Resetting of tubulo-glomerular feedback sensitivity by dietary salt intake

A new view of macula densa cell microanatomy

Although macula densa (MD) cells are chief regulatory cells in the nephron with unique microanatomical features, they have been difficult to study in full detail due to their inaccessibility and limitations in earlier microscopy techniques. The present study used a new mouse model with a comprehensive imaging approach to visualize so far unexplored microanatomical features of MD cells, their regulation, and functional relevance. MD-GFP mice with conditional and partial induction of green fluorescent protein (GFP) expression, which specifically and intensely illuminated only single MD cells, were used with fluorescence microscopy of fixed tissue and live MD cells in vitro and in vivo with complementary electron microscopy of the rat, rabbit, and human kidney. An elaborate network of major and minor cell processes, here named maculapodia, were found at the cell base, projecting toward other MD cells and the glomerular vascular pole. The extent of maculapodia showed upregulation by low dietary salt intake and the female sex. Time-lapse imaging of maculapodia revealed highly dynamic features including rapid outgrowth and an extensive vesicular transport system. Electron microscopy of rat, rabbit, and human kidneys and three-dimensional volume reconstruction in optically cleared whole-mount MD-GFP mouse kidneys further confirmed the presence and projections of maculapodia into the extraglomerular mesangium and afferent and efferent arterioles. The newly identified dynamic and secretory features of MD cells suggest the presence of novel functional and molecular pathways of cell-to-cell communication in the juxtaglomerular apparatus between MD cells and between MD and other target cells.NEW & NOTEWORTHY This study illuminated a physiologically regulated dense network of basal cell major and minor processes (maculapodia) in macula densa (MD) cells. The newly identified dynamic and secretory features of these microanatomical structures suggest the presence of novel functional and molecular pathways of cell-to-cell communication in the juxtaglomerular apparatus between MD and other target cells. Detailed characterization of the function and molecular details of MD cell intercellular communications and their role in physiology and disease warrant further studies.

Effect of salt intake on afferent arteriolar dilatation: role of connecting tubule glomerular feedback (CTGF)

Afferent arteriole (Af-Art) resistance is modulated by two intrinsic nephron feedbacks: 1) the vasoconstrictor tubuloglomerular feedback (TGF) mediated by Na⁺-K⁺-2Cl^- cotransporters (NKCC2) in the macula densa and blocked by furosemide and 2) the vasodilator connecting tubule glomerular feedback (CTGF), mediated by epithelial Na⁺ channels (ENaC) in the connecting tubule and blocked by benzamil. High salt intake reduces Af-Art vasoconstrictor ability in Dahl salt-sensitive rats (Dahl SS). Previously, we measured CTGF indirectly, by differences between TGF responses with and without CTGF inhibition. We recently developed a new method to measure CTGF more directly by simultaneously inhibiting NKCC2 and the Na⁺/H⁺ exchanger (NHE). We hypothesize that in vivo during simultaneous inhibition of NKCC2 and NHE, CTGF causes an Af-Art dilatation revealed by an increase in stop-flow pressure (P_SF) in Dahl SS and that is enhanced with a high salt intake. In the presence of furosemide alone, increasing nephron perfusion did not change the P_SF in either Dahl salt-resistant rats (Dahl SR) or Dahl SS. When furosemide and an NHE inhibitor, dimethylamiloride, were perfused simultaneously, an increase in tubular flow caused Af-Art dilatation that was demonstrated by an increase in P_SF. This increase was greater in Dahl SS [4.5 ± 0.4 (SE) mmHg] than in Dahl SR (2.5 ± 0.3 mmHg; P < 0.01). We confirmed that CTGF causes this vasodilation, since benzamil completely blocked this effect. However, a high salt intake did not augment the Af-Art dilatation. We conclude that during simultaneous inhibition of NKCC2 and NHE in the nephron, CTGF induces Af-Art dilatation and a high salt intake failed to enhance this effect.

Reduced nephron endowment due to fetal uninephrectomy impairs renal sodium handling in male sheep

Reduced nephron endowment is associated with development of renal and cardiovascular disease. We hypothesized this may be attributable to impaired sodium homoeostasis by the remaining nephrons. The present study investigated whether a nephron deficit, induced by fetal uninephrectomy at 100 days gestation (term=150 days), resulted in (i) altered renal sodium handling both under basal conditions and in response to an acute 0.9% saline load (50 ml.kg-1 of body weight.30 min-1); (ii) hypertension and (iii) altered expression of renal channels/transporters in male sheep at 6 months of age. Uninephrectomized animals had significantly elevated arterial pressure (90.1+/-1.6 compared with 77.8+/-2.9 mmHg; P<0.001), while glomerular filtration rate and renal blood flow (per g of kidney weight) were 30% lower than that of the sham animals. Total kidney weight was similar between the groups. Renal gene expression of apical NHE3 (type 3 Na+/H+ exchanger), ENaC (epithelium Na+ channel) beta and gamma subunits and basolateral Na+/K+ ATPase beta and gamma subunits were significantly elevated in uninephrectomized animals, while ENaC alpha subunit expression was reduced. Urine flow rate and sodium excretion increased in both groups in response to salt loading, but this increase in sodium excretion was delayed by approximately 90 min in the uninephrectomized animals, while total sodium output was 12% in excess of the infused load (P<0.05). In conclusion, the present study shows that animals with a congenital nephron deficit have alterations in tubular sodium channels/transporters and cannot rapidly correct for variations in sodium intake probably contributing to the development of hypertension. This suggests that people born with a nephron deficit should be monitored for early signs of renal and cardiovascular disease.

