentin, exhibited vicarious proliferative signaling, and expressed less vascular endothelial growth factor but more profibrotic peptides. The disproportionate failure of regenerating tubules to redifferentiate in rats with 75% renal mass reduction associated with more severe capillary rarefaction and greater tubulointerstitial fibrosis. Furthermore, initially normotensive rats with 75% renal mass reduction developed hypertension and proteinuria, 2-4 weeks postischemia-reperfusion. In summary, severe (>50%) renal mass reduction disproportionately compromised tubule repair, diminished capillary density, and promoted fibrosis with hypertension after ischemia-reperfusion-induced AKI in rats, suggesting that accelerated declines of renal function may occur after AKI in patients with preexisting CKD.

Pressure-Induced Renal Injury in Angiotensin II Versus Norepinephrine-Induced Hypertensive Rats

The susceptibility to renal perfusion pressure (RPP)–induced renal injury was investigated in angiotensin II (Ang II)– versus norepinephrine (NE)-infused hypertensive rats. To determine the magnitude of RPP-induced injury, Sprague-Dawley rats fed a 4% salt diet were instrumented with a servocontrolled aortic balloon occluder positioned between the renal arteries to maintain RPP to the left kidney at baseline levels whereas the right kidney was exposed to elevated RPP during a 2-week infusion of Ang II IV (25 ng/kg per minute), NE IV (0.5, 1.0, and 2.0 &mgr;g/kg per minute on days 1, 2, and 3 to 14, respectively), or saline IV (sham rats). Over the 14 days of Ang II infusion, RPP averaged 161.5±8.0 mm Hg to uncontrolled kidneys and 121.9±2.0 mm Hg to servocontrolled kidneys. In NE-infused rats, RPP averaged 156.3±3.0 mm Hg to uncontrolled kidneys and 116.9±2.0 mm Hg to servocontrolled kidneys. RPP averaged 111.1±1.0 mm Hg to kidneys of sham rats. Interlobular arterial injury and juxtamedullary glomerulosclerosis were largely RPP dependent in both models of hypertension. Superficial cortical glomerulosclerosis was greater and RPP dependent in NE- versus Ang II-infused rats, which was primarily independent of RPP. Outer medullary tubular necrosis and interstitial fibrosis were also primarily RPP dependent in both models of hypertension; however, the magnitude of injury was exacerbated in Ang II-infused rats. We conclude that elevated RPP is the dominant cause of renal injury in both NE- and Ang II-induced hypertensive rats and that underlying neurohumoral factors in these models of hypertension alter the pattern and magnitude of RPP-induced renal injury.

Role of Renal Perfusion Pressure Versus Angiotensin II on Renal Oxidative Stress in Angiotensin II–Induced Hypertensive Rats

Renal oxidative stress is thought to contribute to both the etiology and the associated renal injury in angiotensin (Ang) II–dependent hypertension. The contribution of Ang II versus elevated renal perfusion pressure (RPP) on albuminuria and renal oxidative stress in this model of hypertension was explored in the present study by chronically servocontrolling RPP to the left kidney and comparing responses with the right uncontrolled kidney and the left kidney of sham rats. Hypertension was produced in Sprague-Dawley rats fed a 4% NaCl diet by chronic IV infusion of Ang II (25 ng/kg per minute). The RPP to the left kidney was servocontrolled to mean daily pressures averaging ≈120 mm Hg, whereas the uncontrolled kidneys averaged ≈170 mm Hg over 14 days of Ang II infusion. Ang II infusion resulted in a 2.4-fold increase in albuminuria, which was RPP dependent. Kidneys exposed to both elevated RPP and Ang II (uncontrolled kidneys) displayed a 3.5-fold increase in malondialdehyde excretion and a 37% and 27% increase in renal cortical and outer medullary superoxide production, respectively. Elevated RPP significantly contributed to global renal oxidative stress (70% increase in malondialdehyde excretion) and outer medullary superoxide production. Elevated circulating levels of Ang II, per se, were responsible for a 1.5-fold and 2.0-fold increase in renal cortical and outer medullary NADPH oxidase activity, respectively. In summary, this study demonstrates that elevated RPP is directly responsible for the excess albuminuria in Ang II–infused rats, whereas both elevated RPP and Ang II directly contribute to the observed renal oxidative stress.

