Hydronephrotic Kidney Model Research Articles

Systolic and mean blood pressures and afferent arteriolar myogenic response dynamics: a modeling approach

The afferent arteriolar myogenic response contributes to the autoregulation of renal blood flow (RBF) and glomerular filtration rate (GFR), and plays an essential role in protecting the kidney against hypertensive injury. Systolic blood pressure (SBP) is most closely linked to renal injury, and a myogenic response coupled to this signal would facilitate renal protection, whereas mean blood pressure (MBP) influences RBF and GFR. The relative role of SBP vs. MBP as the primary determinant of myogenic tone is an area of current controversy. Here, we describe two mathematical models, Model-Avg and Model-Sys, that replicate the different delays and time constants of vasoconstrictor and vasodilator phases of the myogenic responses of the afferent arteriole. When oscillating pressures are applied, the MBP determines the magnitude of the myogenic response of Model-Avg, and the SBP determines the response of Model-Sys. Simulations evaluating the responses of both models to square-wave pressure oscillations and to narrow pressure pulses show decidedly better agreement between Model-Sys and afferent arteriolar responses observed in cortical nephrons in the in vitro hydronephrotic kidney model. Analysis showing that the difference in delay times of the vasoconstrictor and vasodilator phases determines the frequency range over which SBP triggers Model-Sys's response was confirmed with simulations using authentic blood pressure waveforms. These observations support the postulate that SBP is the primary determinant of the afferent arteriole's myogenic response and indicate that differences in the delays in initiation vs. termination of the response, rather than in time constants, are integral to this phenomenon.

Effects of amiloride, benzamil, and alterations in extracellular Na+ on the rat afferent arteriole and its myogenic response

Recent studies have implicated epithelial Na+ channels (ENaC) in myogenic signaling. The present study was undertaken to determine if ENaC and/or Na+ entry are involved in the myogenic response of the rat afferent arteriole. Myogenic responses were assessed in the in vitro hydronephrotic kidney model. ENaC expression and membrane potential responses were evaluated with afferent arterioles isolated from normal rat kidneys. Our findings do not support a role of ENaC, in that ENaC channel blockers did not reduce myogenic responses and ENaC expression could not be demonstrated in this vessel. Reducing extracellular Na+ concentration ([Na+]o; 100 mmol/l) did not attenuate myogenic responses, and amiloride had no effect on membrane potential. Benzamil, an inhibitor of ENaC that also blocks Na+/Ca2+ exchange (NCX), potentiated myogenic vasoconstriction. Benzamil and low [Na+]o elicited vasoconstriction; however, these responses were attenuated by diltiazem and were associated with significant membrane depolarization, suggesting a contribution of mechanisms other than a reduction in NCX. Na+ repletion induced a vasodilation in pressurized afferent arterioles preequilibrated in low [Na+]o, a hallmark of NCX, and this response was reduced by 10 micromol/l benzamil. The dilation was eliminated, however, by a combination of benzamil plus ouabain, suggesting an involvement of the electrogenic Na+-K+-ATPase. In concert, these findings refute the premise that ENaC plays a significant role in the rat afferent arteriole and instead suggest that reducing [Na+](o) and/or Na+ entry is coupled to membrane depolarization. The mechanisms underlying these unexpected and paradoxical effects of Na+ are not resolved at the present time.

