Dynamics of tubuloglomerular feedback adaptation to acute and chronic changes in body fluid volume

The nephron-arterial network and its interactions

Tubuloglomerular feedback and the myogenic mechanism form an ensemble in renal afferent arterioles that regulate single-nephron blood flow and glomerular filtration. Each mechanism generates a self-sustained oscillation, the mechanisms interact, and the oscillations synchronize. The synchronization generates a bimodal electrical signal in the arteriolar wall that propagates retrograde to a vascular node, where it meets similar electrical signals from other nephrons. Each signal carries information about the time-dependent behavior of the regulatory ensemble. The converging signals support synchronization of the nephrons participating in the information exchange, and the synchronization can lead to formation of nephron clusters. We review the experimental evidence and the theoretical implications of these interactions and consider additional interactions that can limit the size of nephron clusters. The architecture of the arterial tree figures prominently in these interactions.

Salt sensitivity of renin secretion, glomerular filtration rate and blood pressure in conscious Sprague-Dawley rats

AIM

We hypothesized that in normal rats in metabolic steady state, (i) the plasma renin concentration (PRC) is log-linearly related to Na(+) intake (NaI), (ii) the concurrent changes in mean arterial pressure (MABP) and glomerular filtration rate (GFR) are negligible and (iii) the function PRC = f(NaI) is altered by β₁-adrenoceptor blockade (metoprolol) and surgical renal denervation (DNX).

METHODS

In catheterized, conscious rats on low-Na(+) diet (0.004% Na(+)), NaI was increased by up to 120-fold, in four 3-day steps, by intravenous saline infusion. MABP was recorded continuously, PRC measured in arterial blood, and GFR estimated by inulin clearance.

RESULTS

Steady states were achieved within 3 days. PRC [mIU L(-1)] was log-linearly related to NaI [mmol kg(-1) day(-1)]: PRC = -9.9 log (NaI) + 22. Set point (22 mIU L(-1) at NaI = 1) and slope (9.9 mIU per decade NaI) were independent of metoprolol administration and DNX. MABP and GFR were markedly salt-sensitive: MABP [mmHg] = 4.9 log (NaI) + 99 (P < 0.01), and GFR [mL min(-1)] = 1.4 log (NaI) + 8.3 (P < 0.01). MABP increased similarly (approx. 10%, P < 0.001) irrespective of pre-treatment. Metoprolol, but not DNX, reduced MABP, HR, and GFR (all P < 0.01). Salt sensitivity of GFR was not observed in DNX rats.

CONCLUSION

Log-linear relations to sodium intake exist not only for PRC, but also for MABP and GFR, which per 10-fold increase in sodium intake rose by 5 mmHg and 1.4 mL min(-1) respectively. Steady-state levels of PRC appear independent of renal nerves. MABP and GFR seem markedly salt sensitive in normal rats.

Renal Insufficiency After Contrast Media Administration Trial II (REMEDIAL II): RenalGuard System in high-risk patients for contrast-induced acute kidney injury

BACKGROUND

The RenalGuard System, which creates high urine output and fluid balancing, may be beneficial in preventing contrast-induced acute kidney injury.

METHODS AND RESULTS

The Renal Insufficiency After Contrast Media Administration Trial II (REMEDIAL II) trial is a randomized, multicenter, investigator-driven trial addressing the prevention of contrast-induced acute kidney injury in high-risk patients. Patients with an estimated glomerular filtration rate ≤30 mL · min(-1) · 1.73 m(-2) and/or a risk score ≥11 were randomly assigned to sodium bicarbonate solution and N-acetylcysteine (control group) or hydration with saline and N-acetylcysteine controlled by the RenalGuard System and furosemide (RenalGuard group). The primary end point was an increase of ≥0.3 mg/dL in the serum creatinine concentration at 48 hours after the procedure. The secondary end points included serum cystatin C kinetics and rate of in-hospital dialysis. Contrast-induced acute kidney injury occurred in 16 of 146 patients in the RenalGuard group (11%) and in 30 of 146 patients in the control group (20.5%; odds ratio, 0.47; 95% confidence interval, 0.24 to 0.92). There were 142 patients (48.5%) with an estimated glomerular filtration rate ≤30 mL · min(-1) · 1.73 and 149 patients (51.5%) with only a risk score ≥11. Subgroup analysis according to inclusion criteria showed a similarly lower risk of adverse events (estimated glomerular filtration rate ≤30 mL · min(-1) · 1.73 m(-2): odds ratio, 0.44; risk score ≥11: odds ratio, 0.45; P for interaction=0.97). Changes in cystatin C at 24 hours (0.02±0.32 versus -0.08±0.26; P=0.002) and 48 hours (0.12±0.42 versus 0.03±0.31; P=0.001) and the rate of in-hospital dialysis (4.1% versus 0.7%; P=0.056) were higher in the control group.

