Modulation of the expression of connexins 37, 40, and 43 in endothelial cells in a culture.

Connexin (Cx) 37, 40, and 43 are implicated in vascular function, specifically in the electrical coupling of endothelial cells and vascular smooth-muscle cells. In the present study, we investigated whether factors implicated in vascular dysfunction can modulate the gene expression of Cx37, Cx40, and Cx43 and whether this is associated with changes in endothelial layer barrier function in human microvascular endothelial cells (HMEC-1). First, HMEC-1 were subjected to stimuli for 4 and 8 h. We tested their responses to DETA-NONOate, H2O2, high glucose, and angiotensin II, none of which relevantly affected the transcription of the connexin genes. Next, we tested inflammatory factors IL-6, interferon gamma (IFNγ), and TNFα. IFNγ (10 ng/mL) consistently induced Cx40 expression at 4 and 8 h to 10-20-fold when corrected for the control. TNFα and IL-6 resulted in small but significant depressions of Cx37 expression at 4 h. Two JAK inhibitors, epigallocatechin-3-gallate (EGCG) (100-250 µM) and AG490 (100-250 µM), dose-dependently inhibited the induction of Cx40 expression by IFNγ. Subsequently, HMEC-1 were subjected to 10 ng/mL IFNγ for 60 h, and intercellular and transcellular impedance was monitored by electric cell-substrate impedance sensing (ECIS). In response to IFNγ, junctional-barrier impedance increased more than cellular-barrier impedance; this was prevented by AG490 (5 µM). In conclusion, IFNγ can strongly induce Cx40 expression and modify the barrier properties of the endothelial cell membrane through the JAK/STAT pathway. Moreover, the Cx37, Cx40, and Cx43 expression in endothelial cells is stable and, apart from IFNγ, not affected by a number of factors implicated in endothelial dysfunction and vascular diseases.

Reduced tubuloglomerular feedback activity and absence of its synchronization in a connexin40 knockout rat.

Introduction: Tubuloglomerular feedback (TGF) is the negative feedback component of renal blood flow (RBF) autoregulation. Neighbouring nephrons often exhibit spontaneous TGF oscillation and synchronization mediated by endothelial communication, largely via connexin40 (Cx40). Methods: We had a knockout (KO) rat made that lacks Cx40. One base pair was altered to create a stop codon in exon 1 of Gja5, the gene that encodes Cx40 (the strain is WKY-Gja55em1Mcwi). Blood pressure (BP)-RBF transfer functions probed RBF dynamics and laser speckle imaging interrogated the dynamics of multiple efferent arterioles that reach the surface (star vessels). Results: The distribution of wild type (WT), heterozygote, and KO pups at weaning approximated the Mendelian ratio of 1:2:1; growth did not differ among the three strains. The KO rats were hypertensive. BP-RBF transfer functions showed low gain of the myogenic mechanism and a smaller TGF resonance peak in KO than in WT rats. Laser speckle imaging showed that myogenic mechanism had higher frequency in KO than in WT rats, but similar maximum spectral power. In contrast, the TGF frequency was similar while peak power of its oscillation was much smaller in KO than in WT rats. In WT rats, plots of instantaneous TGF phase revealed BP-independent TGF synchronization among star vessels. The synchronization could be both prolonged and widespread. In KO rats TGF synchronization was not seen, although BP transients could elicit short-lived TGF entrainment. Discussion: Despite the reduced TGF spectral power in KO rats, there was sufficient TGF gain to induce oscillations and therefore enough gain to be effective locally. We conclude that failure to synchronize is dependent, at least in part, on impaired conducted vasomotor responses.

Chronic, Combined Cardiac and Renal Dysfunction Exacerbates Renal Venous Pressure-Induced Suppression of Renal Function in Rats.

