Interlobular arterial resistance: influence of renal arterial pressure and angiotensin II

In Vivo assessment of hypertensive nephrosclerosis using ultrasound localization microscopy

PURPOSE

As a typical chronic kidney disease (CKD), hypertensive nephrosclerosis (HN) is a common syndrome of hypertension, characterized by chronic kidney microvascular damage. Early diagnosis of microvascular damage using conventional ultrasound imaging encounters challenges in sensitivity and specificity owing to the inherent diffraction limit. Ultrasound localization microscopy (ULM) has been developed to obtain microvasculature and microvascular hemodynamics within the kidney, and would be a promising tool for early diagnosis of CKD.

METHODS

In this study, the advantage of quantitative indexes obtained by using ULM (mean arterial blood flow speeds of different segments of interlobular arteries) over indexes obtained using conventional clinical serum (β2-microglobulin, serum urea nitrogen and creatinine) and urine (24-hour urine volume and urine protein) tests and ultrasound Doppler imaging [peak systolic velocity (PSV), end-diastolic velocity (EDV) and resistance index (RI)] and contrast-enhanced ultrasound imaging [CEUS; rise time (RT), peak intensity (IMAX), mean transit time (mTT) and area under the time-intensity curve (AUC)] for early diagnosis of HN was investigated. Examinations were carried out on 6 spontaneously hypertensive rats (SHR) and 5 normal Wistar-Kyoto (WKY) rats at the age of 10 weeks.

RESULTS

The experimental results showed that the indicators derived from conventional clinical inspections (serum and urine tests) and ultrasound imaging (PSV, EDV, RI, RT, IMAX, mTT and AUC) did not show significant difference between hypertensive and healthy rats (p > 0.05), while the TTP of the SHR group (28.52 ± 5.52 s) derived from CEUS is significantly higher than that of the WKY group (18.68 ± 7.32 s; p < 0.05). The mean blood flow speed in interlobular artery of SHR (12.47 ± 1.06 mm/s) derived from ULM is significantly higher than that of WKY rats (10.13 ± 1.17 mm/s; p < 0.01).

CONCLUSION

The advantages of ULM over conventional clinical inspections and ultrasound imaging methods for early diagnosis of HN were validated. The quantitative results showed that ULM can effectively diagnose HN at the early stage by detecting the blood flow speed changes of interlobular arteries. ULM may promise a reliable technique for early diagnosis of HN in the future. This article is protected by copyright. All rights reserved.

Renal haemodynamic and protective effects of renoactive drugs in type 2 diabetes: Interaction with SGLT2 inhibitors

Diabetic kidney disease remains the leading cause of end‐stage kidney disease and a major risk factor for cardiovascular disease. Large cardiovascular outcome trials and dedicated kidney trials have shown that sodium‐glucose cotransporter (SGLT)2 inhibitors reduce cardiovascular morbidity and mortality and attenuate hard renal outcomes in patients with type 2 diabetes (T2D). Underlying mechanisms explaining these renal benefits may be mediated by decreased glomerular hypertension, possibly by vasodilation of the post‐glomerular arteriole. People with T2D often receive several different drugs, some of which could also impact the renal vasculature, and could therefore modify both renal efficacy and safety of SGLT2 inhibition. The most commonly prescribed drugs that could interact with SGLT2 inhibitors on renal haemodynamic function include renin‐angiotensin system inhibitors, calcium channel blockers and diuretics. Herein, we review the effects of these drugs on renal haemodynamic function in people with T2D and focus on studies that measured glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) with gold‐standard techniques. In addition, we posit, based on these observations, potential interactions with SGLT2 inhibitors with an emphasis on efficacy and safety.

This invited review describes the renal haemodynamic and protective effects of commonly prescribed drugs in people with type 2 diabetes and their interaction with SGLT2 inhibitors.

Glomerular Volume and Glomerulosclerosis at Different Depths within the Human Kidney

BACKGROUND

Age, CKD risk factors, and kidney function are associated with larger glomerular volume and a higher percentage of globally sclerotic glomeruli. Knowledge of how these associations may differ by cortical depth is limited.

METHODS

To investigate glomerular volume and glomerulosclerosis across different depths of cortex, we studied wedge sections of the renal parenchyma from 812 patients who underwent a radical nephrectomy (for a tumor), separately characterizing glomeruli in the superficial (subcapsular), middle, and deep (juxtamedullary) regions. We compared the association of mean nonsclerotic glomerular volume and of glomerulosclerosis (measured as the percentage of globally sclerotic glomeruli) with age, obesity, diabetes, smoking, kidney function, and structural pathology in the superficial, middle, and deep regions.

RESULTS

The superficial, middle, and deep regions showed significant differences in glomerular volume (0.0025, 0.0031, and 0.0028 µm³, respectively) and in glomerulosclerosis (18%, 7%, and 11%, respectively). There was a marked increase in glomerulosclerosis with age in the superficial region, but larger glomerular volume was not associated with age at any cortical depth. Glomerulosclerosis associated more strongly with arteriosclerosis and ischemic-appearing glomeruli in the superficial region. Hypertension, lower eGFR, and interstitial fibrosis associated with glomerulosclerosis and glomerular volume to a similar extent at any depth. Diabetes and proteinuria more strongly associated with glomerulosclerosis in the deep and middle regions, respectively, but neither associated with glomerular volume differently by depth. Obesity associated more strongly with glomerular volume in the superficial cortex.

CONCLUSIONS

Most clinical characteristic show similar associations with glomerulosclerosis and glomerulomegaly at different cortical depths. Exceptions include age-related glomerulosclerosis, which appears to be an ischemic process and is more predominant in the superficial region.

Hypertension accelerates age-related intrarenal small artery (IRSA) remodelling and stiffness in rats with possible involvement of AGEs and RAGE

OBJECTIVES

To study changes in morphology, advanced glycation end products (AGEs) and the AGEs receptor, RAGE, that occur with ageing in intrarenal small arteries (IRSAs) of spontaneously hypertensive rats (SHRs) and to investigate the possible roles of hypertension, AGEs and RAGE in the progression of IRSA remodelling and stiffness with ageing in rats.

METHODS

Ageing SHRs and ageing normotensive Wistar Kyoto (WKY) rats were studied. The minimal renal vascular resistance (minRVR) was measured. Renal arcuate arteries (RAAs) and interlobular arteries (RILAs), the expression of α-smooth muscle actin, proliferating cell nuclear antigen, AGEs, RAGE and the plasma concentrations of AGEs were also examined.

RESULTS

The IRSA minRVR, wall thickening, cell proliferation and collagen deposition in RILAs and RAAs gradually increased with age in SHRs and were much higher in 24-week-old SHRs than in age-matched WKY rats (p<0.05); these indexes in WKY rats were only elevated in the 72-week group (p<0.05). The expression of RAGE in the RAA and RILA tunica media in SHRs was upregulated by 24 weeks and 12 weeks (p<0.05), respectively, while AGEs levels in the plasma and in the IRSA tunica media were increased by 48 weeks (p<0.05) and increased gradually with age. The levels of both RAGE and AGEs in WKY rats were increased only at 72 weeks (p<0.05).

