Hypohydration attenuates increases in creatinine clearance to oral protein loading and the renal hemodynamic response to exercise pressor reflex

Insufficient hydration is prevalent among free living adults. This study investigated whether hypohydration alters 1) renal functional reserve, 2) the renal hemodynamic response to the exercise pressor reflex, and 3) urine-concentrating ability during oral protein loading. In a block-randomized crossover design, 22 healthy young adults (11 females and 11 males) underwent 24-h fluid deprivation (Hypohydrated) or 24-h normal fluid consumption (Euhydrated). Renal functional reserve was assessed by oral protein loading. Renal hemodynamics during the exercise pressor reflex were assessed via Doppler ultrasound. Urine-concentrating ability was assessed via free water clearance. Creatinine clearance did not differ at 150 min postprotein consumption between conditions [Hypohydrated: 246 mL/min, 95% confidence interval (CI): 212-280; Euhydrated: 231 mL/min, 95% CI: 196-265, P = 0.2691] despite an elevated baseline in Hypohydrated (261 mL/min, 95% CI: 218-303 vs. 143 mL/min, 95% CI: 118-168, P < 0.0001). Renal artery vascular resistance was not different at baseline (P = 0.9290), but increases were attenuated in Hypohydrated versus Euhydrated at the end of handgrip (0.5 mmHg/cm/s, 95% CI: 0.4-0.7 vs. 0.8 mmHg/cm/s 95% CI: 0.6-1.1, P = 0.0203) and end occlusion (0.2 mmHg/cm/s, 95% CI: 0.1-0.3 vs. 0.4 mmHg/cm/s 95% CI: 0.3-0.6, P = 0.0127). There were no differences between conditions in free water clearance at 150 min postprotein (P = 0.3489). These data indicate that hypohydration 1) engages renal functional reserve and attenuates the ability to further increase creatinine clearance, 2) attenuates increases in renal artery vascular resistance to the exercise pressor reflex, and 3) does not further enhance nor impair urine-concentrating ability during oral protein loading.NEW & NOTEWORTHY Insufficient hydration is prevalent among free living adults. This study found that hypohydration induced by 24-h fluid deprivation engaged renal functional reserve and that oral protein loading did not further increase creatinine clearance. Hypohydration also attenuated the ability to increase renal vascular resistance during the exercise pressor reflex. In addition, hypohydration neither enhanced nor impaired urine-concentrating ability during oral protein loading. These data support the importance of mitigating hypohydration in free living adults.

Renal Functional Reserve in Acute Kidney Injury Patients Requiring Dialysis

The incidence of acute kidney injury (AKI) has increased in the recent past. Patients with AKI have an increased risk of mortality. They are also at increased risk of developing chronic kidney disease (CKD). AKI can lead to irreversible loss of renal function despite complete clinical recovery. Currently, no tools are available to diagnose this subclinical loss of renal function. Renal functional reserve (RFR) can serve as an essential tool for analyzing this subclinical loss of renal function, and patients with loss of RFR post-AKI may be closely followed for the development of CKD. This prospective observational study, conducted at the Department of Nephrology, Jawaharlal Institute of Postgraduate Medical Education & Research (JIPMER), aimed to investigate RFR in 223 patients with AKI requiring dialysis. The study excluded patients with CKD and obstructive uropathy. Methods included RFR assessment three months post-AKI recovery, utilizing technetium-99m (Tc-99m) diethylenetriaminepentaacetic acid (DTPA) plasma clearance during amino acid infusion. Statistical analyses and logistic regression were applied, receiving ethical approval. Results revealed a high in-hospital mortality rate of 78.02%, associated with elevated Sequential Organ Failure Assessment (SOFA) scores. Among 24 patients with complete AKI recovery, the RFR at three months was 10.06% (interquartile range (IQR) 5.60-20.15), with the measured GFR significantly lower than the estimated glomerular filtration rate (GFR). The study concludes that AKI requiring dialysis is linked to high mortality and emphasizes the predictive value of SOFA scores. Additionally, RFR testing at three months post-recovery provides insights into potential long-term impacts on renal function. This study contributes valuable insights into the prognosis of AKI patients requiring dialysis. It underscores the need for further research on RFR as a diagnostic tool and the lasting consequences of AKI.

