Contrasting renal functional reserve in very long-term Type I diabetic patients with and without nephropathy

Brief Early Life Angiotensin Converting Enzyme Inhibition Offers Reno-Protection in Sheep with a Solitary Functioning Kidney at 8 Months of Age

Background: Children born with a solitary functioning kidney (SFK) are predisposed to develop hypertension and kidney injury. Glomerular hyperfiltration and hypertrophy contribute to the pathophysiology of kidney injury. Angiotensin converting enzyme inhibitors (ACEi) can mitigate hyperfiltration and may be therapeutically beneficial in reducing progression of kidney injury in SFK. Methods: SFK was induced in male sheep fetuses at 100 days gestation (term=150 day). Between 4-8 weeks of age, SFK lambs received enalapril (SFK+ACEi; 0.5mg/kg/day, once daily, orally) or vehicle (SFK). At 8 months we examined whether SFK+ACEi reduced elevation in blood pressure (BP) and improved basal kidney function, renal functional reserve (RFR; glomerular filtration rate (GFR) response to combined amino acid and dopamine infusion), GFR response to nitric oxide synthase (NOS) inhibition and basal nitric oxide (NO) bioavailability (basal urinary total nitrate+nitrite (NOx)). Results: SFK+ACEi prevented albuminuria, resulted in lower basal GFR (16%), higher renal blood flow (~22%), and lower filtration fraction ( 35%), but similar BP compared to ~ vehicle-treated SFK sheep. Together with greater recruitment of RFR (~14%) in SFK+ACEi animals than SFK, this indicates reduction in glomerular hyperfiltration-mediated kidney dysfunction. During NOS inhibition, the decrease in GFR ( 14%) was greater among SFK+ACEi than among SFK animals. Increased ( 85%) basal urinary total NOx in SFK+ACEi animals compared to SFK indicates elevated NO bioavailability likely contributing to improvements in kidney function and prevention of albuminuria. Conclusions: Brief and early ACEi in SFK is associated with reduced glomerular hyperfiltration-mediated kidney disease up to 8 months of age in a sheep model.

Functional Reserve of the Kidney

An exploration of the normal limits of physiologic responses and how these responses are lost when the kidney is injured rarely occurs in clinical practice. However, the differences between "resting" and "stressed" responses identify an adaptive reactiveness that is diminished before baseline function is impaired. This functional reserve is important in the evaluation of prognosis and progression of kidney disease. Here, we discuss stress tests that examine protein-induced hyperfiltration, proximal tubular secretion, urea-selective concentration defects, and acid retention. We discuss diseases in which these tests have been used to diagnose subclinical injury. The study and follow-up of abnormal functional reserve may add considerable understanding to the natural history of CKD.

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.

Renal Functional Reserve Revisited

Kidney function, like the function of other organs, is dynamic and continuously adjusts to changes in the internal environment to maintain homeostasis. The glomerular filtration rate, which serves as the primary index of kidney function in clinical practice, increases in response to various physiological and pathological stressors including oral protein intake. The difference between the glomerular filtration rate in the resting state and at maximum capacity has been termed renal functional reserve (RFR). RFR could provide additional information on kidney health and renal function prognosis. Despite longstanding interest in RFR as a biomarker in nephrology, its underlying mechanisms remain inadequately understood. Moreover, no consensus has been reached on how it should be quantified. Previous studies on RFR have used various measurement methods and yielded heterogeneous results. A standardized and clinically feasible approach to quantifying RFR would allow for more rigorous appraisal of its value as a biomarker and could pave the way for adoption of "renal stress tests" into clinical practice.

Renal hemodynamic response to L-arginine in uncomplicated, type 1 diabetes mellitus: the role of buffering anions and tubuloglomerular feedback