CONCLUSION

Hypertension accelerates the development of age-related IRSA remodelling and stiffness in rats, which may be related to upregulation of RAGE in the IRSA tunica media and increased expression of AGEs at the late stage.

Myoendothelial communication in the renal vasculature and the impact of drugs used clinically to treat hypertension

The renal vasculature has many peculiarities including highly irregular branching. Renal blood flow must sustain adequate perfusion and maintain a high glomerular filtration. Renal autoregulation helps control renal blood flow. The local autoregulatory mechanism, tubuloglomerular feedback, elicits a vasoconstriction that can be found not only in neighboring nephrons but over large areas of the kidney indicating that the renal vasculature supports strong conduction of vascular responses. The basis for conduction is intercellular communication through gap junctions. The endothelium is strongly coupled and serves as a vascular conduction highway leading the signal to the vascular smooth muscle cells through myoendothelial coupling. Extensive intercellular coupling is also found in renin secreting cells where gap junctions seem to tie the cells together to improve control of renin secretion. Lack of coupling leads to dysregulation of renin secretion and hypertension. However, the activity of the renin-angiotensin system also controls gap junction expression in the kidney. Treatment reducing angiotensin II activity, as used in hypertension treatment, can affect expression of renal and vascular gap junction.

TRPV4 channels contribute to renal myogenic autoregulation in neonatal pigs

Myogenic response, a phenomenon in which resistance size arteries and arterioles swiftly constrict or dilate in response to an acute elevation or reduction, respectively, in intravascular pressure is a key component of renal autoregulation mechanisms. Although it is well established that the renal system is functionally immature in neonates, mechanisms that regulate neonatal renal blood flow (RBF) remain poorly understood. In this study, we investigated the hypothesis that members of the transient receptor potential vanilloid (TRPV) channels are molecular components of renal myogenic constriction in newborns. We show that unlike TRPV1-3, TRPV4 channels are predominantly expressed in neonatal pig preglomerular vascular smooth muscle cells (SMCs). Intracellular Ca²⁺ concentration ([Ca²⁺]_i) elevation induced by osmotic cell swelling was attenuated by TRPV4, L-type Ca²⁺, and stretch-activated Ca²⁺ channel blockers but not phospholipase A₂ inhibitor. Blockade of TRPV4 channels reversed steady-state myogenic tone and inhibited pressure-induced membrane depolarization, [Ca²⁺]_i elevation, and constriction in distal interlobular arteries. A step increase in arterial pressure induced efficient autoregulation of renal cortical perfusion and total RBF in anesthetized and mechanically ventilated neonatal pigs. Moreover, intrarenal arterial infusion of the TRPV4 channel blockers HC 067047 and RN 1734 attenuated renal autoregulation in the pigs. These data suggest that renal myogenic autoregulation is functional in neonates. Our findings also indicate that TRPV4 channels are mechanosensors in neonatal pig preglomerular vascular SMCs and contribute to renal myogenic autoregulation.

Distinct protein signature of hypertension-induced damage in the renal proteome of the two-kidney, one-clip rat model

BACKGROUND

Hypertensive nephrosclerosis is one of the most frequent causes of chronic kidney failure. Proteome analysis potentially improves the pathophysiological understanding and diagnostic precision of this disorder. In the present exploratory study, we investigated experimental nephrosclerosis in the two-kidney, one-clip (2K1C) hypertensive rat model.

METHODS

The renal cortex proteome from juxtamedullary cortex and outer cortex of 2K1C male Wistar-Hannover rats (n = 4) was compared with the sham-operated controls (n = 6), using mass spectrometry-based quantitative proteomics. We combined a high abundant plasma protein depletion strategy with an extended liquid chromatographic gradient to improve peptide and protein identification. Immunohistology was used for independent confirmation of abundance.

RESULTS

We identified 1724 proteins, of which 1434 were quantified with at least two unique peptides. Comparative proteomics revealed 608 proteins, including the platelet-derived growth factor receptor-β signalling pathway, with different abundances between the non-clipped kidney of hypertensive 2K1C rats and the corresponding kidney of the normotensive controls (P < 0.05, absolute fold change ≥1.5). Among the most significantly altered proteins in the whole cortex were periostin, transgelin, and creatine kinase B-type. Relative abundance of periostin alone allowed clear classification of 2K1C and controls. Enrichment of periostin in 2K1C rats was verified by immunohistology, showing positivity especially around the fibrotic vessels.

CONCLUSION

The proteome is altered in hypertension-induced kidney damage. We propose periostin, especially in combination with transgelin and creatine kinase B-type, as possible proteomic classifier to distinguish hypertensive nephrosclerosis from the normal tissue. This classifier needs to be further validated with respect to early diagnosis of fibrosis, prognosis, and its potential as a novel molecular target for pharmacological interventions.

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.

Hemodynamic basis for the limited renal injury in rats with angiotensin II-induced hypertension

ANG II is thought to increase the susceptibility to hypertension-induced renal disease (HIRD) via blood pressure (BP)-dependent and BP-independent pathways; however, the quantitative relationships between BP and HIRD have not been examined in ANG II-infused hypertensive rats. We compared the relationship between radiotelemetrically measured BP and HIRD in Sprague-Dawley rats (Harlan) chronically administered ANG II (300-500 ng·kg(-1)·min(-1), n = 19) for 4 wk versus another commonly employed pharmacological model of hypertension induced by the chronic administration of N(ω)-nitro-l-arginine methyl ester (l-NAME, 50 mg·kg(-1)·day(-1), n = 23). [DOSAGE ERROR CORRECTED]. Despite the significantly higher average systolic BP associated with ANG II (191.1 ± 3.2 mmHg) versus l-NAME (179.9 ± 2.5 mmHg) administration, the level of HIRD was very modest in the ANG II versus l-NAME model as evidenced by significantly less glomerular injury (6.6 ± 1.3% vs. 11.3 ± 1.5%, respectively), tubulointerstitial injury (0.3 ± 0.1 vs. 0.7 ± 0.1 injury score, respectively), proteinuria (66.3 ± 10.0 vs. 117.5 ± 10.1 mg/day, respectively), and serum creatinine levels (0.5 ± 0.04 vs. 0.9 ± 0.07 mg/dl, respectively). Given that HIRD severity is expected to be a function of renal microvascular BP transmission, BP-renal blood flow (RBF) relationships were examined in additional conscious rats administered ANG II (n = 7) or l-NAME (n = 8). Greater renal vasoconstriction was observed during ANG II versus l-NAME administration (41% vs. 23% decrease in RBF from baseline). Moreover, administration of ANG II, but not l-NAME, led to a unique BP-RBF pattern in which the most substantial decreases in RBF were observed during spontaneous increases in BP. We conclude that the hemodynamic effects of ANG II may mediate the strikingly low susceptibility to HIRD in the ANG II-infused model of hypertension in rats.