Renal microvascular constriction to membrane depolarization and other stimuli: pivotal role for rho-kinase

Contraction of vascular smooth muscle is determined not only by levels of intracellular free calcium but also by the sensitivity of its contractile apparatus. A potential modulator of the latter is rho-kinase. We addressed the question of a possible central role for rho-kinase in the regulation of periglomerular microvascular tone. In the rat hydronephrotic kidney model, diameter changes of distal interlobular arteries, afferent and efferent arterioles were measured using three distinctly different stimuli: intravascular pressure changes, angiotensin II (AngII) and membrane depolarization, which is a physiological component of many signaling pathways, as evoked in two ways. Two selective, structurally different rho-kinase inhibitors, Y-27632 and HA-1077, were employed, as well as a selective protein kinase C alpha inhibitor. Preglomerular vasoconstriction induced by direct membrane depolarization, increases in pressure or AngII all depended for their effect on rho-kinase. A differing role for rho-kinase in efferent arteriolar constriction to AngII as compared to preglomerular microvessels was not found. In conclusion, our data indicate that in the kidney, rho-kinase is involved in a variety of signaling pathways leading to microvascular constriction. It plays a pivotal role not only in preglomerular but also in postglomerular tone.

Governance of arteriolar oscillation by ryanodine receptors.

To investigate the role of ryanodine receptors in glomerular arterioles, experiments were performed using an isolated perfused hydronephrotic kidney model. In the first series of studies, BAYK-8644 (300 nM), a calcium agonist, constricted afferent (19.6 +/- 0.6 to 17.6 +/- 0.5 microm, n = 6, P < 0.01) but not efferent arterioles. Furthermore, BAYK-8644 elicited afferent arteriolar oscillatory movements. Subsequent administration of nifedipine (1 microM) inhibited both afferent arteriolar oscillation and constriction by BAYK-8644 (to 19.4 +/- 0.5 microm). In the second group, although BAYK-8644 constricted afferent arterioles treated with 1 microM of thapsigargin (19.7 +/- 0.6 to 16.8 +/- 0.6 microm, n = 5, P < 0.05), it failed to induce rhythmic contraction. Removal of extracellular calcium with EGTA (2 mM) reversed BAYK-8644-induced afferent arteriolar constriction (to 20.0 +/- 0.5 microm). In the third series of investigations, ryanodine (10 microM) but not 2-aminoethoxyphenyl borate (100 microM) abolished afferent arteriolar vasomotion by BAYK-8644. In the fourth series of experiments, in the presence of caffeine (1 mM), the stronger activation of voltage-dependent calcium channels by higher potassium media resulted in greater afferent arteriolar constriction and faster oscillation. Our results indicate that L-type calcium channels are rich in preglomerular but not postglomerular microvessels. Furthermore, the present findings suggest that either prolonged calcium influx through voltage-dependent calcium channels (BAYK-8644) or sensitized ryanodine receptors (caffeine) is required to trigger periodic calcium release through ryanodine receptors in afferent arterioles.

Effects of barnidipine hydrochloride, a calcium channel blocker, on renal microcirculation in rats: a pilot study

Barnidipine hydrochloride, a dihydropyridine calcium channel blocker, increases both renal blood flow and glomerular filtration while effectively reducing blood pressure. The goal of the present study was to determine the effects of barnidipine on renal microcirculation. We used the rat in vivo split hydronephrotic kidney model to directly observe renal arterioles by light microscopy. When administered into the organ bath (ie, administered from outside of the blood vessels), 10 −7 M of barnidipine significantly dilated both afferent and efferent arterioles and increased glomerular blood flow in both normotensive and hypertensive rats. Systolic blood pressure remained unchanged. Intravenous injection of barnidipine 600 μg/kg body weight dilated afferent arterioles and increased glomerular blood flow in normotensive rats. In contrast, the same treatment in hypertensive rats caused significant sustained dilatation of both afferent and afferent arterioles and gradually reduced systolic blood pressure. Future studies are needed to determine the clinical relevance of these effects of barnidipine

Natriuretic peptide receptors mediate different responses in rat renal microvessels