BACKGROUND AND OBJECTIVE: Increased renal venous pressure (RVP) is common in combined heart and kidney failure. We previously showed that acute RVP elevation depresses renal blood flow (RBF), glomerular filtration rate (GFR), and induces renal vasoconstriction in the absence of changes in blood pressure in healthy rats. We used our established rodent model of chronic combined heart and kidney failure (H/KF) to test whether RVP elevation would impair cardiovascular stability, renal perfusion and exacerbate renal dysfunction. METHODS: Male rats were subjected to 5/6 nephrectomy (SNx or Sham) and 6% high salt diet followed 7 weeks later by ligation of the left anterior descending coronary artery (CL or Sham). Experimental groups: CL + SNx (n = 12), Sham CL + SNx (n = 9), CL+ Sham SNx (n = 6), and Sham Control (n = 6). Six weeks later, anesthetized rats were subjected to an acute experiment whereupon mean arterial pressure (MAP), heart rate (HR), RVP, RBF, and GFR were measured at baseline and during elevation of RVP to 20-25 mmHg for 120 min. RESULTS: Baseline MAP, HR, RBF, and renal vascular conductance (RVC) were comparable among groups. Baseline GFR was significantly depressed in CL + SNx and Sham CL + SNx groups compared to Sham Control and CL + Sham SNx groups. Upon RVP increase, MAP and HR fell in all groups. Increased RVP exacerbated the reduction in RBF in CL + SNx (-6.4 ± 0.9 ml/min) compared to Sham Control (-3.7 ± 0.9 ml/min, p < 0.05) with intermediate responses in Sham CL + SNx (-6.8 ± 1.3 ml/min) and CL + Sham SNx (-5.1 ± 0.4 ml/min) groups. RVP increase virtually eliminated GFR in CL + SNx (-99 ± 1%), Sham CL + SNx (-95 ± 5%), and CL + Sham SNx (-100%) groups compared to Sham Control (-84 ± 15% from baseline; p < 0.05). Renal vascular conductance dropped significantly upon RVP increase in rats with HF (CL + SNx: -0.035 ± 0.011; CL + Sham SNx: -0.050 ± 0.005 ml/min·mmHg-1, p < 0.05) but not Sham CL + SNx (-0.001 ± 0.019 ml/min·mmHg-1) or Control (-0.033 ± mL/min·mmHg-1). CONCLUSION: Chronic combined heart and kidney failure primarily impairs renal hemodynamic stability in response to elevated RVP compared to healthy rats.

Angiotensin II and the Renal Hemodynamic Response to an Isolated Increased Renal Venous Pressure in Rats.

Elevated central venous pressure increases renal venous pressure (RVP) which can affect kidney function. We previously demonstrated that increased RVP reduces renal blood flow (RBF), glomerular filtration rate (GFR), and renal vascular conductance (RVC). We now investigate whether the RAS and RBF autoregulation are involved in the renal hemodynamic response to increased RVP. Angiotensin II (ANG II) levels were clamped by infusion of ANG II after administration of an angiotensin-converting enzyme (ACE) inhibitor in male Lewis rats. This did not prevent the decrease in ipsilateral RBF (-1.9±0.4ml/min, p<0.05) and GFR (-0.77±0.18ml/min, p<0.05) upon increased RVP; however, it prevented the reduction in RVC entirely. Systemically, the RVP-induced decline in mean arterial pressure (MAP) was more pronounced in ANG II clamped animals vs. controls (-22.4±4.1 vs. -9.9±2.3mmHg, p<0.05), whereas the decrease in heart rate (HR) was less (-5±6bpm vs. -23±4bpm, p<0.05). In animals given vasopressin to maintain a comparable MAP after ACE inhibition (ACEi), increased RVP did not impact MAP and HR. RVC also did not change (0.018±0.008ml/minˑmmHg), and the reduction of GFR was no longer significant (-0.54±0.15ml/min). Furthermore, RBF autoregulation remained intact and was reset to a lower level when RVP was increased. In conclusion, RVP-induced renal vasoconstriction is attenuated when ANG II is clamped or inhibited. The systemic effect of increased RVP, a decrease in HR related to a mild decrease in blood pressure, is attenuated also during ANG II clamp. Last, RBF autoregulation remains intact when RVP is elevated and is reduced to lower levels of RBF. This suggests that in venous congestion, the intact RBF autoregulation could be partially responsible for the vasoconstriction.

Surgical trauma is associated with renal immune cell activation in rats: A microarray study.

Acute kidney injury (AKI) is a common perioperative complication that is associated with increased mortality. This study investigates the renal gene expression in male Long-Evans rats after prolonged anesthesia and surgery to detect molecular mechanisms that could predispose the kidneys to injury upon further insults. Healthy and streptozotocin diabetic rats that underwent autoregulatory investigation in an earlier study were compared to rats that were sacrificed quickly for mRNA quantification in the same study. Prolonged surgery caused massive changes in renal mRNA expression by microarray analysis, which was validated by quantitative real-time PCR with good correlation. Furthermore, bioinformatics analysis using gene ontology and pathway analysis identified biological processes involved in immune system activation, such as immune system processes (p = 1.3 × 10-80 ), immune response (p = 1.3 × 10-60 ), and regulation of cytokine production (p = 1.7 × 10-52 ). PCR analysis of specific cell type markers indicated that the gene activation in kidneys was most probably macrophages, while granulocytes and T cell appeared less activated. Immunohistochemistry was used to quantify immune cell infiltration and showed no difference between groups indicating that the genetic activation depends on the activation of resident cells, or infiltration of a relatively small number of highly activated cells. In follow-up experiments, surgery was performed on healthy rats under standard and sterile condition showing similar expression of immune cell markers, which suggests that the inflammation was indeed caused by the surgical trauma rather than by bacterial infection. In conclusion, surgical trauma is associated with rapid activation of immune cells, most likely macrophages in rat kidneys.

