Cyclic GMP mediates influence of macula densa nitric oxide over tubuloglomerular feedback

Responses in Blood Pressure and Kidney Function to Soluble Guanylyl Cyclase Stimulation or Activation in Normal and Diabetic Rats

Introduction:

Agonists of soluble guanylate cyclase (sGC) are being developed as treatment for cardiovascular disease. Most effects of nitric oxide (NO) on glomerular and tubular function are mediated through sGC but whether sGC agonists mimic these effects is unknown.

Methods:

Renal clearance and micropuncture studies were performed in Wistar-Froemter rats (WF), with or without streptozotocin diabetes (STZ-WF), and in Goto-Kakizaki rats (GK) with mild type-2 diabetes to test for acute effects of the sGC “stimulator” BAY 41-2272, which synergizes with endogenous NO, and the “activator” runcaciguat, which generates cGMP independent of NO.

Results:

Both sGC agonists reduced arterial blood pressure (MAP). For MAP reductions <10% the drugs increased GFR in WF and STZ-WF but not in GK. Larger MAP reductions outweighed this effect and GFR declined, with better preserved GFR in STZ-WF. Changes in GFR could not be accounted for by changes in RBF, suggesting parallel changes in ultrafiltration pressure and/or ultrafiltration coefficient. The doses chosen for micropuncture in WF and GK reduced MAP by 2–10% and the net effect on single nephron GFR and ultrafiltration pressure was neutral. Effects of the drugs on tubular reabsorption were dominated by declining MAP and no natriuretic effect observed at any dose.

Discussion/Conclusion:

sGC agonists impact kidney function directly and because they reduce MAP. The direct tendency to increase GFR is most apparent for MAP reductions <10%. The direct effect is otherwise subtle and overridden when MAP declines more. Effects of sGC agonists on tubular reabsorption are dominated by effects on MAP.

Angiotensin-neprilysin inhibition and renal outcomes across the spectrum of ejection fraction in heart failure

AIMS

Patients with heart failure are at higher risk of progression to end-stage renal disease (ESRD), regardless of ejection fraction (EF). We assessed the renal effects of angiotensin-neprilysin inhibition in a pooled analysis of 13 195 patients with heart failure with reduced and preserved EF.

METHODS AND RESULTS

We combined data from PARADIGM-HF (EF ≤40%; n = 8399) and PARAGON-HF (EF ≥45%; n = 4796) in a pre-specified pooled analysis. We assessed the effect of treatment (sacubitril/valsartan vs. enalapril or valsartan) on a composite of either ≥50% reduction in estimated glomerular filtration rate (eGFR), ESRD, or death from renal causes, in addition to changes in eGFR slope. We assessed whether baseline renal function or EF modified the effect of therapy on renal outcomes. At randomization, eGFR was 68 ± 20 ml/min/1.73 m2 in PARADIGM-HF and 63 ± 19 ml/min/1.73 m2 in PARAGON-HF. The composite renal outcome occurred in 70 of 6594 patients (1.1%) in the sacubitril/valsartan group and in 123 of 6601 patients (1.9%) in the valsartan or enalapril group (hazard ratio 0.56, 95% confidence interval [CI] 0.42-0.75; p < 0.001). The mean eGFR change was -1.8 (95% CI -1.9 to -1.7) ml/min/1.73 m2 /year for the sacubitril/valsartan group, compared with -2.4 (95% CI -2.5 to -2.2) ml/min/1.73 m2 /year for the valsartan or enalapril group. The treatment effect on the composite renal endpoint was not modified by categories of baseline eGFR (p-interaction = 0.64), but was most pronounced in those with baseline EF between 30% and 60% (p-interaction = 0.001).

CONCLUSIONS

In patients with heart failure, sacubitril/valsartan reduced the risk of serious adverse renal outcomes and slowed decline in eGFR, compared with valsartan or enalapril, independent of baseline renal function.

Effects of Sacubitril/Valsartan Versus Valsartan on Renal Function in Patients with and without Diabetes and Heart Failure with Preserved Ejection Fraction: Insights From PARAGON-HF

AIMS

Diabetes is associated with a faster rate of renal function decline in patients with heart failure (HF). Sacubitril/valsartan attenuates the deterioration of renal function to a greater extent in patients with diabetes and HF with reduced ejection fraction compared with renin-angiotensin-system inhibitors alone. We assessed whether the same may be true in HF with preserved ejection fraction (HFpEF).

