Role Of Tubuloglomerular Feedback Research Articles

SGLT2 Inhibitors and Kidney Protection: Mechanisms Beyond Tubuloglomerular Feedback.

Sodium-glucose cotransporter 2 (SGLT2) inhibitors reduce the risk for kidney failure and are a key component of guideline-directed therapy for CKD. While SGLT2 inhibitors' ability to activate tubuloglomerular feedback and reduce hyperfiltration-mediated kidney injury is considered to be the central mechanism for kidney protection, recent data from experimental studies raise questions on the primacy of this mechanism. This review examines SGLT2 inhibitors' role in tubuloglomerular feedback and summarizes emerging evidence on following of SGLT2 inhibitors' other putative mechanisms for kidney protection: optimization of kidney's energy substrate utilization and delivery, regulation of autophagy and maintenance of cellular homeostasis, attenuation of sympathetic hyperactivity, and improvement in vascular health and microvascular function. It is imperative to examine the effect of SGLT2 inhibition on these different physiologic processes to help our understanding of mechanisms underpinning kidney protection with this important class of drugs.

Lack of Connexins 40 and 45 Reduces Local and Conducted Vasoconstrictor Responses in the Murine Afferent Arterioles.

The juxtaglomerular apparatus (JGA) is an essential structure in the regulation of renal function. The JGA embodies two major functions: tubuloglomerular feedback (TGF) and renin secretion. TGF is one of the mechanisms mediating renal autoregulation. It is initiated by an increase in tubular NaCl concentration at the macula densa cells. This induces a local afferent arteriolar vasoconstriction and a conducted response that can be measured several 100 μm upstream from the juxtaglomerular segment. This spread of the vasomotor response into the surrounding vasculature likely plays a key role in renal autoregulation, and it requires the presence of gap junctions, intercellular pores based on connexin (Cx) proteins. Several Cx isoforms are expressed in the JGA and in the arteriolar wall. Disruption of this communication pathway is associated with reduced TGF, dysregulation of renin secretion, and hypertension. We examine if the absence of Cx40 or Cx45, expressed in the endothelial and vascular smooth muscle cells respectively, attenuates afferent arteriolar local and conducted vasoconstriction. Afferent arterioles from wildtype and Cx-deficient mice (Cx40 and Cx45) were studied using the isolated perfused juxtamedullary nephron preparation. Vasoconstriction was induced via electrical pulse stimulation at the glomerular entrance. Inner afferent arteriolar diameter was measured locally and upstream to evaluate conducted vasoconstriction. Electrical stimulation induced local vasoconstriction in all groups. The local vasoconstriction was significantly smaller when Cx40 was absent. The vasoconstriction decreased in magnitude with increasing distance from the stimulation site. In both Cx40 and Cx45 deficient mice, the vasoconstriction conducted a shorter distance along the vessel compared to wild-type mice. In Cx40 deficient arterioles, this may be caused by a smaller local vasoconstriction. Collectively, these findings imply that Cx40 and Cx45 are central for normal vascular reactivity and, therefore, likely play a key role in TGF-induced regulation of afferent arteriolar resistance.

Upregulation of renal Na–K–2Cl cotransporter 2 in obese diabetes mellitus via a vasopressin receptor 2-dependent pathway

Na–K–2Cl cotransporter 2 (NKCC2) in thick ascending limb (TAL) in the kidney plays a central role in tubuloglomerular feedback (TGF) system by sensing NaCl delivery to the distal tubules. Although accumulating data indicate that dysregulated TGF contributes to the progression of diabetic complications, the regulation of NKCC2 in diabetes mellitus (DM) remains unclear. We here show that NKCC2 is overactivated via a vasopressin receptor 2 (V2R)-dependent mechanism in db/db mice, a mouse model of obese DM. Compared with db/+ mice, we found that both aquaporin 2 and NKCC2 levels were significantly increased in the kidney in db/db mice. Immunohistochemical analysis of V2R and NKCC2 in the kidney demonstrated that V2R is present in the TAL, as well as in the collecting duct. Moreover, the administration of tolvaptan, a selective V2R antagonist, sharply decreased aquaporin 2 and NKCC2 in db/db mice, confirming the causal role of V2R signaling in NKCC2 induction in this model. Although tolvaptan reduced aquaporin 2 abundance also in db/+ mice, its effect on NKCC2 was modest compared with db/db mice. In total kidney lysates, uromodulin expression was not altered between db/+ and db/db mice, suggesting that V2R signaling alters NKCC2 without altering uromodulin levels. These data implicate the dysregulation of NKCC2 in the pathophysiology of type 2 DM, and underscore the complex nature of fluid volume disorders in diabetic kidney disease.