CONCLUSION

RenalGuard therapy is superior to sodium bicarbonate and N-acetylcysteine in preventing contrast-induced acute kidney injury in high-risk patients.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrial.gov. Unique identifier: NCT01098032.

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.

Tonic and phasic influences of nitric oxide on renal blood flow autoregulation in conscious dogs

The aim of this study was to investigate the influence of the mean level and phasic modulation of NO on the dynamic autoregulation of renal blood flow (RBF). Transfer functions were calculated from spontaneous fluctuations of RBF and arterial pressure (AP) in conscious resting dogs for 2 h under control conditions, after NO synthase (NOS) inhibition [NG-nitro-L-arginine methyl ester hydrochloride (L-NAME)] and after L-NAME followed by a continuous infusion of an NO donor [S-nitroso-N-acetyl-DL-penicillamine (SNAP)]. After L-NAME (n = 7) AP was elevated, heart rate (HR) and RBF were reduced. The gain of the transfer function above 0.08 Hz was increased, compatible with enhanced resonance of the myogenic response. A peak of high gain around 0.03 Hz, reflecting oscillations of the tubuloglomerular feedback (TGF), was not affected. The gain below 0.01 Hz, was elevated, but still less than 0 dB, indicating diminished but not abolished autoregulation. After L-NAME and SNAP (n = 5), mean AP and RBF were not changed, but HR was slightly elevated. The gain above 0.08 Hz and the peak of high gain at 0.03 Hz were not affected. The gain below 0.01 Hz was elevated, but smaller than 0 dB. It is concluded that NO may help to prevent resonance of the myogenic response depending on the mean level of NO. The feedback oscillations of the TGF are not affected by NO. NO contributes to the autoregulation below 0.01 Hz due to phasic modulation independent of its mean level.

Tubuloglomerular feedback in Dahl rats

We have previously demonstrated a loss of autoregulation in Dahl salt-sensitive (Dahl-S) rats rendered hypertensive on a high-salt diet. To determine whether this was due to a decreased activity of either the myogenic or the tubuloglomerular feedback (TGF) response, we tested the TGF response in both Dahl-S and salt-resistant Dahl rats on high- and low-salt diets. TGF was investigated in the closed-loop mode with a videometric technique, in which the response in late proximal flow rate to perturbations in Henle flow rate was measured. All Dahl rats showed a similar compensatory response to perturbations around the natural operating point, with a TGF response that was more efficient than in normotensive Sprague-Dawley rats. No evidence of decreased TGF responsiveness in hypertensive Dahl-S rats was found. The results suggest that the loss of autoregulation in hypertensive Dahl-S rats is due to a compromised myogenic response. We also measured the free-flow proximal intratubular pressure in Dahl rats. Perfectly regular oscillations were demonstrated in all Dahl series, including the hypertensive Dahl-S rats. This is the first demonstration of regular oscillations in an experimental rat model of hypertension.

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.

