Feedback regulation of nephron filtration rate during pharmacologic interference with the renin-angiotensin and adrenergic systems in rats

Concurrent activation of multiple vasoactive signaling pathways in vasoconstriction caused by tubuloglomerular feedback: a quantitative assessment

Tubuloglomerular feedback (TGF) describes the negative relationship between (a) NaCl concentration at the macula densa and (b) glomerular filtration rate or glomerular capillary pressure. TGF-induced vasoconstriction of the afferent arteriole results from the enhanced effect of several vasoconstrictors with an effect size sequence of adenosine = 20-HETE > angiotensin II > thromboxane = superoxide > renal nerves > ATP. TGF-mediated vasoconstriction is limited by the simultaneous release of several vasodilators with an effect size sequence of nitric oxide > carbon monoxide = kinins > adenosine. The sum of the constrictor effects exceeds that of the dilator effects by the magnitude of the TGF response. The validity of the additive model used in this analysis can be tested by determining the effect of combined inhibition of some or all agents contributing to TGF. Multiple independent contributors to TGF are consistent with the variability of TGF and of the factors contributing to TGF resetting.

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.

Renal responses to furosemide are significantly attenuated in male sheep at 6 months of age following fetal uninephrectomy

We have previously shown that fetal uninephrectomy (uni-x) at 100 days of gestation (term = 150 days) in male sheep results in a 30% nephron deficit, reduction in glomerular filtration rate (GFR) and renal blood flow, and elevation in arterial pressure at 6 mo of age. Furthermore, in response to an acute 0.9% saline load, sodium excretion was significantly delayed in uni-x animals leading us to speculate that tubuloglomerular feedback (TGF) activity was reset in uni-x animals. In the present study, we induced TGF blockade by furosemide administration (1.5 mg/kg iv over 90 min) and determined GFR, effective renal plasma flow, and urine and sodium excretion responses in 6-mo-old male sheep. In response to furosemide, a significant diuresis and natriuresis was observed in the sham group; however, the response was significantly delayed and reduced in uni-x animals (both, Ptreatment×time < 0.001). Cummulative urinary and sodium output was significantly less in the uni-x compared with the sham sheep (both, Ptreatment×time < 0.001). GFR was increased in the sham but not the uni-x sheep ( Ptreatment×time < 0.0001). In conclusion, the excretory response to furosemide was attenuated in the uni-x sheep, and this suggests a rightward resetting of the TGF operating point. The TGF mechanism is important in the fine tuning of sodium homeostasis and is likely a contributing factor for the dysfunction in sodium regulation we have previously observed in the uni-x animals.

p22phox in the macula densa regulates single nephron GFR during angiotensin II infusion in rats

Angiotensin II (ANG II) infusion increases renal superoxide (O2−) and enhances renal vasoconstriction via macula densa (MD) regulation of tubuloglomerular feedback, but the mechanism is unclear. We targeted the p22 phox subunit of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) with small-interfering RNA (siRNA) to reduce NADPH oxidase activity and blood pressure response to ANG II in rats. We compared single nephron glomerular filtration rate (SNGFR) in samples collected from the proximal tubule (PT), which interrupts delivery to the MD, and from the distal tubule (DT), which maintains delivery to the MD, to assess MD regulation of GFR. SNGFR was measured in control and ANG II-infused rats (200 ng·kg−1·min−1 for 7 days) 2 days after intravenous injection of vehicle or siRNA directed to p22 phox to test the hypothesis that p22 phox mediates MD regulation of SNGFR during ANG II. The regulation of SNGFR by MD, determined by PT SNGFR-DT SNGFR, was not altered by siRNA in control rats (control + vehicle, 13 ± 1, n = 8; control + siRNA, 12 ± 2 nl/min, n = 8; not significant) but was reduced by siRNA in ANG II-treated rats (ANG II + vehicle, 13 ± 2, n = 7; ANG II + siRNA, 7 ± 1 nl/min, n = 8; P < 0.05). We conclude that p22 phox and NADPH oxidase regulate the SNGFR during ANG II infusion via MD-dependent mechanisms.

Mechanisms of renal blood flow autoregulation: dynamics and contributions

Autoregulation of renal blood flow (RBF) is caused by the myogenic response (MR), tubuloglomerular feedback (TGF), and a third regulatory mechanism that is independent of TGF but slower than MR. The underlying cause of the third regulatory mechanism remains unclear; possibilities include ATP, ANG II, or a slow component of MR. Other mechanisms, which, however, exert their action through modulation of MR and TGF are pressure-dependent change of proximal tubular reabsorption, resetting of RBF and TGF, as well as modulating influences of ANG II and nitric oxide (NO). MR requires < 10 s for completion in the kidney and normally follows first-order kinetics without rate-sensitive components. TGF takes 30-60 s and shows spontaneous oscillations at 0.025-0.033 Hz. The third regulatory component requires 30-60 s; changes in proximal tubular reabsorption develop over 5 min and more slowly for up to 30 min, while RBF and TGF resetting stretch out over 20-60 min. Due to these kinetic differences, the relative contribution of the autoregulatory mechanisms determines the amount and spectrum of pressure fluctuations reaching glomerular and postglomerular capillaries and thereby potentially impinge on filtration, reabsorption, medullary perfusion, and hypertensive renal damage. Under resting conditions, MR contributes approximately 50% to overall RBF autoregulation, TGF 35-50%, and the third mechanism < 15%. NO attenuates the strength, speed, and contribution of MR, whereas ANG II does not modify the balance of the autoregulatory mechanisms.

Oxidative stress and nitric oxide deficiency in the kidney: a critical link to hypertension?