METHODS AND RESULTS

In the PARAGON-HF trial in patients with HF and LVEF of 45% or greater (n=4,796), we characterized the effects of sacubitril/valsartan on changes in estimated glomerular filtration rate (eGFR) over a period of 192 weeks, and on the prespecified renal composite outcome (eGFR reduction of ≥50%, end-stage renal disease, or death attributable to renal causes) in patients with (n=2,388) and without diabetes (n=2,408). The decline in eGFR was greater in patients with diabetes than in those without (-2.6 vs. -1.7 mL/min per 1·73 m² per year, p<0.001), regardless of treatment assignment. Sacubitril/valsartan attenuated decline in eGFR similarly in patients with (-2.2 vs. -2.9 mL/min per 1·73 m² per year, p=0.001) and without diabetes (-1.5 vs. -2.0 mL/min per 1·73 m² per year, p=0.006) (P_interaction for difference in eGFR slopes = 0.40). Compared with valsartan, sacubitril/valsartan reduced the renal composite outcome similarly in patients without diabetes (HR 0.42 (95% CI 0.19-0.91) and those with diabetes (HR 0.54 (95% CI 0.33-0.89); P_interaction =0.59), as well as across a range of baseline HbA1c (P_interaction =0.71).

CONCLUSION

Sacubitril/valsartan, compared with valsartan, attenuates the decline of eGFR and reduces clinically relevant kidney events similarly among patients with HFpEF with and without diabetes. This article is protected by copyright. All rights reserved.

Nitric oxide mediates anomalous tubuloglomerular feedback in rats fed high-NaCl diet after subtotal nephrectomy

Tubuloglomerular feedback (TGF) responses become anomalous in rats fed high-NaCl diet after subtotal nephrectomy (STN), such that stimulating TGF causes single nephron GFR (SNGFR) to increase rather than decrease. Micropuncture experiments were performed to determine whether this anomaly results from heightened nitric oxide response to distal delivery, which is a known mechanism for resetting TGF, or from connecting tubule TGF (cTGF), which is a novel amiloride-inhibitable system for offsetting TGF responses. Micropuncture was done in Wistar Froemter rats fed high-NaCl diet (HS) for 8-10 days after STN or sham nephrectomy. TGF was manipulated by orthograde microperfusion of Henle's loop with artificial tubular fluid with or without NOS inhibitor, LNMMA, or the cell-impermeant amiloride analog, benzamil. SNGFR was measured by inulin clearance in tubular fluid collections from the late proximal tubule. TGF responses were quantified as the increase in SNGFR that occurred when the perfusion rate was reduced from 50 to 8 nl/min in STN or 40 to 8 nl/min in sham animals. The baseline TGF response was anomalous in STN HS (-4 ± 3 vs 14 ± 3 nl/min, P < 0.001). TGF response was normalized by perfusing STN nephron with LNMMA (14 ± 3 nl/min, P < 0.005 for ANOVA cross term) but not with benzamil (-3 ± 4 nl/min, P = 0.4 for ANOVA cross term). Anomalous TGF occurs in STN HS due to heightened effect of tubular flow on nitric oxide signaling, which increases to the point of overriding the normal TGF response. There is no role for cTGF in this phenomenon.

Effect of neprilysin inhibition on renal function in patients with type 2 diabetes and chronic heart failure who are receiving target doses of inhibitors of the renin-angiotensin system: a secondary analysis of the PARADIGM-HF trial

BACKGROUND

Neprilysin inhibition has favourable effects on experimental diabetic nephropathy. We sought to assess the effects of neprilysin inhibition on the course of renal function in patients with type 2 diabetes.

METHODS

In the randomised, double-blind PARADIGM-HF trial, the effects of sacubitril/valsartan (97 mg/103 mg twice daily) were compared with enalapril (10 mg twice daily) in 8399 patients with mild-to-moderate chronic heart failure and systolic dysfunction. In this secondary intention-to-treat analysis, we assessed the change in estimated glomerular filtration rate (eGFR) over a 44-month follow-up period in patients with (n=3784) and those without (n=4615) diabetes. PARADIGM-HF is registered with ClinicalTrials.gov, number NCT01035255.