Targeting adenosine receptors in the development of cardiovascular therapeutics

Adenosine receptor stimulation has negative inotropic and dromotropic actions, reduces cardiac ischemia–reperfusion injury and remodeling, and prevents cardiac arrhythmias. In the vasculature, adenosine modulates vascular tone, reduces infiltration of inflammatory cells and generation of foam cells, and may prevent the development of atherosclerosis as a result. Modulation of insulin sensitivity may further add to the anti-atherosclerotic properties of adenosine signaling. In the kidney, adenosine plays an important role in tubuloglomerular feedback and modulates tubular sodium reabsorption. The challenge is to take advantage of the beneficial actions of adenosine signaling while preventing its potential adverse effects, such as salt retention and sympathoexcitation. Drugs that interfere with adenosine formation and elimination or drugs that allosterically enhance specific adenosine receptors seem to be most promising to meet this challenge.

Role of adenosine antagonism in the cardio-renal syndrome: Pathophysiology and therapeutic potential

The adversarial relationship between the heart and the kidney in heart failure results from normal homeostatic mechanisms which become inappropriate in the setting of heart failure. The cardio-renal syndrome represents the sum total of this adversarial relation and is clinically manifest as worsening renal function limiting diuresis in spite of volume overload. Tubuloglomerular feedback is a major pathophysiologic component of this syndrome. Adenosine, acting via A1 receptors, plays a major role in tubuloglomerular feedback in the normal state and in cardio-renal syndrome. Preclinical studies and initial human studies suggest that adenosine antagonism increases diuresis and preserves glomerular filtration in the setting of decompensated heart failure. Considering the ubiquitous distribution of adenosine receptors in the body, it is crucial to tailor this therapeutic agent to avoid side effects, especially neurotoxicity. Larger studies are presently underway to understand the therapeutic potential of this novel class of agents.

NKCC Inhibitors Furosemide and Bumetanide Directly Attenuate Afferent Arteriolar Myogenic Reactivity Independent of Actions on Tubuloglomerular Feedback.

The Na/K/2Cl-cotransporter (NKCC) plays a key role in tubuloglomerular feedback (TGF) signaling at the macula densa and NKCC inhibitors are thus frequently used to assess the impact of blockade of TGF on renal hemodynamics. However, NKCC is also implicated in establishing the chloride gradient in smooth muscle myocytes and pharmacologic inhibitors can have direct effects on vascular responses. In the present study we investigated the effects of the NKCC inhibitors, furosemide (F) and bumetanide (B), on myogenic responses of the renal afferent arteriole using the in vitro perfused hydronephrotic rat kidney. This preparation has no tubules and thus no TGF. F and B inhibited myogenic responses over concentrations of 0.1–10 μM (IC50 20 μM) and 0.01–1 μM (IC50 1.0 μM), respectively. Responses evoked by increasing renal arterial pressure from 80 to 160 mmHg were significantly attenuated at concentrations as low as 0.1 μM (P=0.03, F; P=0.002 B). At the highest concentrations employed, F (10 μM) and B (1.0 μM) inhibited responses by 72±4% and 68±5%, respectively. The maximal level of inhibition was not affected by nitric oxide synthase inhibition (100 μM L-NAME), but the time course for the dilation was slowed (t½ = 4.0 ±0.5 min to 8.3±1.7 min, P=0.04, B), suggesting an involvement of NO. RT-PCR performed on single renal arterioles isolated from the normal rat kidney revealed the expression of NKCC1. These results indicate that NKCC is expressed in the afferent arteriole and that NKCC inhibition attenuates myogenic responses by both NO-independent and NO-dependent mechanisms. These effects do not involve a modulation of TGF.