There is growing evidence that oxidative stress contributes to hypertension. Oxidative stress can precede the development of hypertension. In almost all models of hypertension, there is oxidative stress that, if corrected, lowers BP, whereas creation of oxidative stress in normal animals can cause hypertension. There is overexpression of the p22(phox) and Nox-1 components of NADPH oxidase and reduced expression of extracellular superoxide dismutase (EC-SOD) in the kidneys of ANG II-infused rodents, whereas there is overexpression of p47(phox) and gp91(phox) and reduced expression of intracellular SOD with salt loading. Several mechanisms have been identified that can make oxidative stress self-sustaining. Reactive oxygen species (ROS) can enhance afferent arteriolar tone and reactivity both indirectly via potentiation of tubuloglomerular feedback and directly by microvascular mechanisms that diminish endothelium-derived relaxation factor/nitric oxide responses, generate a cyclooxygenase-2-dependent endothelial-derived contracting factor that activates thromboxane-prostanoid receptors, and enhance vascular smooth muscle cells reactivity. ROS can diminish the efficiency with which the kidney uses O(2) for Na(+) transport and thereby diminish the P(O(2)) within the kidney cortex. This may place a break on further ROS generation yet could further enhance vasculopathy and hypertension. There is a tight relationship between oxidative stress in the kidney and the development and maintenance of hypertension.

Paracrine factors in tubuloglomerular feedback: adenosine, ATP, and nitric oxide

The tubuloglomerular feedback response, the change in afferent arteriolar tone caused by a change in NaCl concentration at the macula densa, is likely initiated by the generation of a vasoactive mediator within the confines of the juxtaglomerular apparatus. Substantial progress has been made in identifying the nature of this mediator and the factors that modulate its effect on vascular tone. In support of earlier studies using P1 purinergic antagonists, the application of the knockout technique has shown that adenosine 1 receptors are absolutely required for eliciting TGF responses. The background level of angiotensin II appears to be an important cofactor determining the efficiency of A1AR-induced vasoconstriction, probably through a synergistic interaction at the level of the G protein-dependent transduction mechanism. The source of the adenosine is still unclear, but it is conceivable that adenosine is generated extracellularly from released ATP through a cascade of ecto-nucleotidases. There is also evidence that ATP may activate P2 receptors in preglomerular vessels, which may contribute to autoregulation of renal vascular resistance. Nitric oxide (NO), generated by the neuronal isoform of nitric oxide synthase in macula densa cells, reduces the constrictor effect of adenosine, but the regulation of NO release and its exact role in states of TGF-induced hyperfiltration are still unclear.

Role of angiotensin II in dynamic renal blood flow autoregulation of the conscious dog

The influence of angiotensin II (ANGII) on the dynamic characteristics of renal blood flow (RBF) was studied in conscious dogs by testing the response to a step increase in renal artery pressure (RAP) after a 60 s period of pressure reduction (to 50 mmHg) and by calculating the transfer function between physiological fluctuations in RAP and RBF. During the RAP reduction, renal vascular resistance (RVR) decreased and upon rapid restoration of RAP, RVR returned to baseline with a characteristic time course: within the first 10 s, RVR rose rapidly by 40 % of the initial change (first response, myogenic response). A second rise began after 20-30 s and reached baseline after an overshoot at 40 s (second response, tubuloglomerular feedback (TGF)). Between both responses, RVR rose very slowly (plateau). The transfer function had a low gain below 0.01 Hz (high autoregulatory efficiency) and two corner frequencies at 0.026 Hz (TGF) and at 0.12 Hz (myogenic response). Inhibition of angiotensin converting enzyme (ACE) lowered baseline RVR, but not the minimum RVR at the end of the RAP reduction (autoregulation-independent RVR). Both the first and second response were reduced, but the normalised level of the plateau (balance between myogenic response, TGF and possible slower mechanisms) and the transfer gain below 0.01 Hz were not affected. Infusion of ANGII after ramipril raised baseline RVR above the control condition. The first and second response and the transfer gain at both corner frequencies were slightly augmented, but the normalised level of the plateau was not affected. It is concluded that alterations of plasma ANGII within a physiological range do not modulate the relative contribution of the myogenic response to the overall short-term autoregulation of RBF. Consequently, it appears that ANGII augments not only TGF, but also the myogenic response.

Tubuloglomerular feedback in ACE-deficient mice

In these experiments, we used a strain of angiotensin converting enzyme (ACE) germline null mutant mice, generated by J. H. Krege and co-workers (J. H. Krege, S. W. M. John, L. L. Langenbach, J. B. Hodgin, J. R. Hagaman, E. S. Bachman, J. C. Jennette, D. A. O'Brien, and O. Smithies. Nature 375: 146-148, 1995), to examine the effect of chronic ACE deficiency on the magnitude of tubuloglomerular feedback (TGF) responses. The genotype was determined by PCR on DNA extracted from the tail and was verified after each experiment by assessment of the blood pressure response to an injection of ANG I. To assess TGF responsiveness, we determined the change in stop-flow pressure (PSF) caused by increasing NaCl concentration at the macula densa by using micropuncture techniques. When loop of Henle flow rate was increased from 0 to 40 nl/min, PSF fell from a mean of 42.3 +/- 1.95 to 33.6 +/- 2.09 mmHg (n = 6, P = 0.005) in wild-type mice (+/+), fell from 40.6 +/- 2.35 to 38.6 +/- 1.93 mmHg in heterozygous (+/-) mice (n = 7, P = 0.014), and did not change in homozygous ACE (-/-) mice [36.7 +/- 2.02 mmHg vs. 36.4 +/- 2.01 mmHg; n = 4, P = not significant (NS)]. During an infusion of ANG II at a dose that did not significantly elevate blood pressure (70 ng. kg-1. min-1), TGF response magnitude (PSF 0 - PSF 40) increased from 6.5 +/- 1.4 to 9.8 +/- 1.19 mmHg in +/+ (P = 0.006), from 1.14 +/- 0.42 to 4.6 +/- 1.3 mmHg in +/- (P = 0.016), and from 0.42 +/- 0.25 to 4.02 +/- 1.06 in -/- mice (P = 0.05). Absence of TGF responses in ACE null mutant mice and restoration of near-normal responses during an acute infusion of ANG II supports previous conclusions that ANG II is an essential component in the signal transmission pathway that links the macula densa with the glomerular vascular pole.