According to the "tubulocentric" hypothesis of the glomerular hyperfiltration of diabetes mellitus (DM), tubuloglomerular feedback (TGF) is the critical determinant of the related renal hemodynamic dysfunction. To examine the role of TGF in human type 1 DM, 12 salt-replete healthy (C) and 11 uncomplicated DM individuals underwent measurements of glomerular filtration rate (GFR), renal blood flow (RBF), and lithium-derived absolute "distal" sodium delivery (DDNa). Measurements were made during two 3-h infusions of 0.012 mmol·kg(-1)·min(-1) l-arginine (ARG) buffered with either equimolar HCl (ARG.HCl) or citric acid (ARG.CITR). Our hypothesis was that changes in TGF signaling would be directionally opposite ARG.HCl vs. ARG.CITR according to the effects of the ARG-buffering anion on DDNa. Similar changes in C and DM followed ARG.CITR, with declines in DDNa (-0.26 ± 0.07 mmol/min C vs. -0.31 ± 0.07 mmol/min DM) and increases in RBF (+299 ± 25 vs. +319 ± 29 ml·min(-1)·1.73 m(-2)) and GFR (+6.6 ± 0.8 vs. +11.6 ± 1.2 ml·min(-1)·1.73 m(-2)). In contrast, with ARG.HCl, DDNa rose in both groups (P = 0.001), but the response was 73% greater in DM (+1.50 ± 0.15 mmol/min C vs. +2.59 ± 0.22 mmol/min DM, P = 0.001). RBF also increased (P = 0.001, +219 ± 20 ml·min(-1)·1.73 m(-2) C, +105 ± 14 DM), but ΔRBF after ARG.HCl was lower vs. ARG.CITR in both groups (P = 0.001). After ARG.HCl, ΔRBF also was 50% lower in DM vs. C (P = 0.001) and GFR, unchanged in C, declined in DM (-7.4 ± 0.9 ml·min(-1)·1.73 m(-2), P = 0.02 vs. C). After ARG.HCl, unlike ARG.CITR, DDNa increased in C and DM, associated with less ΔRBF and ΔGFR vs. ARG.CITR. This suggests that the renal hemodynamic response to ARG is influenced substantially by the opposite actions of HCl vs. CITR on DDNa and TGF. In DM, the association of ARG.HCl-induced exaggerated ΔDDNa, blunted ΔRBF, and the decline in GFR vs. C shows an enhanced TGF dependence of renal vasodilatation to ARG, in agreement with a critical role of TGF in DM-related renal hemodynamic dysfunction.

Chronic kidney disease in long duration type 1 diabetes lasting more than 50 years

OBJECTIVE

There is evidence that microalbuminuria and overt nephropathy may progress more slowly in long duration (> 20 years) type 1 diabetes (T1DM). To explore this further, we examined the characteristics of chronic kidney disease (CKD) in a large cohort of long duration T1DM in the United Kingdom (UK).

RESEARCH DESIGN AND METHODS

We studied the UK 'Golden Years' cohort--a group of 400 patients from various parts of the UK with T1DM > 50 years duration. Demographic and clinical information were obtained. HbA(1c), lipids, creatinine and urinary albumin-creatinine ratio (ACR) were measured. Microalbuminuria was defined as 2.5-25.0 mg/mmol for males and 3.5-25.0 mg/mmol for females; macroalbuminuria was defined as an ACR > 25.0 mg/mmol for both sexes.

RESULTS

Mean age was 69 years and duration of diabetes 55 years. Nine percent had macroalbuminuria and 27% microalbuminuria. No patient had stage 5 CKD. Microalbuminuria was associated significantly with increased diabetes duration (p = 0.02), male sex (p = 0.02), smoking (p = 0.002), higher HbA(1c) (p < 0.0001), raised triglycerides (p = 0.04), and peripheral vascular disease (PVD) (p < 0.0001). Macroalbuminuria had significant associations with smoking (p = 0.02), raised triglycerides (p = 0.01), raised creatinine (p = 0.02), PVD (p = 0.01) and hypertension (p = 0.01).

CONCLUSIONS

We conclude that microalbuminuria and CKD are common, even at long duration (> 50 years) of T1DM, and have similar characteristics and associations as they do with shorter disease duration. There is a striking absence, however, of stage 5 CKD, but selection bias may be an important confounder since patients with advanced disease may have not survived.

Cystatin C does not detect acute changes in glomerular filtration rate in early diabetic nephropathy

BACKGROUND

The measurement of renal functional reserve (acute change in glomerular filtration rate [GFR] after protein load) allows the detection of sub-clinical renal dysfunction and has prognostic implications in diabetes. Our aim was to test cystatin C as an index of GFR and renal functional reserve.

METHODS

GFR was measured by C(Sinistrin) at baseline and after protein load in 28 diabetic patients with serum creatinine <1.2 mg/dL. The C(Sinistrin) was compared with cystatin C, serum creatinine, creatinine clearance, and Cockcroft-Gault formula.

RESULTS

Baseline C(Sinistrin) ranged from 67-172 mL/min. Regression analysis showed an overall low relationship between C(Sinistrin) and the indirect markers of GFR. The highest correlation with C(Sinistrin) was obtained for cystatin C clearance (R(2) = 0.58, r = 0.76, p < 0.001), the 1/serum cystatin C (R(2) = 0.58, r = 0.76, p < 0.001), and serum cystatin C (R(2) = 0.52, r = 0.72, p < 0.001). Renal functional reserve was preserved in 6 of 28 patients. There was no significant change in cystatin C in response to protein load.

CONCLUSION

Wide variation in baseline GFR emphasizes the need for the early detection of renal dysfunction. Cystatin C correlated best with C(Sinistrin) at baseline, but did not detect renal functional reserve.

Renal functional reserve in patients with Type 1 diabetes mellitus

AIM

Decreased renal functional reserve might precede incipient diabetic nephropathy in patients with Type 1 diabetes. The aim of this study was to assess the relationship between renal functional reserve and easily assessable estimates of systemic endothelial dysfunction in normoalbuminuric patients with Type 1 diabetes and diabetic retinopathy.