Inhibition of tubuloglomerular feedback by the D1 agonist fenoldopam in chronically salt-loaded rats

1. Chronic dietary NaCl loading in rats is paralleled by an increase of the dopamine concentration in the tubular fluid and humorally mediated inhibition of the tubuloglomerular feedback mechanism at the macula densa. Since these two phenomena are causally linked, the alterations in the tubuloglomerular feedback response by the luminal application of dopamine, the D1 agonist fenoldopam, the D2 agonist bromocriptine and the D1 and D2 antagonists SCH 23390 and metoclopramide were further investigated using the micropuncture technique. 2. Very similar, concentration-dependent inhibition of the tubuloglomerular feedback response was observed for dopamine and fenoldopam. Half-maximal inhibition was achieved at 10(-11) M and the slope factors of the sigmoid concentration-response curves were comparable. Bromocriptine was ineffective. 3. The inhibition of TGF by both agonists could be antagonized very similarly and concentration dependently by the D1 antagonist SCH 23390. At equimolar concentrations of 10(-9) M the inhibition was reduced by approximately 50%. Raising the SCH 23390 concentration to 10(-6) M completely abolished the TGF inhibition. In contrast, TGF inhibition by 10(-9) M-fenoldopam or dopamine was not significantly affected by an equimolar concentration of the D2 antagonist metoclopramide. Increasing metoclopramide concentration to 10(-6) M attenuated tubuloglomerular feedback inhibition by approximately 55%. 4. It is concluded that the inhibition of tubuloglomerular feedback seen during chronic dietary salt loading can be ascribed to the binding of endogenous dopamine to luminal D1 receptors on the macula densa cells.

Blood pressure and tubuloglomerular feedback mechanism in chronically salt-loaded spontaneously hypertensive rats

Experiments were performed to qualitatively characterize the effects of tubuloglomerular feedback (TGF) inhibition by chronic salt loading on salt sensitivity of blood pressure in spontaneously hypertensive rats (SHR). After two weeks of salt loading, systolic blood pressure (SBP) was significantly exacerbated and plasma volume (PV) was expanded in salt-loaded SHR compared with those in control SHR (SBP: 182 +/- 1 vs. 159 +/- 2 mm Hg; PV: 4.38 +/- 0.06 vs. 4.04 +/- 0.03 ml/100 g body wt, respectively). Plasma volume of WKY was also but only transiently expanded by salt loading, whereas plasma volume expansion in SHR had persisted over the entire dietary treatment period. TGF activity was assessed as the maximal reduction of single nephron GFR (SNGFR) on increasing loop of Henle perfusion rate from 0 to 40 nl/min using previously collected tubular fluid from salt-loaded rats (TFs) or control rats (TFc). Maximal TGF response in salt-loaded SHR with TFs was 14.9 +/- 2.9% and 57.8 +/- 2.6% with TFc. In control SHR the responses were 16.9 +/- 2.5% with TFs and 52.7 +/- 2.9% with TFc. In salt-loaded WKY the response with TFs were 3.1 +/- 1.6% and 37.4 +/- 2.8% with TFc. And in control WKY, the response with TFs were 8.2 +/- 1.9% and 40.8 +/- 2.8% with TFc, respectively. These results indicate the TGF resetting in chronically salt-loaded SHR and WKY is caused by the activation of humoral TGF inhibitory factor. The suppression of TGF in SHR was, however, far more variable and, on average, less than in WKY.(ABSTRACT TRUNCATED AT 250 WORDS)