FINDINGS

eGFR decreased by 1·1 mL/min per 1·73 m² per year (95% CI 1·0-1·2) in patients without diabetes, but by 2·0 mL/min per 1·73 m² per year (1·9-2·1) in those with diabetes (p<0·0001). Compared with patients treated with enalapril, those treated with sacubitril/valsartan had a slower rate of decline in eGFR (-1·3 vs -1·8 mL/min per 1·73 m² per year; p<0·0001), and the magnitude of the benefit was larger in patients with versus those without diabetes (difference 0·6 mL/min per 1·73 m² per year [95% CI 0·4-0·8] in patients with vs 0·3 mL/min per 1·73 m² per year [0·2-0·5] in those without diabetes; p_interaction=0·038). The greater effect of neprilysin inhibition in patients with diabetes could not be explained by the effects of treatment on the course of heart failure or on HbA_1c. The incremental benefit of sacubitril/valsartan in patients with diabetes was no longer apparent when changes in eGFR were adjusted for urinary cyclic guanosine monophosphate (p=0·41).

INTERPRETATION

In patients in whom the renin-angiotensin system is already maximally blocked, the addition of neprilysin inhibition attenuates the effect of diabetes to accelerate the deterioration of renal function that occurs in patients with chronic heart failure.

FUNDING

Novartis.

Role of the sodium-hydrogen exchanger in mediating the renal effects of drugs commonly used in the treatment of type 2 diabetes

Diabetes is characterized by increased activity of the sodium-hydrogen exchanger (NHE) in the glomerulus and renal tubules, which contributes importantly to the development of nephropathy. Despite the established role played by the exchanger in experimental studies, it has not been specifically targeted by those seeking to develop novel pharmacological treatments for diabetes. This review demonstrates that many existing drugs that are commonly prescribed to patients with diabetes act on the NHE1 and NHE3 isoforms in the kidney. This action may explain their effects on sodium excretion, albuminuria and the progressive decline of glomerular function in clinical trials; these responses cannot be readily explained by the influence of these drugs on blood glucose. Agents that may affect the kidney in diabetes by virtue of an action on NHE include: (1) insulin and insulin sensitizers; (2) incretin-based agents; (3) sodium-glucose cotransporter 2 inhibitors; (4) antagonists of the renin-angiotensin system (angiotensin converting-enzyme inhibitors, angiotensin receptor blockers and angiotensin receptor neprilysin inhibitors); and (5) inhibitors of aldosterone action and cholesterol synthesis (spironolactone, amiloride and statins). The renal effects of each of these drug classes in patients with type 2 diabetes may be related to a single shared biological mechanism.

Augmentation of glucagon-like peptide-1 receptor signalling by neprilysin inhibition: potential implications for patients with heart failure

Augmentation of glucagon-like peptide-1 (GLP-1) receptor signalling is an established approach to the treatment of type 2 diabetes. However, endogenous GLP-1 and long-acting GLP-1 receptor analogues are degraded not only by dipeptidyl peptidase-4, but also by neprilysin. This observation raises the possibilities that endogenous GLP-1 contributes to the clinical effects of neprilysin inhibition and that patients concurrently treated with sacubitril/valsartan and incretin-based drugs may experience important drug-drug interactions. Specifically, potentiation of GLP-1 receptor signalling may underlie the antihyperglycaemic actions of sacubitril/valsartan. Neprilysin inhibitors may also be able to augment the effects of long-acting GLP-1 analogues to increase heart rate and myocardial cyclic AMP, and thus, potentiate these deleterious actions; if so, concomitant treatment with GLP-1 receptor agonists may limit the efficacy of neprilysin inhibitors in patients with both heart failure and diabetes. For patients not concurrently treated with GLP-1 analogues, the action of neprilysin to enhance the effects of GLP-1 may be particularly relevant in the brain, where augmentation of GLP-1 and other endogenous peptides may act to inhibit amyloid-induced neuroinflammation and cytotoxicity and improve memory formation and executive functioning. Experimentally, neprilysin inhibitors may also potentiate the effects of endogenous GLP-1 and GLP-1 receptor agonists on blood vessels and the kidney. The role of neprilysin in the metabolism of endogenous GLP-1 and long-acting GLP-1 analogues points to a range of potential pathophysiological effects that may be clinically relevant to patients with heart failure, with or without diabetes.