Tubuloglomerular Feedback Synchronization in Nephrovascular Networks.

To perform their functions, the kidneys maintain stable blood perfusion in the face of fluctuations in systemic BP. This is done through autoregulation of blood flow by the generic myogenic response and the kidney-specific tubuloglomerular feedback (TGF) mechanism. The central theme of this paper is that, to achieve autoregulation, nephrons do not work as single units to manage their individual blood flows, but rather communicate electrically over long distances to other nephrons via the vascular tree. Accordingly, we define the nephrovascular unit (NVU) to be a structure consisting of the nephron, glomerulus, afferent arteriole, and efferent arteriole. We discuss features that require and enable distributed autoregulation mediated by TGF across the kidney. These features include the highly variable topology of the renal vasculature which creates variability in circulation and the potential for mismatch between tubular oxygen demand and delivery; the self-sustained oscillations in each NVU arising from the autoregulatory mechanisms; and the presence of extensive gap junctions formed by connexins and their properties that enable long-distance transmission of TGF signals. The existence of TGF synchronization across the renal microvascular network enables an understanding of how NVUs optimize oxygenation-perfusion matching while preventing transmission of high systemic pressure to the glomeruli, which could lead to progressive glomerular and vascular injury.

Chronic elevation of renal venous pressure induces extensive renal venous collateral formation and modulates renal function and cardiovascular stability in rats.

Acutely increased renal venous pressure (RVP) impairs renal function, but the long-term impact is unknown. We investigated whether chronic RVP elevation impairs baseline renal function and prevents exacerbation of renal dysfunction and cardiovascular instability upon further RVP increase. RVP elevation (20-25 mmHg) or sham operation (sham) was performed in rats. After 1 wk (n = 17) or 3 wk (n = 22), blood pressure, RVP, renal blood flow (RBF), renal vascular conductance (RVC), and glomerular filtration rate (GFR) were measured at baseline and during superimposed RVP increase. Chronic RVP elevation induced extensive renal venous collateral formation. RVP fell to 6 ± 1 mmHg at 1 wk and 3 ± 1 mmHg at 3 wk. Baseline blood pressure and heart rate were unaltered compared with sham. RBF, RVC, and GFR were reduced at 1 wk but normalized by 3 wk. Upon further RVP increase, the drop in mean arterial pressure was attenuated at 3 wk compared with 1 wk (P < 0.05), whereas heart rate fell comparably across all groups; the mean arterial pressure-heart rate relationship was disrupted at 1 and 3 wk. RBF fell to a similar degree as sham at 1 wk (-2.3 ± 0.7 vs. -3.9 ± 1.2 mL/min, P = 0.066); however, at 3 wk, this was attenuated compared with sham (-1.5 ± 0.5 vs. -4.2 ± 0.7 mL/min, P < 0.05). The drop in RVC and GFR was attenuated at 1 and 3 wk (P < 0.05). Thus, chronic RVP elevation induced by partial renal vein ligation elicits extensive renal venous collateral formation, and although baseline renal function is impaired, chronic RVP elevation in this manner induces protective adaptations in kidneys of healthy rats, which attenuates the hemodynamic response to further RVP increase.

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.

Sodium intake but not renal nerves attenuates renal venous pressure-induced changes in renal hemodynamics in rats.