Glomerular filtration effects of acute volume expansion: importance of chloride

The present studies were done to determine the effect on GFR of acute volume expansion (AVE) using solutions of various sodium salts and to explore if degree of tubuloglomerular feedback (TGF) activation plays a role in any GFR differences. Free-flow micropuncture and inulin clearance studies were combined to investigate anesthetized Munich-Wistar rats expanded to 10% body weight with isotonic solutions of NaCl, Ringers bicarbonate (RB), NaHCO3, Na acetate (NaAc) and Na2SO4 as well as euvolemic controls. In the clearance studies, AVE yielded per gram kidney weight GFR's greater than control (1009 +/- 51 microliter/min) in the NaCl and RB (chloride expanded) groups (1397 +/- 89 and 1389 +/- 64) microliter/min, respectively, P less than 0.05 vs. control) but not in the NaHCO3, NaAc, and Na2SO4 (non-chloride expanded) groups. Proximal minus distal single nephron GFR determinations (P-D), an estimate of the degree of TGF, were less than control 13.2 +/- 2.1 nl/min) in the NaCl and RB groups (4.1 +/- 0.7 and 5.3 +/- 1.9 nl/min, respectively, P less than 0.05 vs. control) but were not significantly different from control in any of the non-chloride expanded groups. Early distal (ED) fluid flow correlated positively with P-D in all groups. ED chloride concentration but not TCO2 nor osmolality correlated with P-D for all groups. The correlation was negative for control and chloride expanded groups and positive for non-chloride expanded groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Tubuloglomerular feedback and glomerular morphology in Goldblatt hypertensive rats on varying protein diets

The present experiments were performed to examine the effect of variation in protein intake on renal function and morphology in the non-clipped kidneys of Goldblatt hypertensive rats. After renal artery clipping, rats were put on diets containing 5 (LP), 17.5 (NP), or 51% (HP) protein. After 4 to 5 weeks, all rats had developed hypertension. GFR was directly correlated with protein intake (1.47 +/- 0.15 in HP, 1.19 +/- 0.14 in NP, and 0.93 +/- 0.08 ml/min in LP), as was SNGFR (44.2 +/- 1.89, 39.1 +/- 2.23, and 27.9 +/- 0.86 nl/min in HP, NP, and LP rats). The response of SNGFR to changes in loop of Henle flow rate was attenuated in NP and HP rats: the maximum decrease was reduced (30.0 +/- 5.2% in NP, 22.1 +/- 4.2% in HP) and higher tubular flow rates were required to elicit responses (V1/2, the flow rate at which the response is half-maximum, was 28.9 +/- 2.6 nl/min in NP and 28.2 +/- 1.4 nl/min in HP). In LP rats, the maximum response was a decrease of 47.7 +/- 2.5%, and V1/2 was 18.1 +/- 1.2 nl/min, values similar to those found in normal control rats. The weights of the non-clipped kidneys were 0.96 +/- 0.04 g (LP), 1.06 +/- 0.05 g (NP), and 1.36 +/- 0.06 g (HP). In the LP rats there was no difference between the non-clipped and clipped kidneys. Light microscopic evaluation showed a high incidence of focal glomerulosclerosis in non-clipped kidneys of HP rats, but no glomerular lesions in the non-clipped kidneys of LP rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphometric studies of the extraglomerular mesangial cell field in volume expanded and volume depleted rats

The extraglomerular mesangial cell field was studied by morphometric techniques in volume expanded and volume depleted rats. The volume density of the extraglomerular mesangial interstitium was found to be significantly different between the two conditions, 16.9 +/- 3.7% in volume depletion and 29.0 +/- 4.1% in volume expansion. No difference in the volume density of the peritubular interstitium could be detected under the same conditions. These findings are interpreted as indicating a specific sensitivity of the extraglomerular mesangial interstitium to changes in body fluid content, a phenomenon which may play a role in the mechanism of resetting the tubulo-glomerular feedback control.

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.