Connexin 37 is localized in renal epithelia and responds to changes in dietary salt intake

Connexins are the main components of gap junction channels, which are important for intercellular communication. In the kidney, several members of the connexin (Cx) family have been identified. Renal vascular expression and hemodynamic impacts have so far been shown for Cx37, Cx40, and Cx43. Additionally, Cx30, Cx30.3, and Cx43 have been identified to be part of tubular epithelial gap junctions and/or hemichannels. However, the localization and role of other Cx family members in renal epithelial structures remain undetermined. We aimed to localize Cx37 in the kidney to obtain information on its epithelial expression and potential functions. Immunohistochemistry in rodent kidney showed characteristic punctate patterns in the vasculature and along the nephron. Strong basolateral expression was found in the thick ascending limb and distal convoluted tubule. Weaker abundances were found in the proximal tubule and the collecting duct also at the basolateral side. In situ hybridization and real-time PCR of isolated nephron segments confirmed this distribution at the mRNA level. Ultrastructurally, Cx37 immunostaining was confined to basolateral cell interdigitations and infoldings. As a functional approach, rats were fed low- or high-salt diets. Compared with control and high-salt diets, rats treated with low-salt diet showed significantly increased Cx37 mRNA and protein levels. This may be indicative of an adaptive tubular response to changes in sodium reabsorption. In summary, renal epithelia express Cx37 in their basolateral membranes. Here, the formation of Cx37 gap junctions may be involved in cellular communication and adjustments of vectorial epithelial transport.

Salt-sensing mechanisms in blood pressure regulation and hypertension

High salt consumption contributes to the development of hypertension and is considered an independent risk factor for vascular remodeling, cardiac hypertrophy, and stroke incidence. In this review, we discuss the molecular origins of primary sensors involved in the phenomenon of salt sensitivity. Based on the analysis of literature data, we conclude that the kidneys and central nervous system (CNS) are two major sites for salt sensing via several distinct mechanisms: 1) [Cl(-)] sensing in renal tubular fluids, primarily by Na(+)-K(+)-Cl(-) cotransporter (NKCC) isoforms NKCC2B and NKCC2A, whose expression is mainly limited to macula densa cells; 2) [Na(+)] sensing in cerebrospinal fluid (CSF) by a novel isoform of Na(+) channels, Na(x), expressed in subfornical organs; 3) sensing of CSF osmolality by mechanosensitive, nonselective cation channels (transient receptor potential vanilloid type 1 channels), expressed in neuronal cells of supraoptic and paraventricular nuclei; and 4) osmolarity sensing by volume-regulated anion channels in glial cells of supraoptic and paraventricular nuclei. Such multiplicity of salt-sensing mechanisms likely explains the differential effects of Na(+) and Cl(-) loading on the long-term maintenance of elevated blood pressure that is documented in experimental models of salt-sensitive hypertension.

Current mechanisms of macula densa cell signalling

Macula densa cells couple renal haemodynamics, glomerular filtration and renin release with tubular fluid salt and water reabsorption. These cells detect changes in tubular fluid composition through a complex of intracellular signalling events that are mediated by membrane transport pathways. Increases in luminal fluid sodium chloride concentration result in alterations in cell sodium chloride concentration, cytosolic calcium, cell pH, basolateral membrane depolarization and cell volume. Macula densa signalling then involves the production and release of specific paracrine signalling molecules at their basolateral membrane. Upon moderate increases in luminal sodium chloride concentration macula densa cells release increasing amounts of ATP and decreasing amounts of prostaglandin E(2), thereby increasing afferent arteriolar tone and decreasing the release of renin from granular cells. On the other hand, further increases in luminal concentration stimulate the release of nitric oxide, which serve to prevent excessive tubuloglomerular feedback vasoconstriction. Paracrine signalling by the macula densa cells therefore controls juxtaglomerular function, renal vascular resistance and participates in the regulation of renin release.