Purinergic signalling in the kidney in health and disease

The involvement of purinergic signalling in kidney physiology and pathophysiology is rapidly gaining recognition and this is a comprehensive review of early and recent publications in the field. Purinergic signalling involvement is described in several important intrarenal regulatory mechanisms, including tuboglomerular feedback, the autoregulatory response of the glomerular and extraglomerular microcirculation and the control of renin release. Furthermore, purinergic signalling influences water and electrolyte transport in all segments of the renal tubule. Reports about purine- and pyrimidine-mediated actions in diseases of the kidney, including polycystic kidney disease, nephritis, diabetes, hypertension and nephrotoxicant injury are covered and possible purinergic therapeutic strategies discussed.

Assessment of renal function; clearance, the renal microcirculation, renal blood flow, and metabolic balance

Historically, tools to assess renal function have been developed to investigate the physiology of the kidney in an experimental setting, and certain of these techniques have utility in evaluating renal function in the clinical setting. The following work will survey a spectrum of these tools, their applications and limitations in four general sections. The first is clearance, including evaluation of exogenous and endogenous markers for determining glomerular filtration rate, the adaptation of estimated glomerular filtration rate in the clinical arena, and additional clearance techniques to assess various other parameters of renal function. The second section deals with in vivo and in vitro approaches to the study of the renal microvasculature. This section surveys a number of experimental techniques including corticotomy, the hydronephrotic kidney, vascular casting, intravital charge coupled device videomicroscopy, multiphoton fluorescent microscopy, synchrotron-based angiography, laser speckle contrast imaging, isolated renal microvessels, and the perfused juxtamedullary nephron microvasculature. The third section addresses in vivo and in vitro approaches to the study of renal blood flow. These include ultrasonic flowmetry, laser-Doppler flowmetry, magnetic resonance imaging (MRI), phase contrast MRI, cine phase contrast MRI, dynamic contrast-enhanced MRI, blood oxygen level dependent MRI, arterial spin labeling MRI, x-ray computed tomography, and positron emission tomography. The final section addresses the methodologies of metabolic balance studies. These are described for humans, large experimental animals as well as for rodents. Overall, the various in vitro and in vivo topics and applications to evaluate renal function should provide a guide for the investigator or physician to understand and to implement the techniques in the laboratory or clinic setting.

Neurohormonal interactions on the renal oxygen delivery and consumption in haemorrhagic shock-induced acute kidney injury

Haemorrhagic shock is a common cause of acute kidney injury (AKI), which is a major risk factor for developing chronic kidney disease. The mechanism is superficially straightforward. An arterial pressure below the kidney's autoregulatory region leads to a direct reduction in filtration pressure and perfusion, which in turn cause renal failure with reduced glomerular filtration rate and AKI because of hypoxia. However, the kidney's situation is further worsened by the hormonal and neural reactions to reduced perfusion pressure. There are three major systems working to maintain arterial pressure in shock: sympathetic signalling, the renin-angiotensin system and vasopressin. These work to retain electrolytes and water and to increase peripheral resistance and cardiac output. In the kidney, the increased electrolyte reabsorption consumes oxygen. At the same time, at the signalling level seen in shock, all of these hormones reduce renal perfusion and thereby oxygen delivery. This creates an exaggerated hypoxic situation that is liable to worsen the AKI. The present review will examine this mechanistic background and identify a number of areas that require further studies. At this time, the ideal treatment of haemorrhagic shock appears to be slow fluid resuscitation, possibly with hyperosmolar sodium, low chloride and no artificial colloids. From the standpoint of the kidney, renin-angiotensin system inhibitors appear fruitful for further study.

Detecting physiological systems with laser speckle perfusion imaging of the renal cortex

Laser speckle perfusion imaging (LSPI) has become an increasingly popular technique for monitoring vascular perfusion over a tissue surface. However, few studies have utilized the full range of spatial and temporal information generated by LSPI to monitor spatial properties of physiologically relevant dynamics. In this study, we extend the use of LSPI to analyze renal perfusion dynamics over a spatial surface of ~5 × 7 mm of renal cortex. We identify frequencies related to five physiological systems that induce temporal changes in renal vascular perfusion (cardiac flow pulse, respiratory-induced oscillations, baroreflex components, the myogenic response, and tubuloglomerular feedback) across the imaged surface and compare the results with those obtained from renal blood flow measurements. We find that dynamics supplied from global sources (cardiac, respiration, and baroreflex) present with the same frequency at all locations across the imaged surface, but the local renal autoregulation dynamics can be heterogeneous in their distribution across the surface. Moreover, transfer function analysis with forced blood pressure as the input yields the same information with laser speckle imaging or renal blood flow as the output during control, intrarenal infusion of N(ω)-nitro-L-arginine methyl ester to enhance renal autoregulation, and intrarenal infusion of the rho-kinase inhibitor Y-27632 to inhibit vasomotion. We conclude that LSPI measurements can be used to analyze local as well as global renal perfusion dynamics and to study the properties of physiological systems across the renal cortex.

Development of structural kidney damage in spontaneously hypertensive rats

The spontaneously hypertensive rat (SHR) is one of the major models of hypertension. This article describes the current state of knowledge about the mechanism behind kidney damage in SHR in the context of human hypertension and hypertensive kidney disease. It will argue that hypertensive damage in the SHR is pressure-dependent and shows how initial vascular damage leads to a loss of autoregulation and arterial hypertrophy in the juxtamedullary cortex while the outer cortical structures are relatively protected. Progressive arteriolar media hypertrophy then leads to the collapse of some glomeruli followed by tubular atrophy. The reduced glomerular filtration, thus, leads to compensatory hyperfiltration in another population of glomeruli which develop proteinuria and glomerulosclerosis. This model provides some important questions for future research. The regulation of media hypertrophy will be of great interest, as it might slow nephron loss and interstitial fibrosis. Finally, the mechanism by which reduced tubular flow leads to tubular atrophy is another important area for future research. Initial findings indicate that cilia activation may be of major importance for maintaining tubular structure.