Inhibition of adenosine-1 receptor-mediated preglomerular vasoconstriction in AT1A receptor-deficient mice

The effect of the adenosine type 1 receptor agonist N6-cyclohexyladenosine (CHA) on glomerular vascular reactivity was studied in male angiotensin II type 1A (AT1A) receptor knockout mice (9). Vascular reactivity was assessed as the response of stop-flow pressure (PSF) to infusion of CHA into loops of Henle using micropuncture techniques. In AT1A +/+ mice at ambient arterial blood pressure (96.7 +/- 2.8 mmHg), the presence of CHA (10 (-5) M) in the perfusate increased PSF responses from 6.8 +/- 0.6 to 14.3 +/- 0.9 mmHg when the loop of Henle of the index nephron was perfused and from 0.7 +/- 0.3 to 12.3 +/- 1.0 mmHg when the loop of an adjacent nephron was perfused. At reduced arterial blood pressure (82.8 +/- 1. 3 mmHg), index nephron perfusion with CHA increased PSF responses from 4.5 +/- 0.3 to 9.4 +/- 0.4 mmHg. In AT1A -/- mice with a mean arterial blood pressure of 80 +/- 1.9 mmHg, CHA increased PSF responses only from 0.1 +/- 0.3 to 3.6 +/- 0.54 mmHg during index nephron perfusion and from 0.25 +/- 0.2 to 2.7 +/- 0.55 mmHg during adjacent nephron perfusion, significantly less than in wild-type animals (P < 0.001). Responses to CHA were intermediate in AT1A +/- mice. Thus AT1A receptor knockout mice show a markedly reduced constrictor response to CHA both in the presence and absence of simultaneous activation of the tubuloglomerular feedback system. These data support the notion of a functional interaction between adenosine and angiotensin II in the regulation of afferent arteriolar tone.

Angiotensinogen gene null-mutant mice lack homeostatic regulation of glomerular filtration and tubular reabsorption

Chronic volume depletion by dietary salt restriction causes marked decrease in glomerular filtration rate (GFR) with little increase in urine osmolality in angiotensinogen gene null mutant (Agt-/-) mice. Moreover, urine osmolality is insensitive to both water and vasopressin challenge. In contrast, in normal wild-type (Agt+/+) mice, GFR remains remarkably constant and urine osmolality is adjusted promptly. Changes in volume status also cause striking divergence in renal structure between Agt-/- and Agt+/+ mice. Thus, in contrast to the remarkably stable glomerular size of Agt+/+ mice, glomeruli of Agt-/- mice are atrophied during a low salt and hypertrophied during a high salt diet. Moreover, the renal papilla, a structure unique to mammals and essential for urine diluting and concentrating mechanisms, is hypoplastic in Agt-/- mice. Thus, angiotensin is essential for the two fundamental homeostatic functions of the mammalian kidney, namely stable GFR and high urine diluting and concentrating capacity during alteration in extracellular fluid (ECF) volume. This is not only accompanied by angiotensin's tonic effects on renal vasomotor tone and tubule transporters, but also accomplished through its capacity to affect the structure of both the glomerulus and the papilla directly or indirectly.

Juxtaglomerular cell complex in the regulation of renal salt excretion

Luminal NaCl concentration at the macula densa (MD) has the two established effects of regulating glomerular arteriolar resistance and renin secretion. Tubuloglomerular feedback (TGF), the inverse relationship between MD NaCl concentration and glomerular filtration rate (GFR), stabilizes distal salt delivery and thereby NaCl excretion in response to random perturbations unrelated to changes in body salt balance. Control of vasomotor tone by TGF is exerted primarily by NaCl transport-dependent changes in local adenosine concentrations. During long-lasting perturbations of MD NaCl concentration, control of renin secretion becomes the dominant function of the MD. The potentially maladaptive effect of TGF under chronic conditions is prevented by TGF adaptations, permitting adjustments in GFR to occur. TGF adaptation is mechanistically coupled to the end point targeted by chronic deviations in MD NaCl, the rate of local and systemic angiotensin II generation. MD control of renin secretion is the result of the coordinated action of local mediators that include nitric oxide synthase (NOS) and cyclooxygenase (COX) products. Thus vascular smooth muscle cell activation during high MD transport and granular cell activation during low MD transport is achieved by different extracellular mediators. The coordinated regulation of NOS I and COX-2 expression in MD cells and of renin expression in granular cells suggests that control of juxtaglomerular regulation of gene transcription or mRNA metabolism may be another consequence of a chronic alteration in MD NaCl concentration.