Macula densa basolateral ATP release is regulated by luminal [NaCl] and dietary salt intake

One component of the macula densa (MD) tubuloglomerular feedback (TGF) signaling pathway may involve basolateral release of ATP through a maxi-anion channel. Release of ATP has previously been studied during a maximal luminal NaCl concentration ([NaCl](L)) stimulus (20-150 mmol/l). Whether MD ATP release occurs during changes in [NaCl](L) within the physiological range (20-60 mmol/l) has not been examined. Also, because TGF is known to be enhanced by low dietary salt intake, we examined the pattern of MD ATP release from salt-restricted rabbits. Fluorescence microscopy, with fura 2-loaded cultured mouse mesangial cells as biosensors, was used to assess ATP release from the isolated, perfused thick ascending limb containing the MD segment. The mesangial biosensor cells, which contain purinergic receptors and elevate intracellular Ca(2+) concentration ([Ca(2+)](i)) on ATP binding, were placed adjacent to the MD basolateral membrane. Elevations in [NaCl](L) between 0 and 80 mmol/l, in 20-mmol/l increments, caused stepwise increases in [Ca(2+)](i), with the highest increase at [NaCl](L) of approximately 60 mmol/l. Luminal furosemide at 10(-4) mol/l blocked ATP release, which suggests that the efflux of ATP required MD Na-2Cl-K cotransport. A low-salt diet for 1 wk increased the magnitude of [NaCl](L)-dependent elevations in biosensor [Ca(2+)](i) by twofold, whereas high-salt intake had no effect. In summary, ATP release occurs over the same range of [NaCl](L) (20-60 mmol/l) previously reported for TGF responses, and, similar to TGF, ATP release was enhanced by dietary salt restriction. Thus these two findings are consistent with the role of MD ATP release as a signaling component of the TGF pathway.

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.

Mechanisms of amphotericin B-induced decrease in glomerular filtration rate in rats

Mechanisms responsible for the reductions in renal blood flow (RBF) and glomerular filtration rate (GFR) in response to acute infusions of amphotericin B were investigated in vivo in rats. The influence of salt status and the roles of adenosine, cyclic AMP, and calcium influx were examined. Amphotericin B was infused into the renal artery in seven groups of rats at 0.025 mg/kg of body weight per min for 15 min. RBF and GFR were measured over 15 min before, during, and after the infusion. Control rats were maintained on a normal salt diet; a second group of rats received a salt-depleted diet, and a third group received a high-salt intake. Four other groups were kept on a normal diet and received theophylline (0.5 mumol/kg/min into the renal artery, intra-arterially [i.a.]), dibutyryl cyclic AMP (85 micrograms/min, i.a.), the 5'-nucleotidase inhibitor adenosine alpha,beta-methylene diphosphate (4 mg/kg, intramuscularly), or diltiazem (20 micrograms/kg/min, i.a.). Control rats had a prompt 50% decrease in RBF in response to amphotericin B. This was sustained over the 15-min infusion period and was accompanied by a decrease in creatinine clearance (CLCR) (from 0.83 +/- 0.08 to 0.40 +/- 0.09 ml/min; P less than 0.05). On stopping the infusion, RBF returned quickly to baseline but CLCR continued to decrease further (to 0.35 +/- 0.07 ml/min; P less than 0.05). Salt loading, theophylline, and diltiazem administration prevented the decreases in both RBF and CLCR. Both RBF and CLCR responses in the remaining groups were not significantly different from those in controls. The results of this study reveal a protective effect of salt loading and theophylline against amphotericin B nephrotoxicity in the rat but deny a role for adenosine in mediating these effects. They further suggest that theophylline inhibits the acute responses by a mechanism unrelated to either adenosine receptor blockade or phosphodiesterase inhibition and that calcium influx into the cells is probably responsible for the acute changes in RBF and GFR in response to amphotericin B.

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)

Renal function in normal and potassium-depleted rats before and after preparation for micropuncture experimentation

Renal function was examined in unrestrained conscious rats maintained on either a control diet or a low-potassium diet, then re-examined in the same animals after thiobutabarbital (Inactin) anaesthesia and preparation for micropuncture studies. In conscious rats, insulin clearance (CIn) was not significantly different in the two groups (control 1012 +/- 43, low-K 904 +/- 58 microliters/min per 100g body wt; mean +/- SE), but lithium clearance (CLi; used as an estimate of end-proximal fluid delivery) and fractional lithium excretion (FELi) were substantially reduced in the low-K group (CLi: 246 +/- 11 vs 126 +/- 8 microliters/min per 100g body wt, P less than 0.001; FELi: 0.245 +/- 0.009 vs 0.143 +/- 0.008, P less than 0.001). Following anaesthesia and preparation for micropuncture, there were significant reductions in urine flow rate and sodium excretion in the control group, but not in the low-K rats. Potassium excretion increased in both groups, but values in the potassium-depleted animals remained extremely low. In neither group of rats was preparation for micropuncture associated with significant changes in CIn, CLi or FELi. Thus, differences in tubular function between control and potassium-depleted rats were still apparent. The results suggest that preparation for micropuncture disturbs the function of the distal nephron, but that rates of glomerular filtration and proximal tubular reabsorption remain similar to values in conscious animals.

