Andreas Patzak

Iodixanol, Constriction of Medullary Descending Vasa Recta, and Risk for Contrast Medium–induced Nephropathy

PURPOSE To determine whether a type of contrast medium (CM), iodixanol, modifies outer medullary descending vasa recta (DVR) vasoreactivity and nitric oxide (NO) production in isolated microperfused DVR. MATERIALS AND METHODS Animal handling conformed to the Animal Care Committee Guidelines of all participating institutions. Single specimens of DVR were isolated from rats and perfused with a buffered solution containing iodixanol. A concentration of 23 mg of iodine per milliliter was chosen to mimic that expected to be used in usual examinations in humans. Luminal diameter was determined by using video microscopy, and NO was measured by using fluorescent techniques. RESULTS Iodixanol led to 52% reduction of DVR luminal diameter, a narrowing that might interfere with passage of erythrocytes in vivo. Vasoconstriction induced by angiotensin II was enhanced by iodixanol. Moreover, iodixanol decreased NO bioavailability by more than 82%. Use of 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (a superoxide dismutase mimetic) prevented both vasoconstriction with iodixanol alone and increased constriction with angiotensin II caused by CM. CONCLUSION Iodixanol in doses typically used for coronary interventions constricts DVR, intensifies angiotensin II-induced constriction, and reduces bioavailability of NO. CM-induced nephropathy may be related to these events and scavenging of reactive oxygen species might exert a therapeutic benefit by preventing the adverse effects that a CM has on medullary perfusion.

Does low frequency power of arterial blood pressure reflect sympathetic tone

We tested whether power spectral analysis of arterial blood pressure (ABP) is a feasible tool to detect differences in peripheral sympathetic nerve activity in normotensive and hypertensive rats with differing basal sympathetic tones. Nine Wistar Kyoto rats (WKY), 10 Sprague-Dawley rats (SD), 10 spontaneously hypertensive rats (SHR) and 9 hypertensive transgenic rats harbouring the mouse Ren-2 gene (TGR) were chronically instrumented with femoral artery catheters and nerve electrodes around the sympathetic major splanchnic nerve. Two days after surgery ABP and splanchnic nerve activity (SpNA) were recorded in the conscious state during basal conditions as well as during alpha 1-adrenergic receptor blockade. Power spectra and squared coherence in the low (LF, 0.02-0.20 Hz), mid (MF, 0.20-0.80 Hz) and high (HF, respiration peak +/- 0.3 Hz) frequency bands were calculated for ABP and SpNA. Mean blood pressure in SHR (133 +/- 8 mmHg) and TGR (142 +/- 8 mmHg) was significantly higher (P < 0.05) than in WKY (115 +/- 3 mmHg) and SD (95 +/- 4 mmHg). SpNA in SHR was higher than in WKY (23.4 +/- 6.4 microV vs. 11.6 +/- 0.8 microV, P < 0.05) while SpNA in TGR was lower than in SD (20.1 +/- 3.9 microV vs. 28.8 +/- 4.2 microV, P < 0.05). LF and MF components of ABP variability were not significantly higher in those rats with high sympathetic tones. However, alpha 1-adrenergic receptor blockade reduced LF and MF components of ABP and SpNA in all strains except SHR. LF and MF coherence was not greater in rats with high sympathetic tones than in those with low sympathetic tones. The reduction of LF and MF components of ABP variability by alpha 1-adrenergic receptor blockade indicates an important contribution of peripheral sympathetic nerve activity to LF and MF blood pressure variability on an acute basis. However, the lack of higher LF and MF power in the ABP spectra of those rats with high SpNA together with the finding that LF and MF coherence was not higher in those rats with high SpNA led to the conclusion that LF and MF spectral components of ABP do not appear to be suitable markers for the prevailing sympathetic nerve activity.