Influence of NaCl concentration at the macula densa on angiotensin II-induced constriction of the afferent arteriole

The macula densa, a plaque of specialized tubular epithelial cells, monitors NaCl concentrations in tubular fluid and controls resistance of the glomerular afferent arteriole (AA). In vivo micropuncture studies suggest that there are significant interactions between angiotensin II (Ang II) and macula densa control of glomerular hemodynamics. We tested the hypothesis that Ang II causes stronger constriction of the AA when NaCl concentration at the macula densa is elevated. Rabbit AAs and the attached macula densa were simultaneously microperfused in vitro, and dose-response curves to Ang II were obtained when the macula densa was not perfused or was perfused with either low NaCl (Na+, 26 mEq/L; Cl-, 7 mEq/L) or high NaCl (Na+, 84 mEq/L; Cl-, 65 mEq/L). Ang II induced stronger constriction when the macula densa was perfused with high NaCl; the decrease in diameter at 100 pmol/L was 29 +/- 5.6% (n= 7) compared with 2.1 +/- 1.2% (n=8) for the nonperfused macula densa or 6.1 +/- 4.2% (n=7) for low NaCl (P < .002). However, there was no such difference in the action of norepinephrine. Adding furosemide (10 micromol/L) to the macula densa perfusate abolished the difference in Ang II action between low and high NaCl at the macula densa. Since AA tone is higher when the NaCl concentration at the macula densa is elevated, we tested whether augmented Ang II action is due to higher AA tone. Preconstriction of the AA by 20% with norepinephrine had no effect on Ang II action. Thus, our results demonstrate that sensitivity of the AA to Ang II increases when NaCl concentration at the macula densa is elevated. Such modulation of Ang II action by macula densa NaCl concentration may be important in the control of glomerular hemodynamics.

Contribution of tubuloglomerular feedback to renal arteriolar angiotensin II responsiveness

The purpose of the present study was to test the hypothesis that a component of the afferent arteriolar vasoconstrictor response to angiotensin II (Ang II) requires an intact tubuloglomerular feedback (TGF) mechanism. Enalaprilat-treated male Sprague-Dawley rats served as tissue donors for study of renal microvascular function using the in vitro blood-perfused juxtamedullary nephron technique. Arteriolar lumen diameter responses to exogenous Ang II were determined before and after TGF blockade (papillectomy or 50 microM furosemide). Before TGF blockage, 10 nM Ang II significantly reduced diameters of both mid-afferent (53 +/- 5%) and efferent (43 +/- 9%) arterioles. TGF blockade did not alter baseline diameter of either arteriole, but significantly blunted the mid-afferent vasoconstriction evoked by 10 nM Ang II (44 +/- 7% inhibition by papillectomy; 43 +/- 10% inhibition by furosemide). Similar behavior was observed at afferent arteriolar sites near the glomerulus; however, efferent arteriolar Ang II responsiveness was not altered by papillectomy. The impact of TGF blockade on afferent arteriolar Ang II responsiveness was most prominent at high peptide concentrations (10 nM), while not significantly influencing the response to 1 nM Ang II. In contrast, the afferent vasoconstrictor effect of norepinephrine was unaffected by papillectomy. These data indicate that the vasoconstrictor influence of exogenous Ang II on afferent, but not efferent, arterioles of intact juxtamedullary nephrons includes both TGF-dependent and TGF-independent components.

Effect of captopril and angiotensin II receptor blockade on pressure natriuresis in transgenic TGR(mRen-2)27 rats

The pressure-natriuresis curve of transgenic rats harboring an extra mouse renin gene [TGR(mRen-2)27] is shifted rightward compared with controls; however, whether intrarenal angiotensin II effects are responsible for the rightward shift is unknown. To clarify this issue we infused the converting enzyme inhibitor captopril or the angiotensin II receptor blocker CV 11974 into transgenic and normotensive Sprague-Dawley Hannover control rats. We eliminated any other neural or endocrine regulatory differences between transgenic and control rats by renal denervation and infusion of vasopressin, aldosterone, corticosterone, and norepinephrine in sufficient quantities to occupy all receptors. Sodium excretion increased from 3.4 +/- 1.2 to 10.1 +/- 0.5 mumol/min per gram kidney weight in transgenic rats when renal perfusion pressure was increased from 158 to 201 mm Hg. Captopril (4 mg/kg) and CV 11974 (0.1 mg/kg) shifted the pressure-natriuresis curve of transgenic rats leftward, so that sodium excretion was threefold higher at similar renal perfusion pressures (150 to 160 mm Hg). Similarly, fractional sodium and water excretion curves were shifted leftward, so that values for transgenic and control rats were no longer different. Over the pressure range, renal blood flow in transgenic rats ranged from 3.1 +/- 0.7 to 4.4 +/- 0.5 mL/min per gram kidney weight and increased (P < .05) with both captopril and CV 11974 to ranges from 4.8 +/- 0.9 to 6.8 +/- 0.6 or from 4.5 +/- 0.7 to 6.9 +/- 1.0 mL/min per gram kidney weight, respectively. Glomerular filtration rate in transgenic rats, on the other hand, was not increased. Transgenic kidneys showed severe hypertension-induced nephrosclerosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between adenosine and angiotensin II in renal microcirculation

In order to examine the possibility of an interaction between adenosine and angiotensin II (A II) in the control of the renal microcirculation, we studied the effects of agonists and antagonists of both substances by means of in vivo microscopy in the split hydronephrotic rat kidney. In a first series of experiments (n = 6), local application of the A II receptor antagonist saralasin (10(-6) mol.liter-1 abolished the vasoconstriction and the reduction of glomerular blood flow induced by the A1-adenosine receptor agonist N6-cyclohexyladenosine (CHA, local concentration 10(-7) mol.liter-1). Without saralasin (second series, n = 6), CHA reduced glomerular blood flow and decreased vessel diameters as previously reported from our laboratory. In a third series of experiments (n = 6), A II significantly reduced vessel diameters and glomerular blood flow both alone and during blockage of the A1-adenosine receptor by the selective antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 10(-5) mol.liter-1). In additional experiments, we excluded nonspecific receptor effects of saralasin and confirmed the inhibitory action of DPCPX on the adenosine-induced vasoconstriction. We suppose that adenosine needs a functioning A II receptor system for its vasoconstrictor action, whereas A II can induce a nonadenosine-dependent vasoconstriction.