Afferent arteriolopathy and glomerular collapse but not segmental sclerosis induce tubular atrophy in old spontaneously hypertensive rats

In chronic renal disease, the temporal and spatial relationship between vascular, glomerular and tubular changes is still unclear. Hypertension, an important cause of chronic renal failure, leads to afferent arteriolopathy, segmental glomerulosclerosis and tubular atrophy in the juxtamedullary cortex. We investigated the pathological changes of hypertensive renal disease in aged spontaneously hypertensive rats using a large number of serial sections, where we traced and analyzed afferent arteriole, glomerulus and proximal tubule of single nephrons. Our major finding was that both afferent arteriolopathy and glomerular capillary collapse were linked to tubular atrophy. Only nephrons with glomerular collapse (n = 13) showed tubules with reduced diameter indicating atrophy [21.66 ± 2.56 μm vs. tubules in normotensive Wistar Kyoto rats (WKY) 38.56 ± 0.56 μm, p < 0.05], as well as afferent arteriolar wall hypertrophy (diameter 32.74 ± 4.72 μm vs. afferent arterioles in WKY 19.24 ± 0.98 μm, p < 0.05). Nephrons with segmental sclerosis (n = 10) did not show tubular atrophy and tubular diameters were unchanged (35.60 ± 1.43 μm). Afferent arteriolar diameter negatively correlated with glomerular capillary volume fraction (r = −0.36) and proximal tubular diameter (r = −0.46) implying reduced glomerular and tubular flow. In line with this, chronically damaged tubules showed reduced staining for the ciliary protein inversin indicating changed ciliary signalling due to reduced urinary flow. This is the first morphological study on hypertensive renal disease making correlations between vascular, glomerular and tubular components of individual nephron units. Our data suggest that afferent arteriolopathy leads to glomerular collapse and reduced urinary flow with subsequent tubular atrophy.

Strain vessel hypothesis: a viewpoint for linkage of albuminuria and cerebro-cardiovascular risk

Albuminuria is closely associated with stroke and cardiovascular diseases (CVDs) as well as the salt sensitivity of blood pressure (BP). Although albuminuria may reflect generalized endothelial dysfunction, there may be more specific hemodynamic mechanisms underlying these associations. Cerebral hemorrhage and infarction occur most frequently in the area of small perforating arteries that are exposed to high pressure and that have to maintain strong vascular tone in order to provide large pressure gradients from the parent vessels to the capillaries. Analogous to the perforating arteries are the glomerular afferent arterioles of the juxtamedullary nephrons. Hypertensive vascular damage occurs first and more severely in the juxtamedullary glomeruli. Therefore, albuminuria may be an early sign of vascular damages imposed on 'strain vessels' such as perforating arteries and juxtamedullary afferent arterioles. Coronary circulation also occurs under unique hemodynamic conditions, in which the entire epicardial segments are exposed to very high pressure with little flow during systolic phases. From the evolutionary point of view, we speculate that such circulatory systems in the vital organs are mandatory for survival under the danger of hypoperfusion due to difficult access to salt and water as well as high risks of wound injuries. In addition, albuminuria would indicate an impairment of renal medullary circulation, downstream from the juxtamedullary glomeruli, and therefore an impaired pressure natriuresis, which would lead to salt sensitivity of BP. Our 'strain vessel hypothesis' may explain why hypertension and diabetes, unforeseen in the concept of evolution, preferentially affect vital organs such as the brain, heart and kidney.

Assessment of renal autoregulation

The kidney displays highly efficient autoregulation so that under steady-state conditions renal blood flow (RBF) is independent of blood pressure over a wide range of pressure. Autoregulation occurs in the preglomerular microcirculation and is mediated by two, perhaps three, mechanisms. The faster myogenic mechanism and the slower tubuloglomerular feedback contribute both directly and interactively to autoregulation of RBF and of glomerular capillary pressure. Multiple experiments have been used to study autoregulation and can be considered as variants of two basic designs. The first measures RBF after multiple stepwise changes in renal perfusion pressure to assess how a biological condition or experimental maneuver affects the overall pressure-flow relationship. The second uses time-series analysis to better understand the operation of multiple controllers operating in parallel on the same vascular smooth muscle. There are conceptual and experimental limitations to all current experimental designs so that no one design adequately describes autoregulation. In particular, it is clear that the efficiency of autoregulation varies with time and that most current techniques do not adequately address this issue. Also, the time-varying and nonadditive interaction between the myogenic mechanism and tubuloglomerular feedback underscores the difficulty of dissecting their contributions to autoregulation. We consider the modulation of autoregulation by nitric oxide and use it to illustrate the necessity for multiple experimental designs, often applied iteratively.

Loss of tubuloglomerular feedback in decompensated liver cirrhosis: physiopathological implications

In healthy subjects, arterial pressure reduction or renal ischemia produces renal artery dilatation through autoregulation and tubuloglomerular feedback (TuGF). Patients with decompensated cirrhosis have reduced kidney perfusion pressure but show renal vasoconstriction instead of autoregulation-mediated vasodilation. This study investigates the consequences of kidney autoregulation loss on renal perfusion, glomerular filtration rate, and tubular handling of electrolytes in both compensated and ascitic nonazotemic cirrhotic patients. Forty-two consecutive patients with diuretic-free liver cirrhosis (32 with preascitic and 10 with ascitic disease) and 10 controls were submitted to the following determinations: (a) basal plasma renin activity and aldosterone levels; (b) endogenous dopaminergic activity measured as incremental aldosterone responses during metoclopramide administration; and (c) renal clearances of sodium, potassium, inulin, para-aminohippurate and lithium. Compared with the other groups, ascitic patients showed lower renal plasma flow (P < 0.01) and lithium clearance (P < 0.05), a higher filtration fraction (P < 0.01), and secondary aldosteronism. Controls and preascitic patients displayed tubuloglomerular feedback (the mechanism increasing the glomerular filtration rate when a reduced sodium load reaches the distal tubule), as demonstrated by negative correlations between fractional excretion of lithium (an expression of fractional delivery of sodium to the distal nephron) and glomerular filtration rate (respectively, r = -0.73, P < 0.03, and r = -0.48, P < 0.01). Conversely, patients with ascites showed a positive correlation between lithium fractional excretion and glomerular filtration rate (r = 0.64, P < 0.05). Reduction in renal perfusion, increased filtration fraction, and TuGF derangement, as found in decompensated patients, are indicative of prevalent postglomerular arteriolar vasoconstriction, with ensuing stimulation of proximal tubular sodium reabsorption.