Renal safety and survival among acutely ill hospitalized patients treated by blockers of the Renin-Angiotensin axis or loop diuretics: a single-center retrospective analysis

BACKGROUND

Concern exists regarding the renal safety of blocking the renin-angiotensin system (RAS) during acute illness, especially in the presence of volume depletion and hemodynamic instability.

METHODS

We explored the impact of loop diuretics and RAS blockers on the likelihood of developing acute kidney injury (AKI) or acute kidney functional recovery (AKR) among inpatients. Adjusted odds ratio for AKI, AKR and mortality was calculated, using logistic regression models, with subgroup analysis for patients with estimated glomerular filtration rate (eGFR) <30 ml/min/1.73 m2, corrected for blood pressure measurements.

RESULTS

53,289 patients were included. RAS blockade was associated with reduced adjusted odds ratio for both AKI (0.76, CI 0.70-0.83) AKR (0.55, 0.52-0.58), and mortality within 30 days (0.44, 0.41-0.48), whereas loop diuretics were associated with increased risk of AKI (3.75, 3.42-4.12) and mortality (1.71, 1.58-1.85) and reduced AKR (0.71, 0.66-0.75). Comparable impact of RAS blockers and loop diuretics on renal outcomes and death was found among 6,069 patients with eGFR < 30 ml/min/1.73m2. RAS inhibition and diuretics tended to increase the adjusted odds ratios for AKI and to reduce the likelihood of AKR in hypotensive patients.

CONCLUSIONS

Reduced blood pressure, RAS blockers and diuretics affect the odds of developing AKI or AKR among inpatients, suggesting possible disruption in renal functional reserve (RFR). As long as blood pressure is maintained, RAS inhibition seems to be safe and renoprotective in this population, irrespective of kidney function upon admission, and is associated with reduced mortality.

Renal Functional Response-Association With Birth Weight and Kidney Volume

Introduction

Renal functional response (RFR) is the acute increase in glomerular filtration rate (GFR) after a protein load. Low RFR is a marker of single nephron hyperfiltration. Low birth weight (LBW) is associated with reduced number of nephrons, lower kidney function, and smaller kidneys in adults. In the present study, we investigate the associations among LBW, kidney volume, and RFR.

Methods

We studied adults aged 41 to 52 years born with either LBW (≤2300 g) or normal birth weight (NBW; 3500-4000 g). GFR was measured using plasma clearance of iohexol. A stimulated GFR (sGFR) was measured on a separate day after a protein load of 100 g using a commercially available protein powder, and RFR was calculated as delta GFR. Kidney volume was estimated from magnetic resonance imaging (MRI) images using the ellipsoid formula.

Results

A total of 57 women and 48 men participated. The baseline mean ± SD GFR was 118 ± 17 ml/min for men and 98 ± 19 ml/min for women. The overall mean RFR was 8.2 ± 7.4 ml/min, with mean RFR of 8.3 ± 8.0 ml/min and 8.1 ± 6.9 ml/min in men and women, respectively (P = 0.5). No birth-related variables were associated with RFR. Larger kidney volume was associated with higher RFR, 1.9 ml/min per SD higher kidney volume (P = 0.009). Higher GFR per kidney volume was associated with a lower RFR, -3.3ml/min per SD (P < 0.001).

Conclusion

Larger kidney size and lower GFR per kidney volume were associated with higher RFR. Birth weight was not shown to associate with RFR in mainly healthy middle-aged men and women.