Endogenous Nitric Oxide Is a Key Promoting Factor for Initiation of Seizure-Like Events in Hippocampal and Entorhinal Cortex Slices

Nitric oxide (NO) modulates synaptic transmission, and its level is elevated during epileptic activity in animal models of epilepsy. However, the role of NO for development and maintenance of epileptic activity is controversial. We studied this aspect in rat organotypic hippocampal slice cultures and acute hippocampal–entorhinal cortex slices from wild-type and neuronal NO synthase (nNOS) knock-out mice combining electrophysiological and fluorescence imaging techniques. Slice cultures contained nNOS-positive neurons and an elaborated network of nNOS-positive fibers. Lowering of extracellular Mg2+ concentration led to development of epileptiform activity and increased NO formation as revealed by NO-selective probes, 4-amino-5-methylamino-2′,7′-difluorofluorescein and 1,2-diaminoanthraquinone sulfate. NO deprivation by NOS inhibitors and NO scavengers caused depression of both EPSCs and IPSCs and prevented initiation of seizure-like events (SLEs) in 75% of slice cultures and 100% of hippocampal–entorhinal cortex slices. This effect was independent of the guanylyl cyclase/cGMP pathway. Suppression of SLE initiation in acute slices from mice was achieved by both the broad-spectrum NOS inhibitor N-methyl-l-arginine acetate and the nNOS-selective inhibitor 7-nitroindazole, whereas inhibition of inducible NOS by aminoguanidine was ineffective, suggesting that nNOS activity was crucial for SLE initiation. Additional evidence was obtained from knock-out animals because SLEs developed in a significantly lower percentage of slices from nNOS−/− mice and showed different characteristics, such as prolongation of onset latency and higher variability of SLE intervals. We conclude that enhancement of synaptic transmission by NO under epileptic conditions represents a positive feedback mechanism for the initiation of seizure-like events.

QUANTITATIVE ANALYSIS OF CARDIORESPIRATORY SYNCHRONIZATION IN INFANTS

We investigate the phase synchronization of heartbeat and respiration in a group of healthy infants. Having presented and compared two quantitative measures of synchronization, we conclude that one of these measures — the conditional probability index — allows reliable detection of synchronous epochs of different order n:m and, thus, makes possible an automatic processing of large data sets. In our analysis of experimental time series, we have found numerous epochs of phase synchronization. It turned out that the average degree of synchronization varies with the age of the newborns.

Role of NOX2 in the regulation of afferent arteriole responsiveness

NADPH oxidases (NOX) are the major source of reactive oxygen species (ROS) in the vasculature and contribute to the control of renal perfusion. The role of NOX2 in the regulation of blood pressure and afferent arteriole responsiveness was investigated in NOX2(-/-) and wild-type mice. Arteriole constrictions to ANG II (10(-14)-10(-6) mol/l) were weaker in NOX2(-/-) compared with wild types. N(omega)-nitro-l-arginine methyl ester (l-NAME; 10(-4) mol/l) treatment reduced basal diameters significantly more in NOX2(-/-) (-18%) than in wild types (-6%) and augmented ANG II responses. Adenosine (10(-11)-10(-4) mol/l) constricted arterioles of wild types but not of NOX2(-/-). However, simultaneous inhibition of adenosine type-2 receptors induced vasoconstriction, which was stronger in NOX2(-/-). Adenosine (10(-8) mol/l) enhanced the ANG II response in wild type, but not in NOX2(-/-). This sensitizing effect by adenosine was abolished by apocynin. Chronic ANG II pretreatment (14 days) did not change the ANG II responses in NOX2(-/-), but strengthened the response in wild types. ANG II pretreatment augmented the l-NAME response in NOX2(-/-) (-33%), but not in wild types. Simultaneous application of l-NAME and ANG II caused a stronger constriction in the NOX2(-/-) (-64%) than in wild types (-46%). Basal blood pressures were similar in both genotypes, however, chronic ANG II infusion elevated blood pressure to a greater extent in wild-type (15 +/- 1%) than in NOX2(-/-) (8 +/- 1%) mice. In conclusion, NOX2 plays an important role in the control of afferent arteriole tone and is involved in the contractile responses to ANG II and/or adenosine. NOX2 can be activated by elevated ANG II and may play an important role in ANG II-induced hypertension. NOX2-derived ROS scavenges nitric oxide, causing subsequent nitric oxide-deficiency.