Renal haemodynamics during hyperchloraemia in the anaesthetized dog: effects of captopril

1. An increase in plasma chloride concentration (PC1) decreases renal blood flow (RBF) and glomerular filtration rate (GFR) and inhibits the intrarenal release of renin and angiotensin II (AII). Captopril was administered to indomethacin-treated dogs to assess the role of AII independent of prostaglandins (PGs) in the haemodynamic response to hyperchloraemia. Studies were performed on kidneys that were denervated by autotransplantation. 2. Anaesthetized greyhounds received an intrarenal infusion of 0.616 M-sodium acetate, which was changed to 0.616 M-NaCl (hyperchloraemia). These infusions increased the plasma sodium and osmolality at the experimental kidney by 7-11% throughout but increased the PCl during the hypertonic NaCl infusions only (122 +/- 3 to 136 +/- 3 mM). 3. In vehicle-treated dogs (n = 8), hyperchloraemia reduced the GFR (1.4 +/- 0.1 to 1.0 +/- 0.1 ml min-1 kg-1; P less than 0.05) and the RBF (13.0 +/- 1.4 to 8.3 +/- 0.6 ml min-1 kg-1; P less than 0.01); these changes were reversible on return to the 0.616 M-sodium acetate infusion. Hyperchloraemia reduced the release of AII into renal lymph (2.5 +/- 0.9 to 1.2 +/- 0.4 pg min-1 kg-1; P less than 0.01). 4. Indomethacin (0.6 mg kg-1 and 0.2 mg kg-1 h-1 intrarenally; n = 4) blunted (P less than 0.05) the Cl--induced fall in RBF (10.4 +/- 1.1 to 8.2 +/- 0.6 ml min-1 kg-1) without changing significantly the fall in GFR or the release of AII into renal lymph.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin influences on tubuloglomerular feedback mechanism in hypertensive rats

The tubuloglomerular feedback (TGF) mechanism was evaluated in the nonclipped kidney of Goldblatt hypertensive rats from both stop flow pressure (SFP) and single nephron glomerular filtration rate (SNGFR) responses to step increases in late proximal perfusion rate from 0 to 40 nl/min. During control conditions, increases in late proximal perfusion rate produced flow dependent decreases in SFP and SNGFR with maximal values of 10.2 +/- 1.0 mm Hg and 12.9 +/- 2.5 nl/min, values similar to those obtained in normal rats. During ACE inhibition (MK 422; 0.6 mg/kg/hr), arterial pressure decreased from 168 +/- 8 to 137 +/- 7 mm Hg and there was a marked attenuation in the magnitude of SFP feedback responses (delta = 2.5 +/- 0.3 mm Hg). SNGFR feedback responses, however, were not significantly impaired. Direct decreases in renal arterial pressure reduced control SFP but SFP feedback responses were maintained, indicating that the attenuated SFP feedback responses during ACE inhibition were not due to decreased arterial pressure. Superimposed infusion of angiotensin II during ACE inhibition partially restored SFP feedback responses. In contrast, norepinephrine infusion did not result in a similar restoration of SFP feedback sensitivity. These results indicate that the nonclipped kidney of Goldblatt hypertensive rats has an intact TGF mechanism as assessed from SFP and SNGFR feedback responses. Furthermore, ACE inhibition attenuates SFP but not SNGFR feedback responses, and systemic angiotensin II infusions can restore SFP feedback responsiveness towards normal.

Renal hemodynamic response to intrarenal infusion of calcitonin gene-related peptide in dogs

The renal hemodynamic and excretory effects of intrarenal infusions of synthetic beta-human calcitonin gene-related peptide (beta-hCGRP) were examined in normal sodium replete dogs (Group 1, n = 6), in sodium replete dogs pretreated with indomethacin (Group 2, n = 6), and in sodium deplete dogs (Group 3, n = 5). In all groups of anesthetized dogs beta-hCGRP was infused at 5 and 10 ng.kg-1.min-1 for 50 min periods each. In the sodium replete group, beta-hCGRP infusions strikingly increased renal blood flow, but this response was markedly attenuated in the other 2 groups. During beta-hCGRP infusions, the clearance of creatinine also increased significantly in the sodium replete and deplete groups, but not in the indomethacin pretreated animals. No consistent changes in urinary sodium excretion or plasma renin activity were observed with beta-hCGRP infusions in any of the 3 groups of dogs. These results indicate that beta-hCGRP is a potent renal vasodilator and can increase renal blood flow and glomerular filtration. The data also suggest that the renal hemodynamic actions of beta-hCGRP are partially mediated by renal prostaglandins, and that the vasodilatory effects of beta-hCGRP may be antagonized by high circulating levels of endogenous angiotensin II in sodium-volume depletion. Finally, beta-hCGRP does not appear to have significant actions on urinary sodium excretion or plasma renin activity under the experimental conditions of the present study.