Enhanced response to AVP in the interlobular artery from the spontaneously hypertensive rat

Arginine vasopressin (AVP) induces exaggerated intracellular free calcium (Cai2+) responses in preglomerular smooth muscle cells from young spontaneously hypertensive rats (SHR) due to increased density of the AVP V1a receptor. The intention of the present paper was to examine the relative contribution of afferent arterioles (AA) and interlobular artery (ILA) in AVP- and norepinephrine-induced calcium signaling. The kidneys were perfused with agar solution in vivo, and thin cortical slices were enzyme digested to produce isolated agar-filled vascular fragments. Calcium responses were recorded in fura 2-loaded cells by Ca2+ imaging. Diameter changes were measured after AVP stimulation and mRNA for V1a was measured on isolated vessel fragments. SHR had a significantly higher baseline calcium ratio and lower resting diameter compared with normotensive Wistar-Kyoto rats (WKY). Stimulation with AVP (10(-7) M) in ILA fragments from SHR induced a ratio increase of 0.49 +/- 0.09, significantly higher than the ratio increase in AA from SHR (0.20 +/- 0.03, P < 0.01) and in ILA from WKY (0.24 +/- 0.03, P < 0.01). Stimulation with norepinephrine (10(-7) M) induced responses homogeneously distributed between the segments and strains. Nifedipine treatment or removal of external calcium (Cao2+) reduced the norepinephrine-induced peak response. Both norepinephrine- and AVP-induced sustained responses were abolished after Cao2+ removal in SHR and WKY (P < 0.01). Measurements of V1a receptor mRNA on isolated segments showed a threefold increase in ILA from SHR. The present findings indicate that the exaggerated Ca2+ and contractile response to AVP in SHR is mainly mediated through ILA vasoconstriction.

Vasoconstrictor and vasodilator effects of adenosine in the kidney

Adenosine is an ATP breakdown product that in most vessels causes vasodilatation and that contributes to the metabolic control of organ perfusion, i.e., to the match between oxygen demand and oxygen delivery. In the renal vasculature, in contrast, adenosine can produce vasoconstriction, a response that has been suggested to be an organ-specific version of metabolic control designed to restrict organ perfusion when transport work increases. However, the vasoconstriction elicited by an intravenous infusion of adenosine is only short lasting, being replaced within 1-2 min by vasodilatation. It appears that the steady-state response to the increase of plasma adenosine levels above normal resulting from the infusion is global renal vasorelaxation that is the result of A2AR activation in most parts of the renal vasculature, including larger renal arteries, juxtamedullary afferent arterioles, efferent arterioles, and medullary vessels. A2AR-mediated vasorelaxation is probably facilitated by endothelial receptors that cause the release of nitric oxide and other endothelial relaxing factors. In contrast, isolated perfused afferent arterioles of superficial and midcortical nephrons of rabbit and mouse, especially in their most distal segment at the entrance to the glomerulus, respond to adenosine with persistent vasoconstriction, indicating predominant or exclusive expression of A1AR. A1AR in afferent arterioles are selectively activated from the interstitial aspect of the vessel. This property can dissociate A1AR activation from changes in vascular adenosine concentration, a characteristic that is ideally suited for the role of renal adenosine as a paracrine factor in the control of glomerular function.

Effect of exogenous and endogenous angiotensin II on intrarenal distribution of glomerular filtration rate in rats

Different changes in glomerular filtration rates (GFR) in deep and superficial glomeruli have been suggested to influence renal NaCl excretion and concentrating ability. Angiotensin II (AngII) has been implicated in such changes, but the experimental evidence has been conflicting, probably because of the methodological limitation of just one 'snapshot' measurement of local GFR per kidney. We have therefore studied the effect of AngII and AT(1)-receptor blockade on glomerular filtration in outer, middle and inner cortex (OC, MC and IC, respectively) in pentobarbitone-anaesthetised rats using the aprotinin (Ap) method, providing control and experimental measurements in the same kidney. Glomerular filtration rate per gram cortical tissue was measured based on 'free' glomerular filtration of Ap followed by complete tubular uptake and a 20 min sojourn in the proximal tubular cells before breakdown and incipient return to the plasma.(125)I-labelled Ap was injected I.V. to determine control Ap clearance, followed after 13 min by injection of AngII or the A1 type AngII receptor blocker losartan and 2 min thereafter by (131)I-labelled Ap to determine clearance in the experimental period. Tracer activity in frequent blood samples and in tissue samples allowed calculation of GFR in the two periods. Mean GFR control values were: 1.13 ml min(-1) in whole kidney and 1.44, 1.27 and 0.76 ml min(-1) per gram cortical tissue in OC, MC and IC, respectively. The most sensitive and comprehensive measure of altered GFR distribution is the ratio between the relative filtration change in inner versus that in outer cortex, F = (IC(E)/IC(C))/(OC(E)/OC(C)), where subscripts E and C stand for experimental and control, respectively. F values greater than 1.00 directly indicate and quantify a relatively greater increase of filtration rate in inner than in outer cortex. We found in salt-replete rats that at practically unchanged total GFR, intravenous and intra-arterial infusion of AngII increased F to 1.07 and 1.04 (P < 0.05) whereas losartan reduced F to 0.99. After pretreatment with the inhibitor of nitric oxide production L-NAME, losartan increased total GFR by 8 % and F fell to 0.95 (P < 0.05). In salt-depleted rats losartan reduced F to 0.95 (P < 0.05) at unchanged total GFR. All IC/OC changes induced by losartan were significantly different from that obtained by AngII infusions. We conclude that deep nephrons have higher postglomerular AngII tone and also higher AngII sensitivity than superficial nephrons. The better preserved GFR in deep cortex during AngII action may contribute towards maintaining the renal concentrating ability by providing NaCl for reabsorption by the ascending limb of the loop of Henle.

Analysis of myogenic mechanisms in renal autoregulation

To examine whether local myogenic mechanisms account for autoregulation of renal blood flow, a theoretical analysis was undertaken on a model of the pre-glomerular vascular tree consisting of a main and a short, narrow juxtaglomerular segment. At atmospheric extravascular pressure in vitro data are consistent with a relationship r=r0(1 + k - pk) between radius (r) and transmural pressure (p) at p > 60 mmHg, where k can be estimated from in vitro data and r=r0 at complete autoregulatory vasodilation. After introducing r=r(0)(1 + k - pk), Poiseuille's formula was integrated along the main segment, Deltax long, between arterial pressure P(1) and P(2) at the end of the main segment. At the lowest autoregulatory pressure P(1)=65 mmHg pre-glomerular blood flow is F=5Kr(0)(4)/Deltax. At P(1)=140 mmHg a pressure drop of only 17 mmHg to P2=123 mmHg is sufficient to fulfil the criterion for complete autoregulation: F=5Kr(0)(4)/Deltax. Thus, 80% of the total pre-glomerular vascular resistance is localized to the juxtaglomerular segment. Loop diuretics may abolish juxtaglomerular contractility. Calculated flow/pressure relationships after eliminating juxtaglomerular contractility are similar to those obtained after administering ethacrynic acid. If a constant tension hypothesis (r=60r(0)/p) rather than the transmural pressure hypothesis [r=r(0)(1 + k - pk)] applies, complete autoregulation is maintained to P(2)=89 mmHg, but the effect of loop diuretics is not mimicked. In conclusion, high juxtaglomerular contractility may be attributed to a myogenic mechanism only if extravascular pressure in the juxtaglomerular segment is subatmospheric.

