Renal functional reserve

Protein intake and risk of urolithiasis and kidney diseases: an umbrella review of systematic reviews for the evidence-based guideline of the German Nutrition Society

PURPOSE

Changes in dietary protein intake metabolically affect kidney functions. However, knowledge on potential adverse consequences of long-term higher protein intake (HPI) for kidney health is lacking. To summarise and evaluate the available evidence for a relation between HPI and kidney diseases, an umbrella review of systematic reviews (SR) was conducted.

METHODS

PubMed, Embase and Cochrane Database of SRs published until 12/2022 were searched for the respective SRs with and without meta-analyses (MA) of randomised controlled trials or cohort studies. For assessments of methodological quality and of outcome-specific certainty of evidence, a modified version of AMSTAR 2 and the NutriGrade scoring tool were used, respectively. The overall certainty of evidence was assessed according to predefined criteria.

RESULTS

Six SRs with MA and three SRs without MA on various kidney-related outcomes were identified. Outcomes were chronic kidney disease, kidney stones and kidney function-related parameters: albuminuria, glomerular filtration rate, serum urea, urinary pH and urinary calcium excretion. Overall certainty of evidence was graded as 'possible' for stone risk not to be associated with HPI and albuminuria not to be elevated through HPI (above recommendations (> 0.8 g/kg body weight/day)) and graded as 'probable' or 'possible' for most other kidney function-related parameters to be physiologically increased with HPI.

CONCLUSION

Changes of the assessed outcomes may have reflected mostly physiological (regulatory), but not pathometabolic responses to higher protein loads. For none of the outcomes, evidence was found that HPI does specifically trigger kidney stones or diseases. However, for potential recommendations long-term data, also over decades, are required.

Renal functional reserve: from physiological phenomenon to clinical biomarker and beyond

Glomerular filtration rate (GFR) is acutely increased following a high-protein meal or systemic infusion of amino acids. The mechanisms underlying this renal functional response remain to be fully elucidated. Nevertheless, they appear to culminate in preglomerular vasodilation. Inhibition of the tubuloglomerular feedback signal appears critical. However, nitric oxide, vasodilator prostaglandins, and glucagon also appear important. The increase in GFR during amino acid infusion reveals a "renal reserve," which can be utilized when the physiological demand for single nephron GFR increases. This has led to the concept that in subclinical renal disease, before basal GFR begins to reduce, renal functional reserve can be recruited in a manner that preserves renal function. The extension of this concept is that once a decline in basal GFR can be detected, renal disease is already well progressed. This concept likely applies both in the contexts of chronic kidney disease and acute kidney injury. Critically, its corollary is that deficits in renal functional reserve have the potential to provide early detection of renal dysfunction before basal GFR is reduced. There is growing evidence that the renal response to infusion of amino acids can be used to identify patients at risk of developing either chronic kidney disease or acute kidney injury and as a treatment target for acute kidney injury. However, large multicenter clinical trials are required to test these propositions. A renewed effort to understand the renal physiology underlying the response to amino acid infusion is also warranted.

A randomized trial comparing the nephrotoxicity of cisplatin/ifosfamide-based combination chemotherapy with or without amifostine in patients with solid tumors

This study evaluates the degree of kidney damage during cisplatin/ifosfamide-based combination chemotherapy and its possible prevention by amifostine. Thirty-one patients with solid tumors stratified according to pretreatment were randomized to receive VIP- or TIP-chemotherapy with or without amifostine (910 mg/m2) given as a short infusion prior to cisplatin. Chemotherapy consisted of cisplatin (50 mg/m2), ifosfamide (4 g/m2) and either etoposide (500 mg/m2) (= VIP) or paclitaxel (175 mg/m2) (= TIP) repeated at 3 weekly intervals. For all patients the glomerular filtration rate (GFR) measured by creatinine-clearance, serum creatinine, electrolytes and differential urinary protein/enzyme excretion were determined prior to, during and after each cycle. A total of 62 cycles of chemotherapy were evaluable. In the amifostine-group GFR was fully maintained after application of two cycles of chemotherapy, whereas in the control group a > 30%-reduction of median GFR (108 to 80 ml/min) was observed (p < 0.001). Patients receiving amifostine had a lower degree of high molecular weight proteins excretion indicating less glomerular damage. In both groups significant increases of tubular marker profiles peaking at day 3 after chemotherapy were observed with a nearly complete reversibility of these changes prior to the next chemotherapy cycle. The number of patients with low magnesium serum levels during treatment was 17% after amifostine application versus 69% in control patients. The results seem to indicate that treatment with amifostine can preserve GFR after application of two cisplatin/ifosfamide-based chemotherapy cycles. This may be advantageous if repetitive cycles of chemotherapy or subsequent administration of high dose chemotherapy is planned.