The control of glomerular filtration rate and renal blood flow in chronically volume-expanded rats

1. Chronic volume expansion by dietary salt loading practically abolishes tubuloglomerular feed-back (TGF) by means of a humoral inhibitor in tubular fluid. Elimination of the vasoconstrictor influence of feed-back does not, however, increase glomerular filtration rate (GFR) and renal blood flow (RBF), implying that chronic salt loading induces additional preglomerular vasoconstriction. This being so, the feed-back response which, although absent in free-flowing nephrons, can still be elicited by loop of Henle perfusion with Ringer solution, should be essentially normal, except that nephron GFR at any loop perfusion rate should be lower than in controls. Persistence of RBF, GFR and nephron GFR autoregulation would imply that autoregulation is achieved by a preglomerular resistance control system independent of feed-back. 2. These hypotheses were tested by clearance and micropuncture experiments in rats chronically fed a diet containing 40 g NaCl (kg food)-1. 3. RBF and GFR autoregulation indeed persisted, the former down to 90 mmHg compared with 105 mmHg in controls. In controls, nephron GFR measured distally was autoregulated down to 90 mmHg whereas that measured proximally was autoregulated only above 105 mmHg. In high-salt rats nephron GFR from both sites was autoregulated to 90 mmHg. 4. Loop of Henle perfusion with homologous tubular fluid in high-salt rats confirmed attenuation of feed-back. Loop perfusion with Ringer solution yielded a response comparable to that in controls (maximal reduction of nephron GFR to 57%, compared with 56% in controls). Absolute nephron GFR at any loop perfusion rate was lower in high-salt rats than in controls. 5. These observations confirm the initial hypotheses. Considering feed-back and autoregulation as independent, preglomerular resistance control mechanisms, together with elementary haemodynamic considerations, allows formulation of a renal haemodynamics model whose quantitative predictions regarding characteristics of RBF, GFR and feed-back control are remarkably consistent with the literature.

Intrarenal hypertonic saline infusions in dogs with thoracic caval constriction

Intrarenal artery infusions of hypertonic saline can activate tubuloglomerular feedback (TGF), decreasing renal blood flow (RBF) and glomerular filtration rate (GFR). The response to infusion of hypertonic saline is enhanced by salt depletion and attenuated by salt loading, but has not previously been investigated in pathophysiological states where expanded extracellular fluid volume due to salt retention is associated with avid, renal sodium reabsorption. The renal response following intrarenal infusions of hypertonic saline was investigated in five control dogs and eleven dogs with partial constriction of the thoracic portion of their inferior vena cava, which resulted in salt retention and the formation of ascites. Intrarenal infusion of hypertonic saline induced significant reductions in RBF and GFR in both control and caval constricted dogs. The extent of these reductions were positively correlated with baseline renal function. An intravenous infusion of 50 ml/kg of 0.9% sodium chloride, which abolished the vasoconstrictor response in normal dogs, failed to abolish the decrease in RBF and GFR in response to intrarenal hypertonic saline infusion in dogs with ascites which had an initial vasoconstrictor response. We conclude that the potential for TGF is preserved in early stages of caval constriction syndrome in dogs, but that this potential activity decreases when basal renal function decreases.

The morphological basis of fluid balance in the interstitium of the juxtaglomerular apparatus

The morphological basis of fluid balance in the interstitium of the juxtaglomerular apparatus (JGA) was reevaluated in rats, mice and Tupaia. Three ultrastructural features in the region of the vascular pole of the renal corpuscle are described that may be important for the fluid balance in this region: (1) podocyte foot processes in the parietal layer of Bowman's capsule, (2) endothelial fenestrations in the wall of the incoming afferent arteriole, both facing Goormaghtigh and epithelioid cells, and (3) the mesangial-type lining of the glomerular stalk. With respect to the relevant pressure gradients, this morphology may provide the basis of bulk-fluid flow directed to the interstitium of the JGA including the Goormaghtigh cell field. Thus, the fluid balance in the lacis area and, consequently, the tubulo-glomerular feedback mechanism, probably does not solely depend upon the reabsorptive transport of the macula densa. Similar considerations may be valid for the humoral control of renin secretion from juxtaglomerular epithelioid cells.

[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.