Atrial natriuretic peptide (ANP) has unique effects on the renal vasculature, in that it dilates preglomerular vessels and constricts efferent arterioles. In the present study we aimed to characterize the natriuretic peptide receptor (NPR) subtypes, which mediate the renovascular effects of ANP, using in vivo microscopy in the split hydronephrotic kidney model of rats. ANP (10(-9) and 3.10(-9)), which binds to NPR-A and NPR-C, dilated preglomerular vessels and constricted efferent arterioles similarly to that found in previous studies. C-type natriuretic peptide (10(-9) to 10(-7)), which binds to NPR-B and NPR-C, dilated pre- and postglomerular vessels and profoundly increased glomerular blood flow. A specific ligand of NPR-C, C-ANP (des-[Gln18,Ser19,Gly20,Leu21,Gly22]ANP 4-23-NH2, 10(-9) to 10(-7)) was devoid of vascular effects. The ANP antagonist A71915 (10(-9) to 10(-6)) induced moderate dilation in renal vessels possibly due to some agonistic activity on NPR-B, ANP-induced preglomerular vasodilation was attenuated by A71915 (10(-6)) to 36 +/- 6% of the initial response, whereas efferent vasoconstriction was completely abolished (-4 +/- 4% of initial response). Our results indicate that ANP dilates preglomerular vessels and constricts efferent arterioles through NPR-A, both responses being antagonized by A71915 with different potencies. Furthermore, our data show that in the rat renal microcirculation stimulation of NPR-B results in vasodilation only, whereas NPR-C does not mediate vascular responses.

Effects of systemic complement activation on renal circulation of rats

In a variety of immunopathological diseases activation of the complement cascade occurs either systemically or localized in the kidney. To elucidate the functional impact of complement activation upon the renal microcirculation, we administered cobra venom factor of Naja naja kaouthia (CVF) i.v. into thiobarbital anaesthetized female rats. CVF is a potent activator of the alternative pathway of complement by forming the C3-convertase CVF, Bb which cannot be downregulated by the natural inhibitor factors H and I and thereby leads to generation of the anaphylatoxins C3a and C5a and formation of the membrane attack complex (MAC). We utilized creatinine clearance and flowmeter measurements in the normal kidney and intravital microscopy of the split hydronephrotic rat kidney model to observe the microvascular changes. Bolus injection of CVF (100 U kg-1) resulted in an immediate reduction of RBF (-68% after 10 min), which remained decreased during the entire experiment (90 min). Systemic blood pressure was significantly reduced only in the early period (-23% of control: 126 mmHg after 10 min). After an initial anuric phase of 30 min duration, the glomerular filtration rate was significantly diminished by 47%. White cell count was decreased by about 50% after the experiments. Application of the competitive thromboxane A2-antagonist, BM 13505, reversed all renal and systemic CVF-effects. Continuous infusion of the competitive leukotriene D4-antagonist, ICI 198615, attenuated the late renal CVF-effects (i.e. 30 min after injection of CVF). Depletion of polymorphonuclear cells (PMN) attenuated the CVF-effects similar to BM 13505. Intravenous administration of CVF in the hydronephrotic kidney model resulted in a massive constriction of the interlobar and arcuate artery, with a fall in glomerular blood flow comparable to the reduction of RBF in the normal kidney. Diameters of the afferent arterioles--most sensitive to many vasoconstricting agents--were not significantly altered. Our results suggest that injection of CVF and the liberation of high amounts of the anaphylatoxins, C3a and C5a, induces the release of TXA2, which contributes to the early renal effects and the formation of cysteinyl-leukotrienes which play an important role in the late phase of systemic complement activation. Utilizing the split hydronephrotic kidney model we demonstrated the predominant action of complement activation on the large preglomerular vessels for the first time. PMN are seemingly involved in the liberation of secondary mediators which appear to reduce renal blood flow and glomerular filtration.