The angiotensin converting enzyme inhibitor enalapril in acute ischemic renal failure in rats

The influence of the renin-angiotensin system on renal hemodynamics, tubular pressure and tubulo-glomerular feedback was investigated with the angiotensin converting enzyme inhibitor MK 421 (enalapril), in uninephrectomized rats with and without ischemia-induced acute renal failure. In animals with normal renal function proximal tubular pressure and tubulo-glomerular feedback response were lowered by enalapril long-term treatment, whereas glomerular filtration rate and renal blood flow were not influenced by the drug. After 45 and 70 minutes ischemia there was no difference between treated and untreated animals in the severely impaired glomerular filtration rate. Renal blood flow remained unaffected by the treatment. The histological damage due to ischemia (tubular casts, tubular necrosis and medullary capillary congestion) was not influenced by enalapril. As tubulo-glomerular feedback had been significantly inhibited during renin-angiotensin inhibition, its importance in mediating acute renal failure remains doubtful; other factors such as tubular obstruction and medullary congestion may be crucial.

Changes in single nephron renin release are mediated by tubular fluid flow rate

In vivo renin release from single nephrons microperfused with artificial tubular fluid was studied in recollection experiments. Renin concentration was measured in systemic arterial plasma (A-PRC) and in either early proximal tubular fluid (TFR), or in plasma from the welling point of the efferent arteriole (SV-PRC) belonging to the microperfused nephron. Micropuncture collections were controlled to maintain the proximal intratubular pressure equal to the control free-flow pressure. Increasing the Henle loop flow rate from 12 to 18, or to 34 nl/min, was followed by a significant decrease in TFR, while reducing the flow rate from 12 to 6 nl/min caused a significant increase in TFR. Similarly, increasing the Henle loop free-flow rate by 6 to 8 nl/min depressed SV-PRC, while reducing the flow rate by fluid aspiration at 8 to 10 nl/min caused a significant increase in SV-PRC. These data indicate: that renin release, to a significant part at least, occurs into the vascular lumen of the afferent arteriole: that modest changes in early distal flow rate may control renin release from the JG-cells; and that increasing the flow rate depresses renin release, and vice versa. It is suggested that the renin system is directly involved in an additional TGF mechanism controlling postglomerular vascular resistances.

Functional basis for the glomerular alterations in uranyl nitrate acute renal failure

We have examined the acute renal failure that occurs after uranyl nitrate administration in the rat and the specific effects of pretreatment of rats with angiotensin converting enzyme inhibitor (CEI), plasma volume expansion (PVE) after uranyl nitrate, and a combination of these treatments. We utilized a combination of micropuncture measurements of glomerular hemodynamics, cage studies, and histologic examination of renal tissue to evaluate the degree of acute renal failure in all groups studied. Uranyl nitrate (UN) (25 mg/kg body wt) administration caused a reduction in the nephron filtration rate (SNGFR) (39.4 +/- 1.6 to 24.8 +/- 2.9 nl X min-1 X g kidney wt-1, P less than 0.02) as a result of a major decrease in the glomerular ultrafiltration coefficient (LpA) from control values (greater than or equal to 0.085 +/- 0.008 to 0.035 +/- 0.007 nl X sec-1 X mm Hg-1 X g kidney wt-1, P less than 0.01). Treatments with CEI, PVE, and the combination of CEI and PVE in rats receiving UN restored 0.38 +/- LpA to normal values (greater than 0.061 +/- 0.009, 0.091 +/- 0.020, and 0.138 +/- 0.020 nl X sec-1 X mm Hg-1 X g kidney wt-1, respectively). Cage studies revealed that CEI treatment prevented oliguria and resulted in major volume losses and reduction in weight. However, rats died after a similar period after UN, but probably by different mechanisms. Analysis of renal ultrastructure revealed equivalent tubular damage in all experimental groups. Alterations in LpA after UN are functional in nature and are potentially preventable and reversible by a combination of treatments with CEI and PVE.

[Angiotensin-converting enzyme inhibition: direct and indirect mechanisms]

The introduction of angiotensin-converting-enzyme (ACE)-inhibitors into the analysis of the renin-angiotensin system (RAS) had broadened our knowledge of the integral role of renin and the kidney in circulatory homeostasis and has provided a pathophysiologically based concept for the treatment of hypertension. When the RAS is activated, as it is when sodium is restricted, the renal blood supply shows the most striking vasodilatation among vascular beds assessed after ACE-inhibition. Sodium excretion rises, there is a fall in blood-pressure, and plasma concentrations of angiotensin II (AII) and aldosterone are reduced. Conversely, with sodium loading the hemodynamic and hormonal effects of ACE-inhibitors are small. In 50-60% of normal or high-renin patients with essential hypertension ACE-inhibitors induce a potentiated acute renal response: renal blood flow and sodium excretion increase more than they do in the remainder of the hypertensives or in normal subjects. The responders of the hypertensive patients fail to increase renal blood flow or to enhance renal vascular responsiveness to infused AII when they shift from a low to a high sodium intake. The altered renal response of these "sodium-sensitive" hypertensives could be related to local activity of the RAS which is insufficiently suppressed by sodium loading. ACE-inhibition reverses this failure of the renal blood supply to respond to sodium loading. Kidneys of spontaneously hypertensive rats and the renin-rich kidney of Goldblatt-hypertensive rats show an increased tubulo glomerular (TG) feedback response as compared to normal kidneys. The change in TG-feedback response might be expected to contribute to the inability of the hypertensive kidney to respond adequately to sodium loading. ACE-inhibition reduces TG-feedback sensitivity. In renal artery stenosis glomerular capillary pressure tends to be maintained by an AII mediated rise in postglomerular resistance. Suppression of AII by ACE-inhibition reduces efferent vascular tone and thus filtration rate. There is a potential for interaction of ACE-inhibitors with the kallikrein and prostaglandin pathways as well as with the sympathetic nervous system and endogenous opioids. This may modify the renal and blood pressure responses to these compounds.