Activation of the tubuloglomerular feedback mechanism in dehydrated rats

To study the influence of the tubuloglomerular feedback control (TGF) on the regulation of glomerular filtration rate (GFR) during dehydration, micropuncture experiments were performed on surface nephrons of dehydrated rats. Dehydration was achieved by withdrawal of food and water for 24 h. The urine flow rate decreased to 1.5 microliters/min (controls 2.9 microliters/min) and GFR decreased in these rats to 0.80 ml/min (controls 1.22). TGF was studied by two different micropuncture procedures. With the first technique the changes in proximal stop-flow pressure in response to changes of the late proximal microperfusion rate were measured. With this technique the perfusion rate necessary to induce a half maximal stop-flow pressure response, the turning point, was also determined. An increased TGF sensitivity was found in dehydrated rats, as indicated by increased stop-flow pressure responses (35 versus 26%) and decreased turning points (16 versus 21 nl/min). With the second micropuncture technique the single nephron GFR (SNGFR) was measured at distal and proximal tubular sites, in the same nephron. Distal SNGFR was decreased during dehydration to 32.2 nl/min, versus 42.7 nl/min in controls. A significant difference between paired SNGFR measurements in the same nephron was observed during dehydration, the proximal value being 5.3 nl/min higher than the distal, whereas this difference was not seen in control rats. This finding indicates that activation of the feedback mechanism takes place to reduce SNGFR. It is concluded that the decrease in whole kidney GFR is partly caused by the observed increase in feedback activity. The present results are also in agreement with our earlier hypothesis that the hydrostatic and oncotic pressure conditions within the interstitial space surrounding the macula densa cells modulate the sensitivity of the tubuloglomerular feedback mechanism.

Chronic salt loading: effects on plasma volume and regulation of glomerular filtration rate in Wistar rats

Experiments were carried out in Wistar rats to determine whether the loss of sensitivity of the tubuloglomerular feedback mechanism (TGF) which is known to occur in volume expansion is due to a change in the functional characteristics of the juxtaglomerular apparatus or to a change in some property of the tubular fluid which influences the feedback signal at the macula densa. Proximal tubular fluid was collected by means of a microperfusion/suction pump from Wistar rats maintained for a minimum of 10 days on a high salt diet and also from rats fed a control low salt diet. Both fluids were then used to perfuse loops of Henle in rats from both groups and the feedback response assessed from the change in early proximal tubular flow rate (EPF). In high salt rats, perfusion of the loop of Henle with homologous tubular fluid confirmed the loss of sensitivity of the TGF mechanism in volume expansion, the response of EPF was practically absent. In contrast, the low salt rat responded with a 50% decrease in EPF to loop perfusion at 40 nl/min with its homologous fluid. On the other hand, when the loop of Henle in high salt rats was perfused at 40 nl/min with heterologous (low salt) tubular fluid, EPF again decreased by some 50% whereas EPF in low salt rats failed to respond to loop perfusion with high salt fluid. From these results it is concluded that in rats chronically volume expanded by a high salt diet an unknown inhibitory principle occurs in the proximal tubular fluid which reduces the sensitivity of the tubuloglomerular feedback mechanism.

Feedback-mediated reduction of glomerular filtration rate during infusion of hypertonic saline

An acute rise in plasma sodium concentration from 146 +/- 2.6 to 155 +/- 1.7 was produced in rats by the intraarterial infusion of 0.6 M sodium chloride (0.25 ml/min for 4 min followed by 0.25 ml/hr). A parallel fall in whole kidney GFR (from 0.45 +/- 0.02 to 0.36 +/- 0.04 ml/min per 100 g of body wt) and SNGFR measured in the distal tubule (31.4 +/- 3.01 to 27.9 +/- 2.40 nl/min) was observed. In contrast, proximally measured SNGFR (with feedback interrupted) rose from 32.7 +/- 2.73 to 37.1 +/- 2.84 nl/min. The loop of Henle flow, determined from distal SNGFR and (TF/P) inulin in late proximal fluid collected without interrupting tubular flow, rose from 13.7 +/- 1.50 to 17.0 +/- 1.42 nl/min as a consequence of a fall in proximal reabsorptive rate from 15.8 +/- 1.87 to 11.0 +/- 1.37 nl/min. Intraarterial infusion of hypertonic sodium bicarbonate resulted in comparable increases in plasma sodium concentration and inhibition of proximal reabsorption but did not produce a fall in filtration rate. We conclude (1) acute infusion of hypertonic sodium chloride results in an inhibition of proximal reabsorption and therefore in an increased rate of loop of Henle flow, (2) this increase in flow causes a fall in GFR through the tubuloglomerular feedback mechanism, and (3) acute infusion of hypertonic sodium bicarbonate does not result in a feedback-mediated fall in GFR, presumably because increased delivery of bicarbonate-rich fluid does not activate the feedback mechanism.