Increased central venous pressure and renal venous pressure (RVP) are associated with worsening of renal function in acute exacerbation of congestive heart failure. We tested whether an acute isolated elevation of RVP in one kidney leads to ipsilateral renal vasoconstriction and decreased glomerular filtration rate (GFR) and whether this depends on dietary salt intake or activation of renal nerves. Male Lewis rats received a normal (1% NaCl, NS) or high-salt (6% NaCl) diet for ≥14 days before the acute experiment. Rats were then randomized into the following three groups: time control and RVP elevation to either 10 or 20 mmHg to assess heart rate, renal blood flow (RBF), and GFR. To increase RVP, the left renal vein was partially occluded for 120 min. To determine the role of renal nerves, surgical denervation was conducted in rats on both diets. Renal sympathetic nerve activity (RSNA) was additionally recorded in a separate group of rats. Increasing RVP to 20 mmHg decreased ipsilateral RBF (7.5 ± 0.4 to 4.1 ± 0.7 ml/min, P < 0.001), renal vascular conductance (0.082 ± 0.006 to 0.060 ± 0.011 ml·min-1·mmHg-1, P < 0.05), and GFR (1.28 ± 0.08 to 0.40 ± 0.13 ml/min, P < 0.05) in NS rats. The reduction was abolished by high-salt diet but not by renal denervation. Furthermore, a major increase of RVP (1.6 ± 0.8 to 24.7 ± 1.2 mmHg) immediately suppressed RSNA and decreased heart rate ( P < 0.05), which points to suppression of both local and systemic sympathetic activity. Taken together, acute elevated RVP induces renal vasoconstriction and decreased GFR, which is more likely to be mediated via the renin-angiotensin system than via renal nerves.

Understanding the Two Faces of Low-Salt Intake.

Fierce debate has developed whether low-sodium intake, like high-sodium intake, could be associated with adverse outcome. The debate originates in earlier epidemiological studies associating high-sodium intake with high blood pressure and more recent studies demonstrating a higher cardiovascular event rate with both low- and high-sodium intake. This brings into question whether we entirely understand the consequences of high- and (very) low-sodium intake for the systemic hemodynamics, the kidney function, the vascular wall, the immune system, and the brain. Evolutionarily, sodium retention mechanisms in the context of low dietary sodium provided a survival advantage and are highly conserved, exemplified by the renin-angiotensin system. What is the potential for this sodium-retaining mechanism to cause harm? In this paper, we will consider current views on how a sodium load is handled, visiting aspects including the effect of sodium on the vessel wall, the sympathetic nervous system, the brain renin-angiotensin system, the skin as "third compartment" coupling to vascular endothelial growth factor C, and the kidneys. From these perspectives, several mechanisms can be envisioned whereby a low-sodium diet could potentially cause harm, including the renin-angiotensin system and the sympathetic nervous system. Altogether, the uncertainties preclude a unifying model or practical clinical guidance regarding the effects of a low-sodium diet for an individual. There is a very strong need for fundamental and translational studies to enhance the understanding of the potential adverse consequences of low-salt intake as an initial step to facilitate better clinical guidance.

A gap junction inhibitor, carbenoxolone, induces spatiotemporal dispersion of renal cortical perfusion and impairs autoregulation.

Renal autoregulation dynamics originating from the myogenic response (MR) and tubuloglomerular feedback (TGF) can synchronize over large regions of the kidney surface, likely through gap junction-mediated electrotonic conduction and reflecting distributed operation of autoregulation. We tested the hypotheses that inhibition of gap junctions reduces spatial synchronization of autoregulation dynamics, abrogates spatial and temporal smoothing of renal perfusion, and impairs renal autoregulation. In male Long-Evans rats, we infused the gap junction inhibitor carbenoxolone (CBX) or the related glycyrrhizic acid (GZA) that does not block gap junctions into the renal artery and monitored renal blood flow (RBF) and surface perfusion by laser speckle contrast imaging. Neither CBX nor GZA altered RBF or mean surface perfusion. CBX preferentially increased spatial and temporal variation in the distribution of surface perfusion, increased spatial variation in the operating frequencies of the MR and TGF, and reduced phase coherence of TGF and increased its dispersion. CBX, but not GZA, impaired dynamic and steady-state autoregulation. Separately, infusion of the Rho kinase inhibitor Y-27632 paralyzed smooth muscle, grossly impaired dynamic autoregulation, and monotonically increased spatial variation of surface perfusion. These data suggest CBX inhibited gap junction communication, which in turn reduced the ability of TGF to synchronize among groups of nephrons. The results indicate that impaired autoregulation resulted from degraded synchronization, rather than the reverse. We show that network behavior in the renal vasculature is necessary for effective RBF autoregulation.

Transient impairment of dynamic renal autoregulation in early diabetes mellitus in rats.