Angiotensin II-nitric oxide interaction in the kidney

Purpose of reviewThe balance of angiotensin II and nitric oxide determines the sensitivity of the tubuloglomerular feedback mechanism, renal vascular resistance and filtration rate. Angiotensin II induces nitric oxide release, but the role of angiotensin II receptors here is not fully understood. Further, the angiotensin II–nitric oxide interaction can be modulated by reactive oxygen species. This review focuses on the angiotensin II–nitric oxide interaction and their modulation by reactive oxygen species in the control of renal blood flow. Recent findingsIdeas about the role of angiotensin II type 1 and angiotensin II type 2 receptors are extended by the observation of angiotensin II type 1-mediated nitric oxide release with direct effects on vascular tone, tubuloglomerular feedback and sympathetic neurotransmission. Angiotensin receptors elicit disparate effects on intrarenal circulation. Angiotensin II–nitric oxide interactions are modulated by reactive oxygen species, as shown by angiotensin II type 1-mediated activation of superoxide and depression of antioxidant enzymes leading to reduced nitric oxide concentration – mechanisms that may be also important in angiotensin II-dependent hypertension. SummaryRecent studies show that angiotensin II stimulates the nitric oxide system via angiotensin II type 1 and angiotensin II type 2 receptors, whereas receptors exert different effects on renal and medullary flow. The interaction via angiotensin II type 1 is modulated by reactive oxygen species.

Adenosine Restores Angiotensin II–Induced Contractions by Receptor-Independent Enhancement of Calcium Sensitivity in Renal Arterioles

Adenosine is coupled to energy metabolism and regulates tissue blood flow by modulating vascular resistance. In this study, we investigated isolated, perfused afferent arterioles of mice, which were subjected to desensitization during repeated applications of angiotensin II. Exogenously applied adenosine restores angiotensin II–induced contractions by increasing calcium sensitivity of the arterioles, along with augmented phosphorylation of the regulatory unit of the myosin light chain. Adenosine restores angiotensin II–induced contractions via intracellular action, because inhibition of adenosine receptors do not prevent restoration, but inhibition of NBTI sensitive adenosine transporters does. Restoration was prevented by inhibition of Rho-kinase, protein kinase C, and the p38 mitogen-activated protein kinase, which modulate myosin light chain phosphorylation and thus calcium sensitivity in the smooth muscle. Furthermore, adenosine application increased the intracellular ATP concentration in LuciHEK cells. The results of the study suggest that restoration of the angiotensin II–induced contraction by adenosine is attributable to the increase of the calcium sensitivity by phosphorylation of the myosin light chain. This can be an important component of vascular control during ischemic and hypoxic conditions. Additionally, this mechanism may contribute to the mediation of the tubuloglomerular feedback by adenosine in the juxtaglomerular apparatus of the kidney.

High-frequency oscillations of the heart rate during ramp load reflect the human anaerobic threshold.

Abstract The aim of this study was to analyze the dynamic behavior of the high-frequency component (HF > 0.15 Hz) of heart rate variability (HRV) and the respiratory frequency in relation to the anaerobic threshold (AT). Twenty-two healthy subjects [mean (SD) age: 24 (6) years, height: 175 (10) cm, body mass: 65 (11) kg] completed a ramp load, with increments of 20 W · min−1, on a cycle ergometer. The AT was determined by the V-slope-method. Respiratory movements of the thorax, and the electrocardiogram were monitored. The instantaneous frequency of the HF component of HRV and of the respiratory signal were obtained by the Hilbert transformation. Both frequencies were closely related, the cross correlation coefficient being between 0.84 and 0.99. Various patterns of HRV and respiration were observed during the protocol. Remarkably, however, in over 90% of these cases, a shift in the instantaneous frequency of the HF component occurred during the transition from aerobic to anaerobic work. The difference between the AT determined by gas analysis and the AT evaluated as the power output (ATf), calculated using the approximation of the curve of the instantaneous frequency of HF by hyperbolic tangent functions, varied between 2 and 14%. In conclusion, the present study demonstrates significant changes in the behavior of the instantaneous frequency of HF in the region of the AT.