A functional role for the tubuloglomerular feedback mechanism

Ever increasing evidence exists that the tubuloglomerular feedback system exerts a major influence on overall renal function. Several examples are potentially pertinent to clinical medicine in which there is reasonable evidence that activation or suppression of tubuloglomerular feedback mechanisms contribute significantly to alterations in normal renal physiology. However, in most examples reported, the feedback mechanism is one of several influences acting in concert to determine the final nephron filtration rate, its respective determinants, and the relationship of filtration to the rate of tubular reabsorption. A more complete understanding of all the factors which influence and modify the functional role of tubuloglomerular feedback mechanisms will aid our understanding significantly and the consequent therapy of a variety of altered physiologic conditions.

Tubuloglomerular feedback, prostaglandins, and angiotensin in the autoregulation of glomerular filtration rate

To define the mechanisms responsible for autoregulation of SNGFR in the subnormal pressure range, the response of SNGFR to graded reductions of arterial pressure was measured before and after interfering with the tubuloglomerular feedback system (TGF), angiotensin II action and prostaglandin (PG) synthesis. Studies were performed in male Sprague-Dawley rats in which estimated surgical plasma losses were replaced, because euvolemic animals were found to have better autoregulatory capacity than hydropenic animals. In control plasma-replaced animals, a pressure reduction from normal to 97.5 mm Hg and a further reduction to 78 mm Hg had no significant effect on SNGFR (31.8 +/- 1.32 to 31.7 +/- 1.6 to 29.3 +/- 1.48 nl/min) when all autoregulatory mechanisms were intact. After eliminating TGF, the same pressure steps were followed by significant reductions in SNGFR (40.8 +/- 1.75 to 36.4 +/- 2.18 to 31.0 +/- 1.56 nl/min). During infusion of saralasin (1 microgram/kg X min), SNGFR did not change significantly during reduction of pressure from normal to 95.5 mm Hg (32.0 +/- 1.02 to 30.7 +/- 1.58 nl/min) but fell when pressure was reduced to 77 mm Hg (26.0 +/- 1.19 nl/min). Infusion of this dose of saralasin was without significant effect on the response of early proximal flow rate to loop of Henle perfusion. During indomethacin-induced inhibition of PG synthesis, SNGFR fell significantly in response to both pressure steps (38.6 +/- 1.4 to 34.0 +/- 1.68 to 25.5 +/- 1.29 nl/min). An analysis of the autoregulatory components indicates that in the higher pressure interval 115 to 95 mm Hg, TGF contributes about 50% and PG's about 30% to autoregulatory adjustments. In the lower pressure interval, 95 to 78 mm Hg, 30% autoregulatory compensation occurs through the TGF mechanism and 20% depends upon the action of angiotensin II. Probably in part by interfering with both of those mechanisms, inhibition of PG synthesis reduces autoregulatory compensation by about 60%.

An oscillating intratubular pressure response to alterations in Henle loop flow in the rat kidney

We describe in gas anesthetized rats an oscillating intratubular pressure response, probably of vascular origin, sensitive to small physiological changes in fluid delivery to the distal tubule. The oscillation apparently indicates that an adjustment of vascular resistance is in operation, but at present it reveals neither the effector site (afferent and/or efferent arteriole) nor the effector mechanism (vasoconstriction and/or dilatation). The renin-angiotensin system seems to be involved in this phenomenon.

Reversal of indomethacin-induced inhibition of tubuloglomerular feedback by prostaglandin infusion

Experiments were performed in rats to study the effect of infusion of PGI2, PGE2, and PGF2 alpha on tubuloglomerular feedback responses (i.e. the change of SNGFR in response to a change of loop of Henle flow rate) in the presence and absence of simultaneous inhibition of endogenous PG synthesis with indomethacin. Infusion of PGI2 or PGE2 at rates that did not alter arterial blood pressure did not significantly modify the magnitude of feedback responses (PGI2 8.5 micrograms/hr, PGE2 85 micrograms/hr). Some inhibition of feedback responses was seen when PGI2 and PGE2 were administered at higher rates that were associated with a reduction of blood pressure (PGI2 20 micrograms/hr, PGE2 200 micrograms/hr). PGI2 (8.5 micrograms/hr) and PGE2 (85 micrograms/hr) largely prevented feedback inhibition induced by indomethacin. When given subsequent to indomethacin PGI2 and PGE2 restored feedback responsiveness almost to normal. In contrast, PGF2 alpha did not influence feedback inhibition caused by indomethacin. Infusion of PGI2 induced partial restoration of feedback responses in DOCA-salt treated animals in which the feedback system is virtually completely inactive. Our results indicate that availability of PGI2 or PGE2 is necessary for the normal operation of the tubuloglomerular feedback mechanism for control of nephron filtration rate.