Beneficial effects of brief early life angiotensin-converting enzyme inhibition wane with time in sheep with solitary functioning kidney

A child with a congenital solitary functioning kidney (SFK) may develop kidney disease from early in life due to hyperfiltration injury. Previously, we showed in a sheep model of SFK that brief angiotensin-converting enzyme inhibition (ACEi) early in life is reno-protective and increases renal functional reserve (RFR) at 8 months of age. Here we investigated the long-term effects of brief early ACEi in SFK sheep out to 20 months of age. At 100 days gestation (term = 150 days) SFK was induced by fetal unilateral nephrectomy, or sham surgery was performed (controls). SFK lambs received enalapril (SFK+ACEi; 0.5 mg/kg, once daily, orally) or vehicle (SFK) from 4 to 8 weeks of age. At 8, 14 and 20 months of age urinary albumin excretion was measured. At 20 months of age, we examined basal kidney function and RFR via infusion of combined amino acid and dopamine (AA+D). SFK+ACEi resulted in lower albuminuria (∼40%) at 8 months, but not at 14 or 20 months of age compared with vehicle-SFK. At 20 months, basal GFR (∼13%) was lower in SFK+ACEi compared with SFK, but renal blood flow (RBF), renal vascular resistance (RVR) and filtration fraction were similar to SFK. During AA+D, the increase in GFR was similar in SFK+ACEi and SFK animals, but the increase in RBF was greater (∼46%) in SFK+ACEi than SFK animals. Brief ACEi in SFK delayed kidney disease in the short-term but these effects were not sustained long-term.

The effects of recruitment of renal functional reserve on renal cortical and medullary oxygenation in non-anesthetized sheep

AIM

Recruitment of renal functional reserve (RFR) with amino acid loading increases renal blood flow and glomerular filtration rate. However, its effects on renal cortical and medullary oxygenation have not been determined. Accordingly, we tested the effects of recruitment of RFR on renal cortical and medullary oxygenation in non-anesthetized sheep.

METHODS

Under general anesthesia, we instrumented 10 sheep to enable subsequent continuous measurements of systemic and renal hemodynamics, renal oxygen delivery and consumption, and cortical and medullary tissue oxygen tension (PO2 ). We then measured the effects of recruitment of RFR with an intravenous infusion of 500 ml of a clinically used amino acid solution (10% Synthamin® 17) in the non-anesthetized state.

RESULTS

Compared with baseline, Synthamin® 17 infusion significantly increased renal oxygen delivery mean ± SD maximum increase: (from 0.79 ± 0.17 to 1.06 ± 0.16 ml/kg/min, p < 0.001), renal oxygen consumption (from 0.08 ± 0.01 to 0.15 ± 0.02 ml/kg/min, p < 0.001), and glomerular filtration rate (+45.2 ± 2.7%, p < 0.001). Renal cortical tissue PO2 increased by a maximum of 26.4 ± 1.1% (p = 0.001) and medullary tissue PO2 increased by a maximum of 23.9 ± 2.8% (p = 0. 001).

CONCLUSIONS

In non-anesthetized healthy sheep, recruitment of RFR improved renal cortical and medullary oxygenation. These observations might have implications for the use of recruitment of RFR for diagnostic and therapeutic purposes.

The Assessment of Renal Functional Reserve in β-Thalassemia Major Patients by an Innovative Ultrasound and Doppler Technique: A Pilot Study

Beta-thalassemia syndromes are the most common inherited monogenic disorders worldwide. The most common pathophysiologic and clinical renal disease manifestations of in β-TM patients is the tubular dysfunctions related to iron overload, chronic anemia, and the need for chronic iron chelation therapy. The aim of this pilot study is to apply an innovative ultrasound and Doppler technique to assess the Renal Functional Reserve (RFR) in β-TM patients, and to evaluate its reliability in iron overload tubulopathy. Ultrasound assessment of intra-parenchymal renal resistive index variation (IRRIV) has recently been proposed as a safe and reproducible technique to identify RFR presence. We define the preserved RFR when the Delta Renal Resistive Index (RRI) is >0.05 (baseline RRI—minimum RRI value during stress) in the Renal Stress Test (RST). Nineteen β-TM patients were enrolled for this study. In our series, we found a strong negative correlation between mean ferritin values and Delta RRI (R = −0.51, p = 0.03). This pilot study suggested the RST as reliable tool for assessing the RFR by ultrasound. Specifically, RST could help in clinical practice suggesting the patient’s management and iron chelation therapy.