Large BP-dependent and -independent differences in susceptibility to nephropathy after nitric oxide inhibition in Sprague-Dawley rats from two major suppliers

The N(ω)-nitro-l-arginine methyl ester (l-NAME) model is widely employed to investigate the role of nitric oxide (NO) in renal injury. The present studies show that Sprague-Dawley rats from Harlan (H) and Charles River (CR) exhibit strikingly large differences in susceptibility to l-NAME nephropathy. After 4 wk of l-NAME (∼50 mg·kg(-1)·day(-1) in drinking water), H rats (n = 13) exhibited the expected hypertension [average radiotelemetric systolic blood pressure (BP), 180 ± 3 mmHg], proteinuria (136 ± 17 mg/24 h), and glomerular injury (GI) (12 ± 2%). By contrast, CR rats developed less hypertension (142 ± 4), but surprisingly no proteinuria or GI, indicating a lack of glomerular hypertension. Additional studies showed that conscious H, but not CR, rats exhibit dose-dependent renal vasoconstriction after l-NAME. To further investigate these susceptibility differences, l-NAME was given 2 wk after 3/4 normotensive nephrectomy (NX) and comparably impaired renal autoregulation in CR-NX and H-NX rats. CR-NX rats, nevertheless, still failed to develop proteinuria and GI despite moderate hypertension (144 ± 2 mmHg, n = 29). By contrast, despite an 80-90% l-NAME dose reduction and lesser BP increases (169 ± 4 mmHg), H-NX rats (n = 20) developed greater GI (26 ± 3%) compared with intact H rats. Linear regression analysis showed significant (P < 0.01) differences in the slope of the relationship between BP and GI between H-NX (slope 0.56 ± 0.14; r = 0.69; P < 0.008) and CR-NX (slope 0.09 ± 0.06; r = 0.29; P = 0.12) rats. These data indicate that blunted BP responses to l-NAME in the CR rats are associated with BP-independent resistance to nephropathy, possibly mediated by a resistance to the renal (efferent arteriolar) vasoconstrictive effects of NO inhibition.

Renal injury in angiotensin II+l-NAME-induced hypertensive rats is independent of elevated blood pressure

The balance between angiotensin II (ANG II) and nitric oxide plays an important role in renal function and is thought to contribute to the progression of renal injury in experimental hypertension. In the present study, we investigated the extent of blood pressure (BP)-dependent and BP-independent pathways of renal injury following 2 wk of hypertension produced by intravenous infusion of ANG II (5 ng·kg⁻¹·min⁻¹)+N(ω)-nitro-l-arginine methyl ester (l-NAME; 1.4 μg·kg⁻¹·min⁻¹) in male Sprague-Dawley rats. An aortic balloon occluder was positioned between the renal arteries to maintain (24 h/day) BP to the left kidney (servo-controlled) at baseline levels, whereas the right kidney (uncontrolled) was chronically exposed to elevated BP. Over the 14-day experimental protocol, the average BP to uncontrolled kidneys (152.7 ± 1.8 mmHg) was significantly elevated compared with servo-controlled (113.0 ± 0.2 mmHg) kidneys and kidneys from sham rats (108.3 ± 0.1 mmHg). ANG II+l-NAME infusion led to renal injury that was focal in nature and mainly confined to the outer medulla. Despite the differences in BP between servo-controlled and uncontrolled kidneys, there was a similar ~3.5-fold increase in renal outer medullary tubular injury, ~2-fold increase in outer medullary interstitial fibrosis, ~2-fold increase in outer medullary macrophage infiltration, and a significant increase in renal oxidative stress, all of which are indicative of BP-independent mediated pathways. The results of this study have important implications regarding the pathogenesis of renal injury in various experimental models of hypertension and provide novel insights regarding the variable association observed between hypertension and renal injury in some human populations.

Blood pressure-renal blood flow relationships in conscious angiotensin II- and phenylephrine-infused rats

Chronic ANG II infusion in rodents is widely used as an experimental model of hypertension, yet very limited data are available describing the resulting blood pressure-renal blood flow (BP-RBF) relationships in conscious rats. Accordingly, male Sprague-Dawley rats (n = 19) were instrumented for chronic measurements of BP (radiotelemetry) and RBF (Transonic Systems, Ithaca, NY). One week later, two or three separate 2-h recordings of BP and RBF were obtained in conscious rats at 24-h intervals, in addition to separate 24-h BP recordings. Rats were then administered either ANG II (n = 11, 125 ng·kg(-1)·min(-1)) or phenylephrine (PE; n = 8, 50 mg·kg(-1)·day(-1)) as a control, ANG II-independent, pressor agent. Three days later the BP-RBF and 24-h BP recordings were repeated over several days. Despite similar increases in BP, PE led to significantly greater BP lability at the heart beat and very low frequency bandwidths. Conversely, ANG II, but not PE, caused significant renal vasoconstriction (a 62% increase in renal vascular resistance and a 21% decrease in RBF) and increased variability in BP-RBF relationships. Transfer function analysis of BP (input) and RBF (output) were consistent with a significant potentiation of the renal myogenic mechanism during ANG II administration, likely contributing, in part, to the exaggerated reductions in RBF during periods of BP elevations. We conclude that relatively equipressor doses of ANG II and PE lead to greatly different ambient BP profiles and effects on the renal vasculature when assessed in conscious rats. These data may have important implications regarding the pathogenesis of hypertension-induced injury in these models of hypertension.