Juxtamedullary afferent and efferent arterioles constrict to renal nerve stimulation

Sympathetic neural control of afferent and efferent arterioles of inner cortical (juxtamedullary) glomeruli has not been established, in part, because of difficulty accessing these vessels, normally located deep below the kidney surface. In this study we utilized the rat hydronephrotic kidney model to visualize the renal microcirculation and to quantitate the responses of juxtamedullary arterioles to brief (30 sec) renal nerve stimulated (RNS). Juxtamedullary afferent and efferent arterioles constricted in a frequency-dependent fashion to RNS, achieving a maximal constriction of 35% to 8 Hz stimulation. In these same kidneys, outer cortical afferent arterioles also constricted to RNS but outer cortical efferent arterioles did not. Microinjection of norepinephrine (NE) around single outer cortical efferent arterioles (to avoid the constriction of preglomerular vessels) constricted the efferent arterioles. However, the afferent arterioles of the same glomeruli were considerably more responsive to microinjected NE. Thus, the lack of constriction of outer cortical efferent arterioles to RNS may relate, in part, to their low sensitivity to NE, the primary neurotransmitter. These direct observations indicate that the juxtamedullary efferent arterioles are responsive to renal nerve stimulation whereas the outer cortical efferent vessels are not. These results, which should be cautiously extrapolated to normal filtering kidneys, indicate that glomerular hemodynamic changes evoked by the sympathetic nervous system are different for outer cortical and inner cortical glomeruli.

Factors affecting renal microvascular blood flow in rat hyperdynamic bacteremia.

To determine whether angiotensin II and alpha-adrenergic activity contribute to the mechanism of impaired renal microvascular blood flow during hyperdynamic live Escherichia coli (E. coli) bacteremia, we used in vivo video microscopy in the chronic unilateral hydronephrotic kidney of decerebrate male Sprague-Dawley rats. Intravenous infusion of E. coli caused arteriolar constriction to 83 +/- 4% of baseline (BL) in cortical radial arteries (CRA), 82 +/- 3% of BL in afferent (AFF) arterioles, and decreased flow to 54 +/- 9% of BL. Subsequent local inhibition of renal prostaglandin synthesis with mefenamate increased preglomerular arteriolar constriction to 55 +/- 6% of BL in CRA and 51 +/- 6% of BL in AFF arterioles and decreased renal microvascular blood flow to 26 +/- 8% of BL values in E. coli animals but had no effect on control animals. Subsequent local renal angiotensin II receptor blockade with saralasin acetate increased renal microvascular blood flow in E. coli animals to 64 +/- 9% of BL by dilating CRA to 78 +/- 5% of BL and AFF arterioles to 89 +/- 5% of BL. Phentolamine caused further dilation of CRA to 104 +/- 7% BL and AFF arterioles to 116 +/- 109% and increased flow to 99 +/- 8% of BL. Acetylcholine increased diameters further to 110 +/- 3% of BL in CRA and 136 +/- 12% of BL in AFF arterioles. These data indicate that in our chronic hydronephrotic kidney model during E. coli bacteremia, renal microvascular tone is due to increased angiotensin II and alpha-adrenergic activity and some other, as yet, undefined factor.

Escherichia coli Bacteremia Exacerbates Cyclosporine-Induced Renal Vasoconstriction

The clinical observation that cyclosporine (CSA) nephrotoxicity is particularly severe in patients during and following bacterial infections has recently been made. Transplant recipients develop a marked deterioration of graft function following Escherichia coli bacteremia secondary to urinary tract infection. CSA causes intrarenal vasoconstriction which may account for its nephrotoxicity. We therefore undertook a study using the split hydronephrotic kidney model to investigate the direct in vivo effects of CSA and E. coli bacteremia on the renal microcirculation. Hydronephrotic kidneys in Sprague-Dawley rats were suspended in an environmentally controlled tissue bath. Interlobular arterial (ILA) and afferent (AFF) and efferent (EFF) arteriolar diameters were measured by in vivo videomicroscopy and red cell velocity by Doppler velocimetry. Topical administration of CSA to the kidney in the tissue bath caused a 23 ± 1% constriction of the ILA and a 67 ± 5% reduction in blood flow. AFF and EFF arterioles were also constricted by 21 ± 3 and 16 ± 2%, respectively. The intravenous infusion of live E. coli was also followed by decreases in ILA diameters and flow (38 ± 4 and 68 ± 4%) and AFF diameters (22 ± 5%) while EFF diameters were unchanged. The infusion of E. coli following addition of CSA to the tissue bath resulted in a dramatically increased constriction of ILA (49 ± 4%) and AFF (31 ± 2%) vessels and almost abolished ILA flow (90 ± 2%). We conclude that in this model, E. coli bacteremia exacerbates CSA-induced preglomerular vasoconstriction and suggests a scientific basis for the severe renal dysfunction noted in transplant recipients during bacterial infection.