Dietary protein suppresses feedback control of glomerular filtration in rats

We have examined the possibility that changes in glomerular filtration rate (GFR) after changes in dietary protein intake may depend on altered function of the tubuloglomerular (TG) feedback system. We studied male Sprague-Dawley rats after dietary pretreatment for 9.6 +/- 3.6 (SD) d with isocaloric diets containing either 6% or 40% casein. We found that GFR in rats fed the high protein diet was 24-29% higher than in rats fed the low protein diet. Simultaneous measurements of single nephron GFR (SNGFR) in the distal tubule were 6.3 nl/min or 21% higher in the rats fed the high protein diet whereas proximally measured SNGFR was not statistically different in the two groups. The higher distally measured SNGFR of rats receiving the high protein diet was associated with a 4.2 nl/min or 50% smaller suppression of SNGFR by TG feedback (-4.3 vs. -8.5 nl/min, P less than 0.001). Loop perfusion experiments demonstrated that in rats fed the high protein diet the TG feedback mechanism was less sensitive than in rats fed the low protein diet. The TG feedback response in rats fed the low protein diet, as assessed by reductions in stop-flow pressure and SNGFR, was half-maximal at flows of 14-15 nl/min. In contrast, the TG feedback response in rats fed the high protein diet was half-maximal at 22-24 nl/min. Maximal suppression of stop-flow pressure and SNGFR and the slope of the TG feedback response to increasing loop flow rates were not different in the two groups. We conclude that the sensing mechanism of the TG feedback system is rendered less responsive by a high protein intake, and that this change permits GFR to increase.

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.

Reduction of renal blood flow and proximal bicarbonate reabsorption in rats by gentamicin

Although aminoglycoside-induced acute renal failure occurs commonly, little is known about the mechanisms which alter renal hemodynamics. In sodium-depleted Sprague-Dawley rats treated with gentamicin, we measured RBF and GFR at the onset of this model of nephrotoxic acute renal failure. After 10 days of sodium chloride depletion, one group of rats received a single injection of gentamicin, 100 mg/kg, while control animals received the gentamicin vehicle. Twenty-four hours later, PCr and UNa/UCr were similar in both groups. CIn was unchanged, but RBF was reduced significantly (12.40 +/- 1.33 vs. 16.89 +/- 1.24 ml/min). Micropuncture studies revealed that although SNGFR was unchanged, end-proximal and early distal flow rates were increased significantly. End-proximal TFCl was reduced significantly in gentamicin-treated animals when compared to controls (130.7 +/- 3.9 vs. 149.5 +/- 4.1 mEq/liter). Early distal TFCl was also reduced significantly (32.4 +/- 2.0 vs. 44.3 +/- 1.4 mEq/liter). In other rats, 24 hr after a second injection of gentamicin, PCr and UNa/UCr were increased significantly and both GFR and RBF were reduced significantly. We conclude that the earliest hemodynamic change in gentamicin-induced acute renal failure is a reduction in RBF which precedes any change in GFR. A single dose of gentamicin also impairs proximal bicarbonate and water reabsorption and reduces end-proximal and early distal chloride concentration.

Regulation of renal blood flow by plasma chloride

Micropuncture studies have shown that glomerular filtration rate (GFR) falls in response to a rise in Na(+) or Cl(-) concentrations in the loop of Henle, whereas studies in isolated kidneys have shown that GFR falls in response to osmotic diuresis. To define the separate effects of an acute increase in plasma sodium (P(Na)), chloride (P(Cl)) or osmolality (P(osmol)), changes in renal blood flow (RBF) and GFR were measured during intrarenal infusions of hypertonic NaCl, NaHCO(3), Na acetate, dextrose, NH(4)Cl or NH(4)acetate to denervated kidneys. The infusions raised P(osmol) at the experimental kidney by 30-45 mosmol. RBF increased abruptly by 10-30% with all hypertonic infusions indicating that an acute increase in plasma tonicity causes renal vasodilatation. Renal vasodilatation persisted or increased further during infusion of dextrose, NaHCO(3) and Na acetate, but GFR was unchanged. In contrast, during infusion of the two Cl-containing solutions, vasodilatation was reversed after 1-5 min and RBF and GFR decreased (P < 0.01) below preinfusion levels. Prior salt depletion doubled the vasoconstriction seen with hypertonic NaCl infusions. Overall, changes in RBF were unrelated to changes in P(Na) or fractional Na or fluid reabsorption but correlated with changes in P(Cl) (r = -0.91) and fractional Cl(-) reabsorption (r = 0.94). The intrafemoral arterial infusion of the two Cl-containing solutions did not increase femoral vascular resistance. In conclusion, hyperchloremia produces a progressive renal vasoconstriction and fall in GFR that is independent of the renal nerves, is potentiated by prior salt depletion and is related to tubular Cl(-) reabsorption. Chloride-induced vasoconstriction appears specific for the renal vessels.