The use of lithium clearance measurements as an estimate of glomerulo-tubular function

Lithium clearance measurements are based on the observation that lithium undergoes isoosmotic reabsorption in the proximal renal tubule to the same extent as salt and water, but undergoes neither reabsorption nor secretion elsewhere in the nephron. Consequently, lithium clearance values estimate the delivery of isoosmotic fluid to the loop of Henle and hence provide an assessment of proximal tubular reabsorption of isoosmotic fluid. If sodium clearance and urinary flow rate are also measured, then this allows the derivation of a number of parameters relating to both the absolute and relative renal handling of isoosmotic fluid in the proximal and distal regions of the kidney. Consequently, lithium clearance techniques can be used in both experimental and clinical studies to evaluate glomerulo-tubular function and provide information regarding the handling of sodium and water by the proximal and distal nephron in both health and disease. The use of lithium clearance measurements in the assessment of glomerulo-tubular function in patients treated with rIL2 for colorectal cancer is described and its application to both drug-induced toxicity and other disease states discussed.

Renal functional reserve in insulin dependent diabetic children

BACKGROUND

Microalbuminuria has been shown to be predictive for clinical diabetic nephropathy. Renal functional reserve (RFR), as a response to protein loading in a short period of time, is a parameter to assess the ability of kidneys to increase the glomerular filtration rate (GFR). The aim of this study was to predict the early phase of diabetic nephropathy by measuring urinary albumin level and RFR capacity in patients with insulin-dependent diabetes mellitus (IDDM).

METHODS

Twenty-two patients with IDDM were studied: 11 with a disease duration of less than 5 years (group 1) and 11 with a disease duration of more than 5 years (group 2). As the control group, 15 healthy children (group 3) were included in the study. At the beginning of the study, glucose was measured and the urinary albumin/creatinine ratio was calculated. Average glycosylated hemoglobin (HbA1c) over 1 year was determined. After protein loading (red meat containing 2 g/kg of protein), the creatinine clearance was calculated at each hour for a duration of 4 h. The RFR was accepted as the peak percentage increase in GFR over the baseline value.

RESULTS

Although metabolic control in group 2 was better, the RFR in group 2 was significantly lower than in group 1 (P < 0.05). Urinary microalbumin levels between the groups did not differ (P > 0.05). In two patients in whom microalbuminuria was detected, the RFR was much lower.

CONCLUSIONS

Detecting lower RFR levels in patients with normal urinary albumin excretion, as well as in patients with microalbuminuria, may support the idea that the RFR capacity is more sensitive than microalbuminuria in assessing the early phase of diabetic nephropathy.

Renal functional reserve in patients with IgA glomerulopathy

Seven patients with histologically proven IgA nephropathy and modest impairment of renal function, and 2 patients with IgA nephropathy and nephrotic syndrome were investigated, compared to a control group of 9 healthy individuals, to study the effects of amino acids on glomerular and tubular function, and to evaluate renal functional reserve in IgA nephropathy with different clinical course. Inulin and PAH clearances were used to evaluate glomerular filtration rate (GFR) and effective renal plasma flow (ERPF); proximal and distal tubular fluid delivery and reabsorption were measured by lithium clearance, before and after submission of a standardized amino acid solution. GFR and ERPF increased significantly during amino acid load in healthy individuals and patients without nephrotic syndrome, while filtration fraction (GFR/ERPF) remained constant. Lithium clearance (CLi) and fractional lithium excretion (CLi/GFR) rose significantly in both groups, whereas the reabsorbed volume of fluid in the proximal tubule did not change. In the distal tubule, fractional volume excretion decreased significantly during amino acid load whereas the reabsorbed volume significantly increased. Baseline values of the two groups did not differ significantly. Two patients with nephrotic course of IgA nephropathy showed a distinct decrease in glomerular and tubular function, and a loss of renal functional reserve after amino acid load.

CONCLUSIONS

Despite distinct alterations in renal biopsy, IgA nephropathy without nephrotic course presents with a still adequately preserved kidney function and renal functional reserve. A single determination of renal function with noninvasive functional tests does not give valid prognostic information concerning glomerular and tubular function. Therefore, a repeated measurement of renal function at defined intervals might reveal clinical progression of renal disease. The results of the lithium clearance might indicate an increase in tubular function after amino acid load, indicating a tubular adaptation in state of hyperfiltration.