Efferent arteriole tubuloglomerular feedback in the renal nephron

Afferent and efferent arteriole resistance exerts critical and opposite actions in the regulation of glomerular capillary pressure (PGC) and glomerular filtration rate (GFR). Tubuloglomerular feedback (TGF) plays an important role in the regulation of afferent arteriole resistance; however, the role of TGF in the regulation of efferent arteriole resistance is less well established. We hypothesized that TGF caused by increased NaCl in the tubular fluid stimulates the macula densa to initiate a cascade of events resulting in efferent arteriole vasodilation, mediated by adenosine via its A2 receptor. Rabbit efferent arterioles and adherent tubular segments with macula densa were simultaneously microperfused in vitro while changing NaCl concentration at the macula densa. To study whether autacoids produced by the glomerulus participate in the effect of TGF on efferent arterioles, they were perfused orthograde or retrograde. To eliminate the hemodynamic influence of the afferent arteriole during orthograde perfusion, the perfusion pipette was advanced to the distal end of the afferent arteriole, and the tip of the pressure pipette was placed beyond the afferent arteriole; for retrograde perfusion, the efferent arteriole was perfused from its distal end. In efferent arterioles perfused orthograde and preconstricted with norepinephrine (NE), increasing NaCl concentration at the macula densa increased the diameter by 33%. In preconstricted efferent arterioles perfused retrograde, increasing NaCl at the macula densa increased the diameter by 33%. Efferent arteriole vasodilation was completely blocked by a selective adenosine A2 receptor antagonist (3, 7-dimethyl-1-propargylxanthine) but not by an adenosine A1 receptor antagonist (FK838). Our data show that in vitro, preconstricted efferent arterioles dilate in response to increased macula densa NaCl, and this process is mediated by activation of adenosine A2 receptors. Thus, TGF changes efferent arteriole resistance in the opposite direction from the afferent arteriole, possibly amplifying TGF regulation of PGC and GFR. In vivo efferent arteriole TGF may only buffer the signals that cause efferent arteriole resistance to parallel changes in afferent arteriole resistance. Effects of TGF on efferent arterioles perfused orthograde or retrograde were similar, suggesting that glomerular autacoids do not participate in this process.

Ultrastructural localization of Na-K-2Cl cotransporter in thick ascending limb and macula densa of rat kidney.

A bumetanide-sensitive Na-K-2Cl cotransporter, BSC-1, is believed to mediate the apical component of transcellular NaCl absorption in the thick ascending limb (TAL) of Henle's loop. To study its ultrastructural localization in kidney, we used an affinity-purified, peptide-derived polyclonal antibody against rat BSC-1. Immunoblots from rat kidney cortex and outer medulla revealed a solitary 161-kDa band in membrane fractions. Immunocytochemistry of 1-micrometer cryosections demonstrated strong BSC-1 labeling of the apical and subapical regions of medullary and cortical TAL cells. Notably, macula densa cells also exhibited distinct labeling. Distal convoluted tubules and other renal tubule segments were unlabeled. Immunoelectron microscopy demonstrated that BSC-1 labeling was associated with the apical plasma membrane and with subapical intracellular vesicles in medullary and cortical TAL and in macula densa cells. Smooth-surfaced TAL cells, in particular, had extensive BSC-1 labeling of intracellular vesicles. These results support the view that BSC-1 provides the apical pathway for NaCl transport across the TAL and that an extensive intracellular reservoir of BSC-1 is present in a subpopulation of TAL cells. Furthermore, the BSC-1 localization in the apical plasma membrane of macula densa cells is consistent with its proposed role in tubuloglomerular feedback.

Progression rate of radiation damage to the mouse kidney: a quantitative analysis of experimental data using a simple mathematical model of the nephron