Renal autoregulation is impaired in early (1 wk) diabetes mellitus (DM) induced by streptozotocin, but effective in established DM (4 wk). Furthermore nitric oxide synthesis (NOS) inhibition with N(G)-nitro-L-arginine methyl ester (L-NAME) significantly improved autoregulation in early DM but not in established DM. We hypothesized that autoregulation is transiently impaired in early DM because of increased NO availability in the kidney. Because of the conflicting evidence available for a role of NO in DM, we tested the hypothesis that DM reduces autoregulation effectiveness by reducing the spatial similarity of autoregulation. Male Long-Evans rats were divided into control (CON) and diabetic (DM; streptozotocin) groups and followed for either 1 wk (CON1, n = 6; DM1, n = 5) or 4 wk (CON4, n = 7; DM4, n = 7). At the end of the experiment, dynamic autoregulation was assessed in isoflurane-anesthetized rats by whole kidney RBF during baseline, NOS1 inhibition, and nonselective NOS inhibition. Kidney surface perfusion, monitored with laser speckle contrast imaging, was used to assess spatial heterogeneity of autoregulation. Autoregulation was significantly impaired in DM1 rats and not impaired in DM4 rats. L-NAME caused strong renal vasoconstriction in all rats, but did not significantly affect autoregulation dynamics. Autoregulation was more spatially heterogeneous in DM1, but not DM4. Therefore, our results, which are consistent with transient impairment of autoregulation in DM, argue against the hypothesis that this impairment is NO-dependent, and suggest that spatial properties of autoregulation may also contribute to reduced autoregulatory effectiveness in DM1.

Laser speckle contrast imaging reveals large-scale synchronization of cortical autoregulation dynamics influenced by nitric oxide.

Synchronization of tubuloglomerular feedback (TGF) dynamics in nephrons that share a cortical radial artery is well known. It is less clear whether synchronization extends beyond a single cortical radial artery or whether it extends to the myogenic response (MR). We used LSCI to examine cortical perfusion dynamics in isoflurane-anesthetized, male Long-Evans rats. Inhibition of nitric oxide synthases by N(ω)-nitro-l-arginine methyl ester (l-NAME) was used to alter perfusion dynamics. Phase coherence (PC) was determined between all possible pixel pairs in either the MR or TGF band (0.09-0.3 and 0.015-0.06 Hz, respectively). The field of view (≈4 × 5 mm) was segmented into synchronized clusters based on mutual PC. During the control period, the field of view was often contained within one cluster for both MR and TGF. PC was moderate for TGF and modest for MR, although significant in both. In both MR and TGF, PC exhibited little spatial variation. After l-NAME, the number of clusters increased in both MR and TGF. MR clusters became more strongly synchronized while TGF clusters showed small highly coupled, high-PC regions that were coupled with low PC to the remainder of the cluster. Graph theory analysis probed modularity of synchronization. It confirmed weak synchronization of MR during control that probably was not physiologically relevant. It confirmed extensive and long-distance synchronization of TGF during control and showed increased modularity, albeit with larger modules seen in MR than in TGF after l-NAME. The results show widespread synchronization of MR and TGF that is differentially affected by nitric oxide.

Segmentation of renal perfusion signals from laser speckle imaging into clusters with phase synchronized dynamics.

Renal perfusion signals contain dynamics arising from the renal autoregulation feedback mechanisms as the contraction and dilation of vessels alter flow patterns. We can capture the time-varying dynamics at points across the renal surface using laser speckle imaging. We segment an imaged area of the renal cortex into clusters with phase synchronized dynamics. Our approach first uses phase coherence with a surrogate data derived threshold to identify synchronized pixel pairs. Non-negative matrix factorization is then applied to segment phase coherence estimates into phase synchronized regions. The method is applied to laser speckle imaging of the renal cortex of anaesthetized rats to identify regions on the renal surface with phase synchronized myogenic activity. In three out of six animals imaged after bolus infusion of N(ω)-nitro-l-arginine methyl ester (NAM), the renal surfaces are segmented into clusters with high phase coherence. No more than two clusters were identified during control period for any animal. In the remaining three animals, a strong myogenic signal could not be detected in surface perfusion during control or NAM. This method can be used to identify synchronization in renal autoregulation dynamics across the renal surface.

Renal blood flow dynamics in inbred rat strains provides insight into autoregulation.