Regulation of renal blood flow by plasma chloride

Micropuncture studies have shown that glomerular filtration rate (GFR) falls in response to a rise in Na(+) or Cl(-) concentrations in the loop of Henle, whereas studies in isolated kidneys have shown that GFR falls in response to osmotic diuresis. To define the separate effects of an acute increase in plasma sodium (P(Na)), chloride (P(Cl)) or osmolality (P(osmol)), changes in renal blood flow (RBF) and GFR were measured during intrarenal infusions of hypertonic NaCl, NaHCO(3), Na acetate, dextrose, NH(4)Cl or NH(4)acetate to denervated kidneys. The infusions raised P(osmol) at the experimental kidney by 30-45 mosmol. RBF increased abruptly by 10-30% with all hypertonic infusions indicating that an acute increase in plasma tonicity causes renal vasodilatation. Renal vasodilatation persisted or increased further during infusion of dextrose, NaHCO(3) and Na acetate, but GFR was unchanged. In contrast, during infusion of the two Cl-containing solutions, vasodilatation was reversed after 1-5 min and RBF and GFR decreased (P < 0.01) below preinfusion levels. Prior salt depletion doubled the vasoconstriction seen with hypertonic NaCl infusions. Overall, changes in RBF were unrelated to changes in P(Na) or fractional Na or fluid reabsorption but correlated with changes in P(Cl) (r = -0.91) and fractional Cl(-) reabsorption (r = 0.94). The intrafemoral arterial infusion of the two Cl-containing solutions did not increase femoral vascular resistance. In conclusion, hyperchloremia produces a progressive renal vasoconstriction and fall in GFR that is independent of the renal nerves, is potentiated by prior salt depletion and is related to tubular Cl(-) reabsorption. Chloride-induced vasoconstriction appears specific for the renal vessels.

Modulation of renal tubular function by renal interstitium

Rat micropuncture experiments have been performed to investigate the influence of renal interstitial pressure conditions on proximal tubular fluid absorption and on the tubulo-glomerular feedback mechanism. The feedback mechanism works by sensing the distal tubular fluid flow rate at the macula densa segment and adjusting the tonus of the arterioles with resulting changes in blood flow and glomerular filtration rate. It was found that in situations with high hydrostatic and low oncotic pressure within the interstitium like in saline volume expansion and post-nephrectomy situations, proximal tubular fluid absorption and the sensitivity of the tubulo-glomerular feedback control are reduced giving rise to increased urine production without activating the feedback mechanism. In situations with low hydrostatic and high oncotic interstitial pressures like in dehydration, hypotension and hypovolemia proximal fluid absorption and feedback sensitivity is increased, feedback sensitivity even so much that the mechanism is activated to reduce GFR even though the load to the distal portion of the nephron is reduced below normal. In this way renal interstitial pressure-volume conditions can modulate renal function in response to extracellular fluid needs and blood pressure level.

Glomerular filtration rate and salt and water reabsorption during pregnancy in the conscious rat

1. Glomerular filtration rate (g.f.r.) and salt and water reabsorption were measured in age-matched conscious virgin rats and rats at different stages of pregnancy that were infused with saline at 40 and 200 microliters min-1. 2. G.f.r. and salt and water reabsorption were significantly higher at all stages of pregnancy irrespective of the the rate of infusion. 3. The similarity in both time course and magnitude, of changes in renal function between conscious and anaesthetized rats at the two infusion rates suggests that the detection of differences in the anaesthetized rat during pregnancy is not dependent on either the anaesthetic/surgery employed or the rate of expansion of the extracellular fluid volume. 4. During the induction of the diuresis in 20-day pregnant rats infused at 40 microliters min-1 urine flow was higher and osmolality lower than in virgin rats and rats at earlier stages of pregnancy, suggesting that late-pregnant rats have an altered ability for elaborating concentrated or dilute urine.

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.

Connection between the changes in tubular reabsorption and in glomerular filtration rate induced by replacement of plasma sodium and chloride by isotonic mannitol

Glomerular filtration and tubular reabsorption have been investigated comparatively in isolated blood-perfused dog kidneys, after addition to the blood of equal volumes of isotonic sodium chloride or isotonic mannitol. In the absence of antidiuretic hormone activity, the urine output showed no significant difference, while the glomerular filtration rate was lower in the presence of the mannitol load. The inhibition of fluid reabsorption was compensated by the decrease of filtration; a quantitative tubulo-glomerular equilibrium was evidenced. The fractional sodium excretion was decreased following the dilution by the mannitol load of plasma sodium and -chloride; this difference disappeared when the reabsorption in the diluting segment was inhibited by furosemide, suggesting that it depended on the rate of chloride reabsorption in this part of the tubule.

Comparison of directly measured and calculated glomerular capillary pressure in the dog kidney at varying perfusion pressure

To gain access to glomeruli of the dog kidney, an approximately 0.3 mm thick superficial tissue layer was cut off from the convexity of the kidney. The surface was superfused with warm Ringer's. Renal perfusion pressure (PP) was changed by an aortic clamp. The afferent oncotic pressure (pi A) was calculated from arterial plasma protein concentration, the efferent (pi E) from pi A and single nephron filtration fraction. At PP 17 and 13kPa, directly measured GCP was significantly lower than that calculated from stop-flow pressure (SFP + pi A). At PP8kPa, the difference was not significant. Hydraulic pressure differences across the glomerular capillary wall was still significantly different from pi E at PP 17 and 13 kPa. Hence, filtration pressure equilibrium was achieved in the dog kidney only at PP 8kPa.