. Mouse kidneys were irradiated bilaterally with a range of single or fractionated X-ray doses. After an interval of 2 weeks or 26 weeks, the animals were reirradiated with a range of single X-ray doses. The rate of development of functional kidney damage was assessed repeatedly by the 51Cr-EDTA clearance assay. The rate at which the damage is expressed was found to depend on the primary dose, on the interval between primary treatment and retreatment, and on the retreatment dose. A subset of the data was analysed using a mathematical model of nephron function. In the model, the residual activity of 51Cr-EDTA depends on the glomerular filtration rate (GFR). The GFR is related to the cellularities of three target cell populations. The filtration capacity of the glomerulus is assumed to depend on the numbers of glomerular endothelial cells and mesangial cells. The reabsorption capacity of the tubule is related to the number of tubular epithelial cells. The impact of tubuloglomerular feedback and the reserve capacity of the kidney on residual activity is considered. The target cell populations are assumed to be of a flexible type, i.e. to consist of cells which are all both functional and self-renewing. Free parameters of the model were optimized by minimizing the residual sum of squares. With the optimized parameter values, the measured and the model-predicted rates of progression of the functional damage correspond well for a wide range of irradiation schedules. The model analysis suggests a pronounced role of tubulo-glomerular feedback in the development of functional injury in the kidney. It is concluded that the model represents a good starting point for quantitative studies of the cellular basis of radiation nephropathy.

Effect of P-450 omega-hydroxylase metabolites of arachidonic acid on tubuloglomerular feedback

The role of endogenous P-450 metabolites of arachidonic acid (AA) on the tubuloglomerular feedback (TGF) response was examined. Under control conditions stop-flow pressure (SFP) fell by 17.0 +/- 2.1 mmHg when the perfusion rate of the loop of Henle was increased from 0 to 50 nl/min. Addition of AA (50 microM) to the perfusate lowered basal SFP by 11.4 +/- 1.1 mmHg and potentiated the TGF response. This effect was blocked by addition of a P-450 inhibitor, 17-octadecynoic acid (17-ODYA) (10 microM), to the perfusate. Perfusion of the loop of Henle with 17-ODYA elevated basal SFP by 3.7 +/- 0.3 mmHg and reduced the TGF response by 80%. After blockade of endogenous P-450 activity with 17-ODYA, addition of 20-hydroxyeicosatetraenoic acid (20-HETE, 10 microM) to the perfusate produced a flow rate-dependent fall in SFP. The effect of 20-HETE was not altered by pretreating the animal with meclofenamate (2 mg/kg iv) or by perfusing the nephron segment with furosemide (50 microM). These results indicate that endogenous P-450 metabolites of AA, particularly 20-HETE, may play a role in TGF and the regulation of renal vascular tone.

A single nephron model of acute tubular injury: Role of tubuloglomerular feedback

A single nephron model of nephrotoxic tubular injury was established to examine the mechanism whereby acute tubular damage contributes to reductions in nephron filtration rate (SNGFR). Acute microperfusion of 0.5 ng of uranyl nitrate (UN) into the early proximal tubule produced a significant reduction (16 to 30%) in SNGFR measured in both distal and proximal tubules of the same nephron and a decrease in absolute proximal reabsorption. Microperfused inulin was retained in the tubule suggesting this finding reflected a true reduction in SNGFR. Concurrent infusion of high dose furosemide (2 x 10(-4)M) and bumetanide (2 x 10(-5) M), but not low dose furosemide (2 x 10(-5) M), prevented the UN induced reduction in SNGFR. High dose furosemide begun after UN perfusion also prevented reduction in SNGFR. Continuous direct measurement of glomerular capillary hydrostatic pressure revealed no change. Distal intratubular Na+ and Cl- concentration increased significantly after UN perfusion. Activation of tubuloglomerular feedback mechanisms best explains the reduction in glomerular ultrafiltration that is characteristic of nephrotoxic forms of tubular injury.

The role of tubuloglomerular feedback in acute impairment of renal function in obstructive jaundice.

Acute renal functional impairment not infrequently accompanies liver dysfunction and, particularly with bile duct obstruction, may be extremely severe. Recent studies suggest that tubuloglomerular feedback (TGF) activated by circulating non-electrolyte factors which occur during liver dysfunction may contribute to the intense renal vasoconstriction thought to be central to the functional renal impairment. In this study, serum from two patients with obstructive jaundice (OJ) and renal impairment, and from rats with OJ due to bile duct ligation were either dialysed or treated with furosemide, known to block electrolyte-mediated TGF. These sera, when perfused into loops of Henle in rat nephrons, induced a significant fall of 28% in stop-flow pressure, an indirect measure of glomerular capillary pressure thus implying arteriolar vasoconstriction. These findings are consistent with the hypothesis that circulating, non-electrolyte factors, which stimulate TGF, occur in cases of obstructive jaundice and that these may contribute to the renal impairment.