Renal functional recovery among inpatients: A plausible marker of reduced renal functional reserve

Renal functional reserve (RFR) reflects the ability of the kidney to enhance glomerular filtration rate (GFR) in response to a protein load. Chronic kidney disease (CKD) leads to diminished RFR, since the capacity for whole-body GFR to increase through hyperfiltration of remaining nephrons is limited. Evaluating 41,456 inpatients following computerised tomography we reported many exhibiting acute kidney injury (AKI) but more patients with recovering kidney function (AKR), presumably reflecting resolution of their critical conditions. The incidences of AKI and AKR were closely co-associated and were both inversely correlated with baseline kidney function. We discuss this phenomenon, arguing that AKR among inpatients with an acute illness, like AKI, may often reflect underlying subtle CKD with diminished RFR.

Current Concepts of Pediatric Acute Kidney Injury-Are We Ready to Translate Them into Everyday Practice?

Pediatric acute kidney injury (AKI) is a major cause of morbidity and mortality in children undergoing interventional procedures. The review summarizes current classifications of AKI and acute kidney disease (AKD), as well as systematizes the knowledge on pathophysiology of kidney injury, with a special focus on renal functional reserve and tubuloglomerular feedback. The aim of this review is also to show the state-of-the-art in methods assessing risk and prognosis by discussing the potential role of risk stratification strategies, taking into account both glomerular function and clinical settings conditioned by fluid overload, urine output, or drug nephrotoxicity. The last task is to suggest careful assessment of eGFR as a surrogate marker of renal functional reserve and implementation of point-of-care testing, available in the case of biomarkers like NGAL and [IGFBP-7] × [TIMP-2] product, into everyday practice in patients at risk of AKI due to planned invasive procedures or treatment.

A Systematic Review of Renal Functional Reserve in Adult Living Kidney Donors

Introduction

The kidney’s capacity to increase its glomerular filtration rate (GFR) in response to a higher functional demand is known as the renal functional reserve (RFR). Good short-term outcomes after living kidney donation have led to more acceptance of borderline donors (with hypertension, obesity, older age) due the ongoing shortage of donor organs. Given recent concerns about increased long-term risk in some donor subgroups, better donor stratification is needed. Measurement of RFR could inform assessment of donor risk.

Methods

A systematic literature review of studies that assessed RFR in donors pre- and/or post-donation was performed. Given study heterogeneity, descriptive analysis and narrative synthesis was conducted.

Results

Sixteen of 3250 identified studies published between 1956 and 2019 met inclusion criteria. Most studies were cross-sectional and conducted before (n = 8) and/or after (n = 16) kidney donation. Methods for measurement of GFR, effective renal plasma flow (ERPF) and RFR were not standardized. Changes in filtration fraction (FF) and ERPF relative to GFR observed after donation varied depending on stimulus used to induce RFR. Overall, RFR fell after donation; however, over the shorter term, RFR was largely preserved in young healthy donors. RFR was more significantly reduced in donors with hypertension, obesity, or older age.

Conclusion

Existing data suggest possible blunting of RFR post-donation in older, obese, and hypertensive donors, which may represent increased single-nephron GFR at baseline. The long-term implications of these changes deserve further study to determine utility in informing selection of borderline kidney donors.

Measurement of renal functional response using iohexol clearance-a study of different outpatient procedures

Background

Glomerular filtration rate (GFR) increases after a heavy protein load; an increase termed renal functional response (RFR). Decreased RFR could be a marker of early kidney damage, but published methods are cumbersome in the outpatient setting. The present study investigates the use of iohexol clearance to measure RFR in outpatients using both one- and two-sample methods.

Methods

Fourteen healthy volunteers with a mean ± SD age of 42 ± 12 years were included (six males and eight females). GFR was measured using plasma iohexol clearance with one- and two-sample methodologies. Four measurements in each individual were performed: one baseline test and three protein loading tests containing 80 g protein (commercially available protein supplementations from Myo Nutrition and Proteinfabrikken and 350 g chicken breast). RFR was calculated as percentage increase in GFR from the baseline test.