Attenuated buffering of renal perfusion pressure variation in juxtamedullary cortex in SHR

Renal tissue damage is substantially more pronounced in the juxtamedullary than in the superficial cortex in hypertensive rats, and the pathogenesis of the morphological changes are only partly understood. Glomerular capillary pressure (P(GC)) is increased, and steady-state autoregulation is normal in the deep renal cortex. We tested the hypothesis that the transient period from one pressure level to another may induce greater variation in local perfusion before stable autoregulation is established. An acute increase in local perfusion was compared in the superficial and juxtamedullary cortex of spontaneously hypertensive (SHR) and Wistar-Kyoto rats (WKY) after an abrupt increase in perfusion pressure. Total renal blood flow (RBF) was measured by a Transonic flow probe and local renal perfusion by laser Doppler flowmetry. Renal perfusion pressure was lowered to 50% of initial values and released abruptly. The maximal RBF increased from 6.3 +/- 0.4 to a maximal value of 7.6 +/- 0.3 ml/min (P < 0.001) in SHR and from 7.3 +/- 0.3 to 8.2 +/- 0.6 ml/min (P < 0.001) in WKY. These changes were not significantly different from each other. The change in superficial cortical perfusion was also not different between SHR and WKY. Pressure release increased juxtamedullary perfusion in SHR from 146 +/- 8 to a maximal value of 228 +/- 17 units (P < 0.001) and in WKY from 160 +/- 13 to 179 +/- 11 units (P < 0.001). The results were significantly different from each other (P < 0.001). The time for maximal flow response was shorter in the deep cortex of SHR, and the time for normalization was longer than in WKY. These data indicate that the buffering of perfusion pressure variation is significantly attenuated in the juxtamedullary cortex, and significantly more so in SHR than in WKY, assuming a covariation of RBF and P(GC), and this finding may explain the extensive morphological damage in the juxtamedullary cortex of SHR.

Reduced reactivity of renal microvessels to pressure and angiotensin II in fawn-hooded rats

Fawn-Hooded rats possess an increased risk to develop glomerular damage. Both an impaired control of preglomerular resistance and an elevated postglomerular resistance have been implicated. In the present study, we directly assessed the myogenic reactivity of distal interlobular arteries and afferent arterioles from hypertensive and normotensive Fawn-Hooded rats compared with Sprague-Dawley and Wistar rats, which are known to be resistant for developing renal disease. Pressure-response curves were made in isolated perfused hydronephrotic kidneys from these rats. In addition, increasing concentrations of angiotensin II were added to the perfusate to determine the reactivity of interlobular arteries, afferent arterioles, and efferent arterioles to this peptide. Preglomerular vessels from hypertensive and normotensive Fawn-Hooded rats exhibited an impaired reactivity to both pressure and angiotensin II compared with that of Sprague-Dawley and Wistar rats. Basal efferent arteriolar diameters were similar among the 4 strains of rat. In addition, efferent arterioles from hypertensive and normotensive Fawn-Hooded rats displayed a reduced sensitivity to angiotensin II. Our observations demonstrate that in Fawn-Hooded rats, 2 components of preglomerular resistance control are impaired: the myogenic and the angiotensin II response. In addition, efferent arteriolar reactivity to angiotensin II is not elevated but lowered in these rats. Therefore, a deficit in preglomerular resistance control is the most important intrinsic factor involved in the increased susceptibility of Fawn-hooded rats to develop renal disease.

Chronic angiotensin converting enzyme inhibition enhances renal vascular responsiveness to acetylcholine in anaesthetized rabbits

OBJECTIVE

To determine whether 6 weeks continuous treatment with an angiotensin converting enzyme (ACE) inhibitor reduced renal vascular responsiveness in vivo, since this treatment results in extensive phenotypic conversion of afferent arteriolar cells from contractile to endocrine-like, renin secretory cells.

METHODS

Enalapril (10 microg/kg per h s.c.) was delivered continuously for 6 weeks. In anaesthetized rabbits (treated or sham), arterial blood pressure and renal blood flow were measured and renal responsiveness tested by constructing dose-response curves to bolus doses of phenylephrine, angiotensin II and acetylcholine delivered directly into the renal artery.

RESULTS

ACE inhibition resulted in a significant shift to the left in the renal vascular conductance responses to acetylcholine (P < 0.005) and angiotensin II (P < 0.05), indicating enhanced, not reduced, responsiveness to these agents. There were no significant effects of chronic ACE inhibition on the conductance responses to phenylephrine.

CONCLUSIONS

Contrary to our hypothesis, 6 weeks ACE inhibition did not reduce renal vascular responsiveness to three vasoactive agents, suggesting that the phenotypic changes observed in the afferent arterioles and to a lesser extent the interlobular arteries, were either insignificant or compensated for by other changes in renal circulatory control.

Effects of angiotensin II on regional afferent and efferent arteriole dimensions and the glomerular pole

The diversity of renal arteriole diameters in different cortical regions has important consequences for control of glomerular capillary pressure. We examined whether intrarenal angiotensin II (ANG II; 0.1, 1, or 5 ng. kg(-1). min(-1)) in anesthetized rabbits acts preferentially on pre- or postglomerular vessels using vascular casting. ANG II produced dose-related reductions in afferent and efferent diameters in the outer, mid, and inner cortex, without effecting arterial pressure. Afferent diameter decreased more than efferent in the outer and mid cortex (P < 0.05) but by a similar extent in juxtamedullary nephrons (P = 0.58). Calculated efferent resistance increased more than afferent, especially in the outer cortex (127 vs. 24 units; 5 ng. kg(-1). min(-1) ANG II). ANG II produced significant dose-related increases in the distance between the arterioles at the entrance to the glomerular pole in all regions. Thus afferent diameter decreased more in response to ANG II, but efferent resistance rose more due to smaller resting luminal dimensions. The results also indicate that glomerular pole dimensions change in response to ANG II.

Integrating multiple paracrine regulators of renal microvascular dynamics

There has been tremendous growth in our knowledge about the multiple interacting mechanisms that regulate renal microvascular function. Paracrine signals originating from endothelial and epithelial cells exert profound influences on the basal tone and reactivity of the pre- and postglomerular arterioles. Selective responsiveness of these arterioles to various stimuli is possible because of differential activating mechanisms in vascular smooth muscle cells of afferent and efferent arterioles. Afferent arterioles rely predominantly on voltage-dependent calcium channels, while efferent arterioles utilize other mechanisms for calcium entry as well as intracellular calcium mobilization. The autoregulatory responses of preglomerular arterioles exemplify the selectivity of these complex control mechanisms. The myogenic mechanism responds to increases in renal perfusion pressure through "stretch-activated" cation channels that lead to depolarization, calcium entry, and vascular contraction. Autoregulatory efficiency is enhanced by the tubuloglomerular feedback (TGF) mechanism which responds to flow-dependent changes in tubular fluid composition at the level of the macula densa and transmits signals to the afferent arterioles to alter the activation state of voltage-dependent calcium channels. Recent studies have implicated extracellular ATP as one paracrine factor mediating TGF and autoregulatory related signals to the afferent arterioles. Other paracrine agents including nitric oxide, angiotensin II, adenosine, and arachidonic acid metabolites modulate vascular responsiveness in order to maintain an optimal balance between the metabolically determined reabsorptive capabilities of the tubules and the hemodynamically dependent filtered load.