Assessment of the filtration reserve capacity of the kidney in workers exposed to cadmium

It has been assessed whether an internal dose of cadmium (Cd), as reflected by a Cd concentration in urine not yet sufficient to induce a significantly increased urinary excretion of various plasma proteins (microproteinuria defined as beta 2-microglobulin in urine greater than 300 micrograms/g creatinine, or retinol-binding protein in urine greater than 300 micrograms/g creatinine, or albumin in urine greater than 15 mg/g creatinine, or a combination of these), may affect the filtration reserve capacity of the kidney. The last was determined by measuring the difference between the baseline creatinine clearance and the maximal creatinine clearance after an acute oral load of protein (400 g of cooked red meat). In total 215 men were examined of whom eventually 87 Cd exposed workers (concentration of Cd in urine greater than 2 micrograms/g creatinine) from zinc/cadmium smelters and 92 control workers (concentration of Cd in urine less than 2 micrograms/g creatinine, absence of microproteinuria, normal fasting serum creatinine) were retained for data analysis performed separately for workers aged less or more than 50 years. Microproteinuria was present in 20 Cd workers, all older than 50. This study confirmed the previous observation that the age related decline of the baseline glomerular filtration rate (GFR) is accelerated in male workers with Cd induced microproteinuria; the same observation was made for the maximal GFR. It was found, however, that a renal Cd burden that had not yet caused microproteinuria did not impair the filtration reserve capacity of the kidney. This study therefore validates the previous estimate of the threshold effect concentration of Cd in urine (10 micrograms/g creatinine) that is intended to prevent the occurrence of microproteinuria in male Cd workers. It should be kept in mind, however, that because of the likely interference of the healthy worker effect, this conclusion may not be directly extrapolated to the general population.

The effect of unilateral nephrectomy on the subsequent radiation response of the pig kidney

The left kidney of 14 Large White female pigs, approximately 14 weeks of age, was surgically removed. Thirty weeks after unilateral nephrectomy (UN) the remaining kidney was irradiated with a single dose of between 11.9 and 15.6 Gy of 60Co gamma-rays; three pigs received sham irradiation. Following irradiation glomerular filtration rate (GFR), effective renal plasma flow (ERPF) and haematocrit (Hct) were determined for up to 48 weeks after irradiation. Irradiation resulted in a dose-dependent decline in GFR, evident 8 weeks after irradiation. This was followed by a gradual improvement in GFR, although after doses of 14.0 Gy GFR remained below control values throughout the study. A similar pattern of response was seen in terms of ERPF, but this was not dose-related. Doses of greater than or equal to 14.0 Gy also caused a significant reduction (p less than 0.001) in Hct within 4-8 weeks. Minimal levels were evident 16 weeks after irradiation; Hct then increased, but remained below preirradiation values. Dose-effect curves were obtained by determining the percentage of irradiated kidneys which showed a greater than or equal to 50% reduction in GFR and ERPF, fitted by probit analysis, and ED50 values (+/- SE) were calculated for each parameter. The ED50 values for GFR and ERPF were 14.49 +/- 0.27 Gy and 12.56 +/- 0.98 Gy, respectively. These values were not significantly different from those obtained from intact age-matched pigs in which the right kidney alone was irradiated; UN did not compromise or alter the radiation response of the kidney to irradiation. However, the ED50 values obtained for the UN pigs were significantly greater (p less than 0.001) than the values of 9.76 +/- 0.17 Gy and 6.19 +/- 0.93 Gy, seen for GFR and ERPF in intact age-matched pigs in which both kidneys were irradiated. Thus although both experimental situations involved irradiating the entire renal tissue, the relative radiosensitivity of the kidneys varied considerably. These findings show that the radiosensitivity of the pig kidney can vary markedly, depending on the physiological status of the kidney at the time of irradiation.

Cisplatin-induced reductions in renal functional reserve uncovered by unilateral nephrectomy: an experimental study in the pig

Groups of mature Large White female pigs, approximately 10 months of age, received single intravenous infusions of 1.5, 2 or 2.5 mg/kg body weight (equivalent to approximately 90, approximately 120 and approximately 150 mg/m2) cisplatin. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured before and at 4 weeks after cisplatin infusion by renography using [99 mTc]-DTPA (diethylenetriamminepentaacetic acid and iodohippurate sodium I 131, respectively. The left kidney of each cisplatin-treated animal plus that of four age-matched control pigs was then removed surgically, and GFR and ERPF were measured in the remaining kidney at 4 weekly intervals for up to 24 weeks after unilateral nephrectomy (UN). The pigs treated with cisplatin exhibited no consistent change in either GFR or ERPF at 4 weeks after treatment. A histological evaluation of kidneys from animals treated with greater than or equal to 2 mg/kg cisplatin that had been removed at UN revealed both tubular and glomerular lesions. The latter consisted of cell proliferation on the parietal surface of the urinary space; damage to the S1 portion of the proximal convolution was also noted. Following UN there was a pronounced dose-dependent reduction in the functional status of the remaining kidney such that the increase in GFR and ERPF in pigs initially receiving 2.5 mg/kg cisplatin was less than 50% of that seen in age-matched UN controls. Moreover, the glomerular lesions observed at 4 weeks after cisplatin infusion had apparently progressed to glomerular hyalinisation by 24 weeks after UN. Thus, prior treatment with cisplatin may cause a permanent reduction in renal functional reserve that may be clinically "silent" until exposure to an additional nephrotoxic insult.