Attenuation of nephrotoxic acute renal failure in the dog with angiotensin-converting enzyme inhibitor (SQ-20,881)

Angiotensin-converting enzyme inhibitor was used in dogs with uranyl nitrate-induced acute renal failure to evaluate (1) a possible protective effect of angiotensin blockade and (2)the role of angiotensin II in the generation of renal failure in this model. Angiotensin-converting enzyme inhibitor treatment attenuated the fall in glomerular filtration rate and renal blood flow during the first 6 hours after injection of the nephrotoxic agent. A protective effect of similar magnitude was observed whether angiotensin-converting enzyme inhibitor treatment preceded, or shortly followed, the administration of uranyl nitrate. This indicates that angiotensin-converting enzyme inhibitor delivery to its intrarenal site of action remains effective after administration of the nephrotoxin. In addition, protection of glomerular filtration rate correlated with sodium and renal solute excretion. However, combined treatment with angiotensin-converting enzyme inhibitor and furosemide enhanced solute excretion but did not further improve the protection of renal function. Finally, the protective effects of angiotensin-converting enzyme inhibitor on renal function and hemodynamics were abolished by intravenous indomethacin. In conclusion, early, continuous blockade of angiotensin II protects partially against th initiation of acute renal failure. These findings support a major pathogenic role for angiotensin II in the generation phase of acute renal failure in this model. Furthermore, they suggest that an imbalance between vasoconstrictive (angiotensin II) and vasodilating factors (prostaglandins) may be operative in the early phase of uranyl nitrate-induced acute renal failure in the dog.

Ultracytochemistry of aminopeptidase A (angiotensinase A) in the kidney glomerulus and juxtaglomerular apparatus

Ultracytochemical studies of the Aminopeptidase A (APA; angiotensinase A; E.C. 3.4.11.7) were performed on perfusion fixed rat and mouse kidneys (low concentration aldehydes) using simultaneous azo coupling with alpha-L-Glu-MNA as substrate and HPR as well as HNF as coupling agents. The studies of glomeruli and juxtaglomerular apparatus (JGA) show that APA is localized mainly at the cell membranes of podocytes and endothelial cells (rat and mouse) and of epitheloid cells (mouse) and Goormaghtigh's cells (rat). Increased APA activities are found in the region of cell contacts of epitheloid cells (mouse) and Goormaghtigh's cells (rat). In the epitheloid cells of mice, reaction product is also observed intracellularly in lysosomal structures. Concerning the functional significance of APA in the glomerulus and JGA, it would appear that this enzyme modifies or regulates angiotensin effects in the glomerulus and JGA through the angiotensin degradation.

Influence of uranyl nitrate upon tubular reabsorption and glomerular filtration in blood perfused isolated dog kidneys

The early changes in tubular reabsorption, glomerular filtration, blood flow and sodium excretion brought about by uranyl nitrate were investigated in isolated, blood-perfused dog kidneys during water diuresis. No significant changes in urine volume were observed; the decrease in fluid reabsorption was counterbalanced quantitatively by a reduction in glomerular filtration rate; only a small diminution of renal blood flow was found. The balance between reabsorption and filtration was observed as well when angiotensin action or prostaglandin synthesis were inhibited. The intrarenal venous pressure rose, suggesting that an increase in proximal intratubular hydrostatic pressure caused the decrease in filtration. Tubular back-leak of fluid, or back-diffusion, induced by the toxin, were excluded. The presence of natriuretic compounds in the urine was confirmed.

Indirect evidence against a role of the kinin system in the renal hemodynamic effect of captopril in the rat

The effects of acute saralasin (SAR) and captopril (SQ) administration on arterial pressure (AP), plasma renin activity (PRA), urinary excretion of water and electrolytes, glomerular filtration rate (GFR), renal blood flow (RBF), and glomerular blood flow (GBF) distribution (microsphere technique) were assessed in rats with activation of the renal renin and kallikrein systems (that is, chronic sodium depletion). In both groups AP decreased, and PRA and RBF increased markedly. Blood flow to outermost (C1) glomeruli was (in nl/min/g of kidney wt) 270 +/- 35 in SAR and 219 +/- 20 in the SQ group (NS when compared to 208 +/- 9 in control chronically sodium-depleted rats). Blood flow to innermost glomeruli (C4) strikingly increased from 95 +/- 10 (control) to 216 +/- 21 (SAR) and 180 +/- 13 (SQ group). Hence, preferential vasodilatation of innermost glomeruli occurred (C1/C4 ratio of 2.18 +/- 0.27 in control, 1.26 +/- 0.11 in SAR, and 1.25 +/- 0.07 in SQ rats). Chronic (6 days) administration of SQ was associated with a rapid and marked increase in water and sodium excretion. At the end of the study, RBF was higher than control, and GBF distribution was similar to that observed in acutely treated rats (C1/C4 ratio of 1.16 +/- 0.10). These results suggest that angiotensin plays a significant role in the systemic and renal hemodynamic changes associated with chronic sodium depletion. The similarity of the changes induced by SAR and SQ provides an indirect evidence against an effective role of the renal kallikrein system in the effect of captopril.

The glomerulus, passive filter or regulatory organ?

This review summarizes recent evidence that glomerular filtration rate is highly regulated and not merely the passive consequence of uncontrolled renal and non-renal factors. Changes in the rate of nephron plasma flow and, under certain circumstances, the glomerular permeability coefficient are the major determining factors which influence the rate of glomerular ultrafiltration. Recent studies suggest that a variety a hormonal substances, when infused, share the capacity to affect glomerular filtration rate by influencing nephron plasma flow and specifically by decreasing the glomerular permeability coefficient. Angiotensin II appears to be the important "final common pathway" mediating many of these hormonal effects on the glomerular permeability coefficient. Of the hormonal substances examined, only ADH appears to exert an independent effect. Also, in certain normal and altered physiologic states, it has been demonstrated that certain hormonal substances, notably angiotensin II, participate in the active regulation of the rate of glomerular filtration through the capacity to influence and regulate the rate of nephron plasma flow and effect reduction in the glomerular permeability coefficient.