Renal autoregulation maintains stable renal blood flow in the face of constantly fluctuating blood pressure. Autoregulation is also the only mechanism that protects the delicate glomerular capillaries when blood pressure increases. In order to understand autoregulation, the renal blood flow response to changing blood pressure is studied. The steadystate response of blood flow is informative, but limits investigation of the individual mechanisms of autoregulation. The dynamics of autoregulation can be probed with transfer function analysis. The frequency-domain analysis of autoregulation allows investigators to probe the relative activity of each mechanism of autoregulation. We discuss the methodology and interpretation of transfer function analysis. Autoregulation is routinely studied in the rat, of which there are many inbred strains. There are multiple strains of rat that are either selected or inbred as models of human pathology. We discuss relevant characteristics of Brown Norway, Spontaneously hypertensive, Dahl, and Fawn-Hooded hypertensive rats and explore differences among these strains in blood pressure, dynamic autoregulation, and susceptibility to hypertensive renal injury. Finally we show that the use of transfer function analysis in these rat strains has contributed to our understanding of the physiology and pathophysiology of autoregulation and hypertensive renal disease.Interestingly all these strains demonstrate effective tubuloglomerular feedback suggesting that this mechanism is not sufficient for effective autoregulation. In contrast, obligatory or conditional failure of the myogenic mechanism suggests that this component is both necessary and sufficient for autoregulation.

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.

Time-frequency approaches for the detection of interactions and temporal properties in renal autoregulation.

We compare the influence of time-frequency methods on analysis of time-varying renal autoregulation properties. Particularly, we examine if detection probabilities are similar for amplitude and frequency modulation for a modulated simulation signal among five time-frequency approaches, and if time-varying changes in system gain are detected using four approaches for estimating time-varying transfer functions. Detection of amplitude and frequency modulation varied among methods and was dependent upon background noise added to the simulated data. Three non-parametric time-frequency methods accurately detected modulation at low frequencies across noise levels but not high frequencies; while the converse was true for a fourth, and a fifth non-parametric approach was not capable of modulation detection. When applied to estimation of time-varying transfer functions, the parametric approach provided the most accurate estimations of system gain changes, detecting a 1 dB step increase. Application of the appropriate methods to laser Doppler recordings of cortical blood flow and arterial pressure data in anesthetized rats reaffirm the presence of time-varying dynamics in renal autoregulation. An increase in the peak system gain and detection of amplitude modulation of the Myogenic mechanism both occurred after inhibition of nitric oxide synthase, suggesting a connection between the operation of underlying regulators and system performance.

Systemic arterial and venous determinants of renal hemodynamics in congestive heart failure.

Heart and kidney interactions are fascinating, in the sense that failure of the one organ strongly affects the function of the other. In this review paper, we analyze how principal driving forces for glomerular filtration and renal blood flow are changed in heart failure. Moreover, renal autoregulation and modulation of neurohumoral factors, which can both have repercussions on renal function, are analyzed. Two paradigms seem to apply. One is that the renin-angiotensin system (RAS), the sympathetic nervous system (SNS), and extracellular volume control are the three main determinants of renal function in heart failure. The other is that the classical paradigm to analyze renal dysfunction that is widely applied in nephrology also applies to the pathophysiology of heart failure: pre-renal, intra-renal, and post-renal alterations together determine glomerular filtration. At variance with the classical paradigm is that the most important post-renal factor in heart failure seems renal venous hypertension that, by increasing renal tubular pressure, decreases GFR. When different pharmacological strategies to inhibit the RAS and SNS and to assist renal volume control are considered, there is a painful lack in knowledge about how widely applied drugs affect primary driving forces for ultrafiltration, renal autoregulation, and neurohumoral control. We call for more clinical physiological studies.

Nephron blood flow dynamics measured by laser speckle contrast imaging.

Tubuloglomerular feedback (TGF) has an important role in autoregulation of renal blood flow and glomerular filtration rate (GFR). Because of the characteristics of signal transmission in the feedback loop, the TGF undergoes self-sustained oscillations in single-nephron blood flow, GFR, and tubular pressure and flow. Nephrons interact by exchanging electrical signals conducted electrotonically through cells of the vascular wall, leading to synchronization of the TGF-mediated oscillations. Experimental studies of these interactions have been limited to observations on two or at most three nephrons simultaneously. The interacting nephron fields are likely to be more extensive. We have turned to laser speckle contrast imaging to measure the blood flow dynamics of 50-100 nephrons simultaneously on the renal surface of anesthetized rats. We report the application of this method and describe analytic techniques for extracting the desired data and for examining them for evidence of nephron synchronization. Synchronized TGF oscillations were detected in pairs or triplets of nephrons. The amplitude and the frequency of the oscillations changed with time, as did the patterns of synchronization. Synchronization may take place among nephrons not immediately adjacent on the surface of the kidney.