Increased glomerular capillary pressure and size mediate glomerulosclerosis in SHR juxtamedullary cortex

To gain insight into the mechanisms in the development of glomerulosclerosis in juxtamedullary cortex, the degree of glomerulosclerosis, glomerular tuft diameter, glomerular capillary pressure (Pgc), and local renal blood flow (RBF) autoregulation were measured in superficial and juxtamedullary cortex of 10- and 70-wk-old spontaneously hypertensive rat (SHR), using aged matched Wistar-Kyoto (WKY) rats as controls. Pgc was measured after corticotomy by direct micropuncture of glomeruli in superficial and juxtamedullary cortex. Total RBF was measured by a transit-time flowmeter (Transonic) and local blood flow by use of laser-Doppler flowmetry. The degree of glomerulosclerosis measured by a semiquantitative histological technique was significantly increased in juxtamedullary compared with superficial cortex in all groups. The difference was most pronounced in the juxtamedullary cortex of 70-wk-old SHR. Pgc was significantly increased in juxtamedullary cortex compared with superficial cortex in 70-wk SHR (57.1 +/- 2.7 vs. 46.5 +/- 0.5 mmHg, P < 0.01). The corresponding data set from 70-wk WKY was 45.5 +/- 0.43 vs. 41.6 +/- 1.5 (P < 0.05). The Pgc in juxtamedullary cortex of 10-wk SHR was slightly higher than in superficial cortex (45.1 +/- 2.3 vs. 50.1 +/- 1.2 mmHg, P = 0.05), whereas there was no difference in 10-wk WKY. Glomerular diameter was larger in juxtamedullary cortex in old animals but not significantly different in 10-wk WKY rats and 10-wk SHR. Total RBF was reset to higher perfusion pressures in hypertensive rats. Juxtamedullary and superficial blood flow autoregulation were not significantly different from total RBF autoregulation in all groups. These results suggest that hypertrophy as well as increased Pgc might contribute to the development of manifest glomerulosclerosis. Changes in local blood flow autoregulation do not seem to play a major role in the development of glomerulosclerosis.

Autoregulation of total and zonal glomerular filtration rate in spontaneously hypertensive rats during antihypertensive therapy

The effects of angiotensin II type 1 receptor antagonist (losartan), angiotensin 1-converting enzyme (ACE) inhibitor (enalapril), and calcium channel blocker (nifedipine) on autoregulation of total and zonal glomerular filtration rate (GFR) were studied in spontaneously hypertensive rats (SHRs), 10 and 40 weeks of age, and 10-week-old Wistar-Kyoto (WKY) rats. Untreated animals in each group served as controls. Renal blood flow (RBF) was measured by a transit-time flow probe (Transonic) on the left renal artery. Total and zonal GFR (outer, middle, and inner cortex) were estimated from tubular uptake of 125iodine-labeled aprotinin (125IAp) injected i.v. at control renal arterial pressure (RAP), and 131IAp injected at a RAP reduced to the lower limit of RBF autoregulation. Autoregulation of RBF was reset to higher pressure levels in untreated hypertensive rats. Enalapril normalized this resetting in 10-week-old SHRs, but not in aged SHRs 40 weeks. Losartan did not completely normalize this resetting in either 10-or 40-week-old SHRs, whereas nifedipine impaired RBF autoregulation in both WKYs and SHRs. A decreased autoregulatory compensation of GFR after pressure reduction was observed in losartan-treated 10-week-old SHRs and after all drug regimens in 40-week SHRs. GFR autoregulation in outer, middle, and inner cortex was impaired in losartan-treated 10-and 40-week-old SHRs. With all treatments, the autoregulation in 10- and 40-week-old SHRs was better preserved in the inner than in the outer cortex. The impaired autoregulation may indicate that a part of the dilatory capacity of preglomerular vessels has already been taken out by hypotensive treatment. Renal vascular abnormalities may have an additional effect.

Ascending myogenic autoregulation: interactions between tubuloglomerular feedback and myogenic mechanisms

A mathematical model of the renal vascular and tubular systems was used to examine the possibility that synergistic interactions might occur between the tubuloglomerular feedback (TGF) and myogenic autoregulatory mechanisms in the kidney. To simulate the myogenic mechanism, the renal vasculature was modelled with a resistance network where the total preglomerular resistance varies with intravascular pressure. In addition, a steady-state model of glomerular filtration, proximal and Henle's loop reabsorption, and TGF-modulation of afferent arteriolar resistance was derived. The results show that, if TGF acts on the distal portion of the preglomerular vasculature, then any TGF-induced vasoconstriction should raise upstream intravascular pressure and, thereby, trigger a myogenic (AMYO) response. The model further predicts that the magnitude of the AMYO response can be similar in magnitude to the TGF-induced increment in afferent resistance. Hence, the effects of TGF excitation on whole kidney hemodynamics may be much greater than predicted from measurements in single nephrons. Moreover, a significant fraction of the intrinsic myogenic autoregulatory response to increased renal perfusion pressure may result from a synergistic interaction between the TGF and myogenic mechanisms.

Effects of bradykinin and papaverine on renal autoregulation and renin release in the anaesthetized dog

The present study on six anaesthetized dogs investigates the influences of two different vasodilators, bradykinin and papaverine, on the relationship between autoregulation of renal blood flow and glomerular filtration rate, sodium excretion and renin release. At control conditions renal blood flow and glomerular filtration rate was autoregulated to the same levels of renal arterial pressure, 55 +/- 3 and 58 +/- 3 mmHg, respectively. Renin release increased from 0.3 +/- 0.1 to 22 +/- 4 micrograms AI min-1, and sodium excretion decreased from 99 +/- 29 to 4.6 +/- 3.3 mumol min-1 when renal arterial pressure was reduced from 122 +/- 6 to 44 +/- 2 mmHg. Infusion of bradykinin (50 ng kg-1 min-1) increased renal blood flow by 50% at control blood pressure without changing glomerular filtration rate, and both renal blood flow and glomerular filtration rate autoregulated to the same pressure levels as during control. Sodium excretion increased threefold at control renal arterial pressure, but was unchanged at low renal arterial pressure. Bradykinin did not change renin release neither at control nor low renal arterial pressure. Papaverine infusion at a rate of 4 mg min-1 increased renal blood flow 50% without changing glomerular filtration rate. The lower pressure limits of renal blood flow and glomerular filtration rate autoregulation were increased to 94 +/- 6 and 93 +/- 6 mmHg, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