Results

Mean RFR was 11.4 ± 5.4% and 12.1 ± 6.4% using one- and two-sample methods, respectively. The three different protein loads resulted in similar mean RFR but there was considerable intra-individual variability. One- and two-sample methods for measurement of RFR showed similar results with near-identical means, but there was some intra-individual variation that was similar for different protein loads. The overall 95% limit of agreement between one- and two-sample methods for calculating RFR was -8.7 to 7.3.

Conclusions

RFR can be investigated using plasma iohexol clearance in an outpatient setting. Protocols using commercially available protein supplementation showed a mean RFR of about 12%. One- and two-sample methods for measuring RFR yield similar results.

Haemodynamic or metabolic stimulation tests to reveal the renal functional response: requiem or revival?

Renal stimulation tests document the dynamic response of the glomerular filtration rate (GFR) after a single or a combination of stimuli, such as an intravenous infusion of dopamine or amino acids or an oral protein meal. The increment of the GFR above the unstimulated state has formerly been called the renal functional reserve (RFR). Although the concept of a renal reserve capacity has not withstood scientific scrutiny, the literature documenting renal stimulation merits renewed interest. An absent or a blunted response of the GFR after a stimulus indicates lost or diseased nephrons. This information is valuable in preventing, diagnosing and prognosticating acute kidney injury and pregnancy-related renal events as well as chronic kidney disease. However, before renal function testing is universally practiced, some shortcomings must be addressed. First, a common nomenclature should be decided upon. The expression of RFR should be replaced by renal functional response. Second, a simple protocol must be developed and propagated. Third, we suggest designing prospective studies linking a defective stimulatory response to emergence of renal injury biomarkers, to histological or morphological renal abnormalities and to adverse renal outcomes in different renal syndromes.

Overweight young female kidney donors have low renal functional reserve postdonation

Maintenance of adequate renal function after living kidney donation is important for donor outcome. Overweight donors, in particular, may have an increased risk for end-stage kidney disease (ESKD), and young female donors have an increased preeclampsia risk. Both of these risks may be associated with low postdonation renal functional reserve (RFR). Because we previously found that higher body mass index (BMI) was associated with lower postdonation RFR, we now studied the relationship between BMI and RFR in young female donors. RFR, the rise in glomerular filtration rate (GFR) (¹²⁵I-iothalamate clearance) during dopamine, was measured in female donors (<45 yr) before and after kidney donation. Donors who are overweight (BMI >25) and nonoverweight donors were compared by Student's t-test; the association was subsequently explored with regression analysis. We included 105 female donors [age 41 (36-44) median(IQR)] with a BMI of 25 (22-27) kg/m². Predonation GFR was 118 (17) ml/min [mean(SD)] rising to 128 (19) ml/min during dopamine; mean RFR was 10 (10) ml/min. Postdonation GFR was 76 (13) ml/min, rising to 80 (12); RFR was 4 (6) ml/min ( P < 0.001 vs. predonation). In overweight donors, RFR was fully lost after donation (1 ml/min vs. 10 ml/min predonation, P < 0.001), and BMI was inversely associated with RFR after donation, independent of confounders (standardized β 0.37, P = 0.02). Reduced RFR might associate with the risk of preeclampsia and ESKD in kidney donors. Prospective studies should explore whether RFR is related to preeclampsia and whether BMI reduction before conception is of benefit to overweight female kidney donors during and after pregnancy.

Estimation of Renal Functional Reserve in Children with Different Grades of Vesicoureteric Reflux

Background

Vesicoureteric reflux (VUR) is one of the most common anomalies encountered in pediatric urology. The concept of renal functional reserve (RFR) as the ability of the kidney to increase glomerular filtration rate (GFR) following a protein load was introduced in the 1980s.

Aim

This study aims to evaluate RFR using ⁹⁹Tc diethylenetriamine pentaacetic acid (DTPA) as the filtration agent for GFR estimation in children with VUR.

Materials and Methods

RFR was estimated in 53 children, of which 31 patients had unilateral VUR (Group I) and 22 patients had bilateral VUR (Group II), by subtracting baseline GFR from stimulated GFR following an intravenous protein load. GFR was determined by double compartment-2 sample method using ⁹⁹Tc DTPA radioisotope as the filtration agent. Both the groups were further subgrouped into low-grade (IA, IIA) and high-grade VUR (IB, IIB).