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

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

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Role of soluble guanylate cyclase in renal hemodynamics and autoregulation in the rat

We studied the influence of soluble guanylate (sGC) on renal blood flow (RBF), glomerular filtration rate (GFR), and RBF autoregulation and its role in mediating the hemodynamic effects of endogenous nitric oxide (NO). Arterial pressure (AP), heart rate (HR), RBF, GFR, urine flow (UV), and the efficiency and mechanisms of RBF autoregulation were studied in anesthetized rats during intravenous infusion of sGC activator cinaciguat before and (except GFR) also after inhibition of NO synthase (NOS) by Nω-nitro-l-arginine methyl ester. Cinaciguat (0.1, 0.3, 1, 3, 10 μg·kg−1·min−1, n = 7) reduced AP and increased HR, but did not significantly alter RBF. In clearance experiments (FITC-sinistrin, n = 7) GFR was not significantly altered by cinaciguat (0.1 and 1 μg·kg−1·min−1), but RBF slightly rose (+12%) and filtration fraction (FF) fell (−23%). RBF autoregulatory efficiency (67 vs. 104%) and myogenic response (33 vs. 44 units) were slightly depressed ( n = 9). NOS inhibition ( n = 7) increased AP (+38 mmHg), reduced RBF (−53%), and greatly augmented the myogenic response in RBF autoregulation (97 vs. 35 units), attenuating the other regulatory mechanisms. These changes were reversed by 77, 78, and 90% by 1 μg·kg−1·min−1 cinaciguat. In vehicle controls ( n = 3), in which cinaciguat-induced hypotension was mimicked by aortic compression, the NOS inhibition-induced changes were not affected. We conclude that sGC activation leaves RBF and GFR well maintained despite hypotension and only slightly impairs autoregulation. The ability to largely normalize AP, RBF, RBF autoregulation, and renovascular myogenic response after NOS inhibition indicates that these hemodynamic effects of NO are predominantly mediated via sGC.

Renal NMDA receptors independently stimulate proximal reabsorption and glomerular filtration

N-methyl-D-aspartate receptors (NMDA) are expressed in the kidney, where little is known of their functional role. Several series of micropuncture experiments were performed in hydropenic rats using the NMDA channel blocker, MK801, and the NMDA coagonist, L-glycine, to probe NMDA for effects on single-nephron glomerular filtration rate (SNGFR) and proximal reabsorption (J(prox)). During intravenous infusion of MK801 or L-glycine, Henle's loop was perfused to manipulate SNGFR via tubuloglomerular feedback (TGF), thereby facilitating analysis of glomerulotubular balance. To confirm local actions on the kidney, MK801 was delivered to the glomerulus by microperfusion past the macula densa and to the proximal tubule by microperfusion into the early S1 segment. By all measures, MK801 acted on the glomerulus to reduce SNGFR, and acted on the proximal tubule to suppress J(prox), while having no effect on the responsiveness of TGF. L-Glycine raised SNGFR, dampened the TGF response, and could not be proved to independently stimulate proximal reabsorption. NMDA exerts a tonic vasodilatory influence on the glomerulus and a proreabsorptive effect on the proximal tubule. These combined effects allow NMDA to modulate SNGFR with minimal impact on late proximal flow. The full effects of L-glycine infusion on proximal tubule and TGF response do not extrapolate from the response to NMDA blockade.

Flash photolysis of caged nitric oxide inhibits proximal tubular fluid reabsorption in free-flow nephron

A nonobstructing optical method was developed to measure proximal tubular fluid reabsorption in rat nephron at 0.25 Hz. The effects of uncaging luminal nitric oxide (NO) on proximal tubular reabsorption were investigated with this method. Proximal fluid reabsorption rate was calculated as the difference of tubular flow measured simultaneously at two locations (0.8-1.8 mm apart) along a convoluted proximal tubule. Tubular flow was estimated on the basis of the propagating velocity of fluorescent dextran pulses in the lumen. Changes in local tubular flow induced by intratubular perfusion were detected simultaneously along the proximal tubule, indicating that local tubular flow can be monitored in multiple sites along a tubule. The estimated tubular reabsorption rate was 5.52 +/- 0.38 nl.min(-1).mm(-1) (n = 20). Flash photolysis of luminal caged NO (potassium nitrosylpentachlororuthenate) was induced with a 30-Hz UV nitrogen-pulsed laser. Release of NO from caged NO into the proximal tubule was confirmed by monitoring intracellular NO concentration using a cell-permeant NO-sensitive fluorescent dye (DAF-FM). Emission of DAF-FM was proportional to the number of laser pulses used for uncaging. Photolysis of luminal caged NO induced a dose-dependent inhibition of proximal tubular reabsorption without activating tubuloglomerular feedback, whereas uncaging of intracellular cGMP in the proximal tubule decreased tubular flow. Coupling of this novel method to measure reabsorption with photolysis of caged signaling molecules provides a new paradigm to study tubular reabsorption with ambient tubular flow.