Feedback activation in rat nephrons by sera from patients with acute renal failure

Tubuloglomerular feedback (TGF) may be triggered by varying the distal electrolyte load. TGF may contribute to acute renal failure (ARF), but maximal TGF stimulation induces only relatively small alterations in single nephron GFR (SNGFR) and stop-flow pressure (SFP), whereas in ARF, GFR may cease, raising the possibility that substances eliciting greater TGF responses than do elelectrolytes occur in ARF. We have sought such substances in serum, urine, and peritoneal fluid from patients with ARF. Sera samples from 1 healthy subject, 4 ARF patients, and 2 other anuric patients were dialyzed against glucose to reduce electrolyte concentrations and perfused through Henle's loops in salt-deplete rats. With ARF sera, SFP fell from 35.6 +/- (SD) 3.5 to 20.9 +/- 3.4 mm Hg (P less than 0.005) and SNGFR from 28.5 +/- 8.7 to 12.6 +/- 7.3 nl/min (P less than 0.005). Furosemide (10(-4) M) did not abolish the response. Perfusion with other patients' sera had no effect on SFP. ARF urine or peritoneal fluid induced similar responses. Conclusion. In some ARF patients, substances present in serum activate TGF in the rat. These substances might contribute to development of certain forms of human ARF, particularly ARF associated with liver dysfunction (hepatorenal syndrome).

Influence of replacement of chloride by sulphate upon urine excretion and glomerular filtration rate in blood perfused isolated dog kidneys

Tubular reabsorption was inhibited in isolated dog kidneys by the progressive substitution of plasma chloride by sulphate. In the absence of antidiuretic hormone activity, urine output remained unchanged owing to an equivalent decrease in glomerular filtration rate. This equilibrium was demonstrated under conditions of "saline natriuresis" and was not disturbed by furosemide. Although the impairment of glomerular filtration rate was accompanied by a decrease of total renal blood flow, the equilibrium was not disrupted by angiotensin antagonism. Sodium excretion was enhanced by low plasma chloride concentrations in the absence, but not in the presence of furosemide. The results are not compatible with a specific role of osmolality, sodium or chloride concentrations in the tubular fluid in the adjustment of glomerular filtration. Simultaneous changes in blood flow and tubular flow resistances might explain the results. It is suggested that, in contrast to the mechanism of tubulo-glomerular feedback found in individual nephrons of hydropenic animals, this intrarenal mechanism might serve to protect the organism against sodium loss under conditions of high intake.

Renal micropuncture study of normotensive and Milan hypertensive rats before and after development of hypertension

Earlier studies of renal transplantation and of sodium metabolism indicated that the cause of high blood pressure in the Milan strain of genetically hypertensive rats (MHS) was altered renal function. To pinpoint the active factors, we used micropuncture to study several indices of renal function in normal (NR) and MHS rats at three different ages: A) 26 to 30 days, before development of hypertension (pre-MHS); B) 35 to 40 days; and C) 75 to 90 days, after the development of hypertension. The indices studied and the important differences found between the two strains were: 1) Single nephron filtration rate (SNFR) and late proximal tubular fluid delivery to the distal nephron (LPF). In group A, the pre-MHS rats had significantly lower values than did the NR (SNFR = 6.3 +/- 0.8 nl/min [MHS] vs. 8.3 +/- 1.2 [NR], P less than 0.01; LPF = 3.14 +/- 0.25 nl/min [MHS] vs. 4.1 +/- 0.35 [NR], p less than 0.05). In group C, the values in the MHS rats were significantly higher than those of the NR (SNFR = 17.3 +/- 1.4 nl/min [MHS] vs. 12.1 +/- 0.8 [NR], P less than 0.05; LPF - 7.4 +/- 0.5 nl/min [MHS] vs. 5.3 +/- 0.3 [NR], P less than 0.01). 2) Number of glomeruli. In group C only, the MHS rats had significantly fewer than did the NR rats (MHS = 55, 253 +/- 2,821 vs. NR = 64,527 +/- 2,900, P less than 0.05). 3) Glomerular filtration rate (GFR) and SNFR as a function of the mean blood pressure (MAP). In group A, the GFR of the MHS rats was lower than that of the NR rats (GFR = 0.38 +/- 0.03 ml/min . 100 g of body wt [MHS], 0.50 +/- 0.03 [NR] P less than 0.05). In group C, there was no longer any significant difference. At equal MAP, SNFR was equal in all the groups, except group A, where SNFR was significantly lower in MHS. 4) Pressure differences: Glomerular capillary pressure (gP). GP was significantly higher in MHS rats than in NR rats (group A, + 5.2 mm Hg; group C, + 6.7 mm Hg). In the pre-MHS rats, anesthesia significantly increased (P less than 0.001) the blood pressure difference between the two strains. This effect was not seen in the adult MHS rats. This may increase the differences in GP between pre-MHS and NR. 5) Afferent effective filtration pressure (EFPA). EFPA values were also higher in MHS rats (+ 2.9 mm Hg in group A, + 6.8 mm Hg in group C), but once again the effects of anesthesia probably account for the differences in magnitude seen between pre-MHS and NR. Only 22% of the absolute differences in systemic arterial pressure in the adult MHS and NR rats was transmitted to the glomerular capillary, while 33% of the difference was transmitted in the younger rats. These values suggest a reduced glomerular hydraulic conductivity, even though other explanations could not be excluded, and they are consistent with the hypothesis that the primary cause of development of hypertension in the MHS rats may be a decrease in SNFR.