Results

The RFR was significantly lower in unilateral high-grade VUR (Group IB) as compared to unilateral low-grade VUR (Group IA) (P = 0.024). RFR was significantly lower in bilateral high-grade VUR patients (IIB) as compared to unilateral low-grade VUR group (IA) (P = 0.0226). Furthermore, the stimulated GFR shows very strong correlation to baseline GFR in both major groups (r = 0.9659 and P = 0.001 in Group I and r = 0.9856 and P = 0.001 in Group II) concluding that the baseline GFR and the stimulated GFR increase or decrease in tandem in both the groups.

Conclusion

The RFR is impaired in children with both unilateral high-grade VUR and bilateral high-grade VUR while it is relatively preserved in unilateral low-grade VUR and bilateral low-grade VUR.

Renal recovery

Recovery patterns after acute kidney injury (AKI) have increasingly become the focus of research, because currently available preventive measures and specific therapeutic intervention are limited. Moreover, changes in renal functional reserve are recognized as a "hidden" indicator of kidney susceptibility to either acute kidney injury or chronic kidney disease. Understanding these phenomena and their association with outcome may enable the initiation of strategies that facilitate fast and sustained recovery during the time course of AKI and limit AKI progression towards chronic kidney disease. Different interventions may be required during various phases of AKI continuum. Early recognition and prevention of second hit by kidney stress, treatment of cause and prevention of aggravation in the early phase of AKI and facilitation of recovery in the phase of acute kidney disease may together represent the key aspects of modern AKI management.

Do trandolapril and indomethacin influence renal function and renal functional reserve in hypertensive patients?

AIMS

To assess the effect of trandolapril (2 mg once daily) and indomethacin (25 mg three times daily), alone and in combination, on renal function and renal functional reserve in hypertensive patients (DBP 95-115 mmHg) requiring regular non-steroidal anti-inflammatory drugs (NSAIDs).

METHODS

Randomized, double-blind, placebo-controlled, four way crossover design. After 3 weeks treatment renal plasma flow (RPF) and glomerular filtration rate (GFR) were measured using the p-aminohippurate (PAH) and inulin methods. Renal functional reserve was estimated by measuring RPF and GFR at the end of an intravenous infusion of dopamine 2 microg kg(-1) and 10% amino acid solution.

RESULTS

There was no significant difference in RPF between treatments: -22.79 ml min(-1) (95% CI -54.82, 9.24) for placebo and trandolapril, -10.37 ml min(-1) (95% CI -30.7, 9.96) for placebo and indomethacin, -14.78 ml min(-1) (95% CI -50.33, 20.77) for placebo and trandolapril with indomethacin. There was no significant difference in functional reserve RPF between treatments: -34.96 ml min(-1) (95% CI -119.8, 49.88) for placebo and trandolapril, 29.78 ml min(-1), -15.18, 74.74) for placebo and indomethacin, and -25.84 ml min(-1) (95% CI -87.62, 35.94) for placebo and trandolapril with indomethacin. There was no significant difference in GFR between treatments: -1.01 ml min(-1) (95% CI -7.45, 5.42) for placebo and trandolapril, -7.88 ml min(-1) (95% CI -15.08, -0.68) for placebo and indomethacin, and -0.36 ml min(-1) (95% CI -7.58, 6.86) for placebo and trandolapril with indomethacin. There was no significant difference in functional reserve GFR between treatments: 5.13 ml min(-1) (95% CI -4.97, 15.23) for placebo and trandolapril, 6.31 ml min(-1) (95% CI -1.88, 14.5) for placebo and indomethacin, 7.21 ml min(-1) (95% CI 1.26, 13.16) for placebo and trandolapril with indomethacin.

CONCLUSION

In hypertensives chronic treatment with NSAIDs or ACEI alone or in combination did not change RPF or GFR and did not change renal functional reserve capacity of RPF or GFR.