Superoxide Dismutase 1 Limits Renal Microvascular Remodeling and Attenuates Arteriole and Blood Pressure Responses to Angiotensin II via Modulation of Nitric Oxide Bioavailability

Oxidative stress is associated with vascular remodeling and increased preglomerular resistance that are both implicated in the pathogenesis of renal and cardiovascular disease. Angiotensin II induces superoxide production, which is metabolized by superoxide dismutase (SOD) or scavenged by NO. We investigated the hypothesis that SOD1 regulates renal microvascular remodeling, blood pressure, and arteriolar responsiveness and sensitivity to angiotensin II using SOD1-transgenic (SOD1-tg) and SOD1-knockout (SOD1-ko) mice. Blood pressure, measured telemetrically, rose more abruptly during prolonged angiotensin II infusion in SOD1-ko mice. The afferent arteriole media:lumen ratios were reduced in SOD1-tg and increased in SOD1-ko mice. Afferent arterioles from nontreated wild types had graded contraction to angiotensin II (sensitivity: 10−9 mol/L; responsiveness: 40%). Angiotensin II contractions were less sensitive (10−8 mol/L) and responsive (14%) in SOD1-tg but more sensitive (10−13 mol/L) and responsive (89%) in SOD1-ko mice. Arterioles from SOD1-ko had 4-fold increased superoxide formation with angiotensin II at 10−9 mol/L. NG-nitro-l-arginine methyl ester reduced arteriole diameter of SOD1-tg and enhanced angiotensin II sensitivity and responsiveness of wild-type and SOD1-tg mice to the level of SOD1-ko mice. SOD mimetic treatment with Tempol increased arteriole diameter and normalized the enhanced sensitivity and responsiveness to angiotensin II of SOD1-ko mice but did not affect wild-type or SOD1-tg mice. Neither SOD1 deficiency nor overexpression was associated with changes in nitrate/nitrite excretion or renal mRNA expression of NO synthase, NADPH oxidase, or SOD2/SOD3 isoforms and angiotensin II receptors. In conclusion, SOD1 limits afferent arteriole remodeling and reduces sensitivity and responsiveness to angiotensin II by reducing superoxide and maintaining NO bioavailability. This may prevent an early and exaggerated blood pressure response to angiotensin II.

CXCR4-overexpressing bone marrow-derived mesenchymal stem cells improve repair of acute kidney injury

Transplantation of bone marrow-derived mesenchymal stem cells (BMSCs) can repair acute kidney injury (AKI), but with limited effect. We test the hypothesis that CXCR4 overexpression improves the repair ability of BMSCs and that this is related to increased homing of BMSCs and increased release of cytokines. Hypoxia/reoxygenation-pretreated renal tubular epithelial cells (HR-RTECs) were used. BMSCs, null-BMSCs, and CXCR4-BMSCs were cocultured with HR-RTECs. The number of migrating BMSCs was counted. Proliferating cell nuclear antigen (PCNA) expression, cell death, and expressions of cleaved caspase-3 and Bcl-2 in cocultured HR-RTECs were measured. Cytokeratin 18 (CK18) expression and cytokine secretions of the BMSCs cultured with HR-RTEC supernatant were detected. BMSC homing, renal function, proliferation, and cell death of tubular cells were assayed in the AKI mouse model. CXCR4-BMSCs showed a remarkable expression of CXCR4. Stromal cell-derived factor-1 in the HR-RTEC supernatant was increased. Migration of BMSCs was CXCR4-dependent. Proportions of CK18(+) cells in BMSCs, null-BMSCs, and CXCR4-BMSCs showed no difference. However, CXCR4 overexpression in BMSCs stimulated secretion of bone morphogenetic protein-7, hepatocyte growth factor, and interleukin 10. The neutralizing anti-CXCR4 antibody AMD3100 abolished this. In cocultured HR-RTECs the proportions of PCNA(+) cells and Bcl-2 expression were enhanced; however, the proportion of annexin V(+) cells and expression of cleaved caspase-3 were reduced. The in vivo study showed increased homing of CXCR4-BMSCs in kidneys, which was associated with improved renal function, reduced acute tubular necrosis scoring, accelerated mitogenic response of tubular cells, and reduced tubular cell death. The enhanced homing and paracrine actions of BMSCs with CXCR4 overexpression suggest beneficial effects of such cells in BMSC-based therapy for AKI.