Tubuloglomerular feedback in rat kidneys of different renin contents

Variations in flow rate through the loop of Henle in the range of 0--50 nl/min were induced using pressure controlled microperfusion. Simultaneously, with the aid of a second pressure-microperfusionsystem, the glomerular function of the same nephron was studied by continuous measurement of two parameters, early proximal flow rate (EPFR) and/or stop flow pressure (SFP). Elevation of loop perfusion above physiological values (40 nl/min) resulted in a drop of EPFR and SFP, whereas lowering perfusion rates had no effect. This feedback behaviour was studied in kidneys with different renin contents to test the role of the renin-angiotensin system in the mediation of the macula densa signal to the adjacent glomerular vessels. Renal renin content, measured after micropuncture experiments by incubation with substrate followed by radioimmunoassay of angiotensin I, was unaltered in control (Ia) and heminephrectomized rats (Ib), lowered in contralateral kidneys of 2 kidneys Goldblatt hypertensive rats (IIa), in DOCA- and salt-loaded rats (IIb), and in DOCA-, salt-loaded and heminephrectomized rats (IIc), and it was evaluated in clipped kidneys of Goldblatt hypertension rats (IIIa). Micropuncture evaluation of the tubuloglomerular feedback behaviour in these experimental groups revealed the following results: 1. a feedback response under all conditions independent of the widely varying renin contents (1000-fold), 2. an asymmetrical behaviour of the feedback response in all kidneys as demonstrated by suppression of EPFR and SFP at elevated loop flow rates, but no change of these parameters when loop flow was interrupted. 3. compared to controls the decrease of each GFR parameter between 0 and 40 nl/min loop perfusion was lower in DOCA- and salt-loaded rats (IIb, IIc). Additional heminephrectomy (IIc) had no further influence on the reduced feedback response in DOCA- and salt-loaded rats, whereas this maneuver reduced the renal renin content drastically. A somewhat higher response than in controls was found in heminephrectomized rats (IIb) and in clipped kidneys of Goldblatt hypertensive rats (IIIa). These different magnitudes of feedback responses do not correlate with the renal renin content. It has been concluded, therefore, that renal renin activity is not the sole determinant of the effectiveness of the tubuloglomerular feedback response.

Maintenance of feedback regulation of filtration dynamics in the absence of divalent cations in the lumen of the distal tubule

In the present experiments we have studied the hypothesis that the feedback responses of glomerular capillary pressure and glomerular filtration rate to elevated distal fluid delivery depend to some extent on the luminal concentration of calcium or magnesium [1]. Loops of Henle were therefore perfused with the following solutions which were designed to yield wide variations of distal divalent cation concentration: 1. Ringer, 2. 140mM NaCl, 3. 125mM NaCl + 10mM CaCl2, 4. 125 mM NaCl + 10 mM MgCl2, 5. 125 mM NaCl + 10 mM Na citrate, and 6. 125 mM NaCl + 10 mM EDTA. During orthograde perfusion with these solutions stop flow pressure (SEP) and early proximal flow rate (EPFR) were measured in each nephron at perfusion rates of 0, 15, 30, and 45 nl/min. We found that perfusion with solutions 2 to 6 did not significantly modify the flow induced change of SFP or EPFR observed during Ringer perfusion. To expose the macula densa cells to chemically well defined solutions loops of Henle were retrogradely perfused from the distal tubule and EPFR was measured in a given nephron with and without perfusion. Identical reductions of EPFR were induced by retrograde perfusions with 140 mM NaCl, 125 mM NaCl + 10 mM CaCl2, and 125 mM NaCl + 10 mM EDTA. Furthermore, an almost complete blunting of the feedback response was noted during retrograde perfusion with 25 mM NaCl. Addition of 5 mM CaCl2 failed to restore the feedback reaction. These results do not support the concept that luminal divalent cations participate in the initiation of tubulo-glomerular feedback responses.