In Vivo Effects of Endothelin on the Renal Microcirculation

Endothelin-1 (ET) is a recently discovered vasoconstrictor peptide which is released by renal vascular endothelial cells in response to a number of pathologic insults including ischemia, endotoxemia, bacteremia, and cyclosporine nephrotoxicity. Because microvascular vasoconstriction is an integral component of the acute renal dysfunction associated with these conditions, this study was undertaken to determine the in vivo effects of ET on the renal microcirculation. We used the split hydronephrotic kidney model in decerebrate Sprague-Dawley rats to study vessel diameter and red cell velocity responses to ET using intravital videomicroscopy and doppler velocimetry. Topical administration of increasing concentrations of ET caused a dose-dependent constriction of interlobular arteries which reached a maximum of 27 ± 5% at an ET concentration of 10 -8 M. A corresponding decrease of 64 ± 8% in interlobular arterial blood flow was observed. Afferent and efferent arteriole diameters were reduced by 39 ± 2% and 27 ± 5%, respectively. These vascular effects were completely prevented by the systemic preinfusion of anti-endothelin antiserum. Infusion of antiserum alone had no effect on systemic hemodynamics or renal microvascular variables, suggesting that ET has little or no role in maintaining basal vascular tone in the kidney. We conclude that ET is a potent in vivo constrictor of the renal microcirculation and may be involved in mediating pathologic vasoconstriction.

Selective preglomerular constriction to nerve stimulation in rat hydronephrotic kidneys.

The purpose of this study was to quantitate the constrictor responses of pre- vs. postglomerular microvessels during brief increases in renal nerve activity by directly observing the intact microcirculation. To validate the use of the rat hydronephrotic kidney model for this purpose, the vascular reactivity of hydronephrotic kidneys, and normal kidneys was assessed to adrenergic and neural stimulation by quantitating whole kidney blood flow velocity changes elicited by intravenous administration of norepinephrine, electrical stimulation of the posterior hypothalamus, and direct renal nerve stimulation. Hydronephrotic and nonhydronephrotic kidneys responded comparably to norepinephrine and posterior hypothalamic stimulation; however, the hydronephrotic kidneys were less responsive than normal kidneys to direct renal nerve stimulation. In the microcirculatory experiments, stimulation of the splanchnic nerve (2-8 Hz) induced a frequency-dependent constriction of interlobular arteries and afferent arterioles. Preglomerular vessel diameters decreased by 42-53% during the 8-Hz stimulation, whereas the efferent arteriolar diameters did not significantly change. Thus, constriction of preglomerular vessels mediates, in large part, the changes in renal hemodynamics evoked by increases in renal sympathetic nerve activity.

Renal growth in response to unilateral ureteral obstruction

Unilateral renal disease or a unilateral nephrectomy results in a stimulus to growth of the contralateral kidney that returns the total kidney mass and the glomerular filtration rate to a value near normal [1–12]. Unilateral ureteral obstruction also initiates growth of both the obstructed and the contralateral untouched kidney [6, 13–20]. Since the growth characteristics of the two kidneys differ, we would like to examine the literature on kidney growth in the hydronephrotic kidney model and direct attention to the differences between the hydronephrotic and the contralateral intact kidney.