A multinephron model of renal blood flow autoregulation by tubuloglomerular feedback and myogenic response

Tubuloglomerular feedback implies that a primary increase in arterial pressure, renal blood flow, glomerular filtration and increased flow rate in the distal tubule increase preglomerular resistance and thereby counteract the primary rise in glomerular filtration rate and renal blood flow. Tubuloglomerular feedback has therefore been assumed to play a role in renal autoregulation, i.e., the constancy of renal blood flow and glomerular filtration at varying arterial pressure. In evaluating this hypothesis, the numerous tubular and vascular mechanisms involved have called for mathematical models. Based on a single nephron model we have previously concluded that tubuloglomerular feedback can account for only a small part of blood flow autoregulation. We now present a more realistic multinephron model, consisting of one interlobular artery with an arbitrary number of evenly spaced afferent arterioles. Feedback from the distal tubule was simulated by letting glomerular blood flow exert a positive feedback on preglomerular resistance, in each case requiring compatibility with experimental open-loop responses in the most superficial nephron. The coupling together of 10 nephrons per se impairs autoregulation of renal blood flow compared to that of a single nephron model, but this effect is more than outweighed by greater control resistance in deep arterioles. Some further improvement was obtained by letting the contractile response spread from each afferent arteriole to the nearest interlobular artery segment. Even better autoregulation was provided by spreading of full strength contraction also to the nearest upstream or downstream afferent arteriole, and spread to both caused a renal blood flow autoregulation approaching experimental observations. However, when the spread effect was reduced to 25% of that in each stimulated afferent arteriole, more compatible with recent experimental observations, the autoregulation was greatly impaired. Some additional mechanism seems necessary, and we found that combined myogenic response in interlobular artery and tubuloglomerular feedback regulation of afferent arterioles can mimic experimental pressure-flow curves.

Pressure-induced vasoconstriction of renal microvessels in normotensive and hypertensive rats. Studies in the isolated perfused hydronephrotic kidney

The capacity of small arteries to respond to increased intravascular pressure may be altered in hypertension. In the kidney, hypertension is associated with a compensatory shift in the autoregulatory response to pressure. To directly determine the effects of established hypertension on the renal microvascular response to changes of perfusion pressure, we evaluated pressure-induced vasoconstriction in hydronephrotic kidneys isolated from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Vessel diameters of interlobular arteries (ILAs) and afferent and efferent arterioles were determined by computer-assisted videomicroscopy during alterations in renal arterial pressure (RAP) from 80 to 180 mm Hg. Increased RAP induced a pressure-dependent vasoconstriction in preglomerular vessels (afferent arterioles and ILAs), but not in postglomerular vessels (efferent arterioles). The calcium antagonist nifedipine prevented pressure-induced afferent arteriolar vasoconstriction with a similar half-maximal inhibitory concentration (IC50) (WKY, 63 +/- 27 vs. SHR, 60 +/- 32 nM). The pressure-activation curves for ILAs in SHR and WKY were similar. In contrast, the pressure-activation curve for afferent arterioles in SHR kidneys exhibited a rightward shift, which was observed at every segment of the afferent arteriole (i.e., near ILA, at midportion, and near glomerulus). These findings demonstrate that the ILA and the afferent arteriole both possess the ability to constrict in response to increased pressure, whereas this property is lacking in the efferent arteriole. Hypertension was associated with a compensatory shift in the pressure response of the afferent arteriole, such that higher RAPs were required to elicit vasoconstriction in this vessel.

Haemodynamic regulation of renal prostaglandin and renin release

To examine the relationship between renal release of the prostaglandins E2 (PGE2) and I2 (PGI2) and renin during autoregulatory vasodilation, experiments were performed in anaesthetized dogs with denervated kidneys. Autoregulatory vasodilation was induced by reducing renal arterial pressure (RAP) or by raising ureteral pressure in steps. During progressive renal arterial constriction, PGE2 and PGI2 release reached maximal values (10.6 +/- 1.7 for PGE2 and 6.6 +/- 1.1 pmol min-1 for PGI2 release) at RAP of 70-80 mmHg, associated with almost no increase in renin release. By further reduction of RAP, prostaglandin release was not significantly altered, whereas renin release reached maximal values (18.7 +/- 2.4 micrograms AI min-1) when autoregulatory vasodilation was complete at RAP below 55-60 mmHg. During progressive elevation of ureteral pressure, the release of PGE2, PGI2 and renin increased in concert in a curvilinear fashion, reaching maximal values at a ureteral pressure of 85 mmHg. There was no further increase during ureteral occlusion and the plateau values averaged 23.6 +/- 3.7 pmol min-1 for PGE2, 8.0 +/- 1.6 pmol min-1 for PGI2 and 16.6 +/- 3.4 micrograms AI min-1 for renin. We conclude that vascular dilation enhances both prostaglandin and renin release. During reduction of RAP, preglomerular arteries are dilated at higher RAP than are afferent arterioles. Release of prostaglandins synthetized in arteries consequently occurs at higher RAP than release of renin, which is not enhanced until afferent arterioles ultimately dilate at RAP approaching 60 mmHg. In contrast, elevation of ureteral pressure provides nearly uniform enhancement of prostaglandin and renin release, indicating a more uniform dilation of the whole preglomerular vascular tree.

Angiotensin II receptors in rabbit renal preglomerular vessels

Controversy exists regarding the specific sites within the renal microcirculation affected by angiotensin II (ANG II). Under some conditions, ANG II can elicit direct vasoconstrictor responses in the preglomerular vessels and efferent arterioles. These experiments were designed to evaluate the binding of 125I-ANG II in preglomerular vessels. Arcuate and interlobular arteries, with attached proximal segments of afferent arterioles, were microdissected from rabbit renal cortexes. A membrane preparation was obtained from the pooled freshly dissected vessels and utilized in an ANG II radioreceptor assay on the same day. Binding site concentrations [N] and dissociation constants [KD] were obtained by Scatchard analyses of binding inhibition data. Specific binding was saturable and reversible. The dissociation of bound ANG II was enhanced in the presence of a nonhydrolyzable analogue of GTP. Linear Scatchard plots were obtained, indicating the presence of a single class of high-affinity binding sites. The KD and N are similar to those for ANG II receptors in extrarenal vascular tissue. The order of binding inhibition potencies of ANG analogues was [Sar1,Ile8]-ANG II much greater than [Sar1,Ala8]ANG II = ANG II = ANG III much greater than ANG I, which is consistent with in vivo observations of the effects of these analogues on renal blood flow. The binding inhibition potencies of ANG III and [Sar1,Ile8]ANG II were greater in renal compared with reported values for extrarenal vasculature and rabbit glomeruli. Furthermore, there were no differences in ANG II receptor parameters in preglomerular vessels obtained from pregnant and nonpregnant rabbits.(ABSTRACT TRUNCATED AT 250 WORDS)