METHODS

Renal functional reserve was calculated as the relative change in glomerular filtration rate after protein ingestion. Glomerular filtration rate was measured using pharmacokinetic compartmental analysis of single-shot plasma sinistrin clearance. We measured the activity of von Willebrand factor and concentrations of C-reactive protein and apolipoprotein B, as easily assessable estimates of systemic endothelial dysfunction.

RESULTS

Twenty-two patients were studied. Renal functional reserve was inversely associated with activity of von Willebrand factor (R=-0.431, p=0.045) and, in a multivariate model, with concentration of C-reactive protein (R=0.652, p=0.031).

CONCLUSION

Renal functional reserve is inversely associated with concentration of C-reactive protein in normoalbuminuric patients with Type 1 diabetes and diabetic retinopathy. This finding provides evidence that decreased renal functional reserve might reflect endothelial dysfunction. We speculate that decreased renal functional reserve might possibly show as an early marker of diabetic nephropathy.

Microalbuminuria and hypertension with focus on type 1 and type 2 diabetes

Microalbuminuria and hypertension with Over the past decade, there has been considerable focus on the concept of microalbuminuria, not only because it predicts renal disease in type 1 and type 2 diabetes, but also because it relates to premature mortality in the diabetic and in the general population. More importantly, intervention at this stage is now possible with the perspective of preserving glomerular filtration rate (GFR) and ameliorating cardiovascular disease and ensuing strong end-points. INITIAL STUDIES: The concept of microalbuminuria was introduced about 20 years ago and since then there has been a multitude of studies and papers on this subject using the original definition, but not always, in the US. Before that time it was suggested, mainly from the US, that diabetic renal disease was an untreatable relentlessly progressive condition.

GENETIC STUDIES

There is an overwhelming number of studies on genetics and diabetes and also covering the genetics of diabetic complications including nephropathy. However, so far the results are extremely disappointing. Patients at risk cannot be identified and genetic analyses are of no value as a guide to treatment. The notion that the development of complications is controlled mainly by a special genetic pattern is increasingly doubtful. In genetic studies, it is rather phenotypic well-accepted risk factors that dominate. STRUCTURAL BASIS OF MICROALBUMINURIA: Patients with microalbuminuria have significant abnormalities in the kidney, including glomeruli. This is quite clear in patients with type 1 diabetes, but is also seen in type 2 diabetes, where on the other hand, other risk factors such as hypertension and dyslipidaemia also seem to be of importance, including loss of autoregulation. Renal biopsies are generally not indicated in the management of diabetic patients. MICROALBUMINURIA AND EARLY MORTALITY: It is quite clear that microalbuminuria predicts early mortality both in type 1 and type 2 diabetes. The association to other risk factors may partly explain this--but this does not account for the whole picture. Endothelial dysfunction as well as inflammatory and arteriosclerotic abnormalities in blood vessels may be a relevant hypothesis that needs to be further explored along with other possibilities. CLINICAL COURSE AND ASSOCIATED ABNORMALITIES: The risk factor for progression in normoalbuminuric patients to microalbuminuria is higher than normal albumin excretion (strongest factor), poor glycaemic control, elevated blood pressure, and to some extent smoking. The clinical course of microalbuminuria is usually progressive, but with the more effective intervention now available we encounter that the so-called natural history (without intervention) is increasingly difficult to study. Microalbuminuria is clearly associated with a number of abnormalities, almost in all organs, but GFR is generally well preserved in spite of more advanced structural lesions. Therefore, microalbuminuria is an important marker for more pronounced diabetic vascular disease in general as well as for nephropathy. Regression to normoalbuminuria only rarely occurs during standard unchanged nonintensive treatment. TREATMENT STRATEGIES: The best possible glycaemic control is important in preventing and ameliorating the course of normo- and micro-albuminuria. Another major treatment strategy, especially in microalbuminuric patients, is antihypertensive treatment including inhibition of the renal angiotensin aldosterone system. Numerous new studies are available, both in type 1 and type 2 diabetes, documenting that not only microalbuminuria but also renal and cardiovascular complications in these patient are also far better controlled by early detection and treatment. Therefore, screening for microalbuminuria should be a strategy in all diabetes management followed by effective intervention as outlined in this paper.

Defective nitric oxide production and functional renal reserve in patients with type 2 diabetes who have microalbuminuria of African and Asian compared with white origin

Diabetic nephropathy is a leading cause of end-stage renal failure. Its incidence is higher and is increasing in persons of Indo-Asian and African-Caribbean (African-Asian) compared with those of white origin. Nitric oxide deficiency is associated with progressive renal disease. It was hypothesized that differences in the capacity to increase glomerular filtration (functional renal reserve) would exist between these racial groups in relation to nitric oxide availability. Patients with type 2 diabetes of African-Asian (n = 9) and white (n = 9) origin with microalbuminuria were studied under euglycemic conditions. Glomerular filtration, renal plasma flow, and clearance of the stable metabolites of nitric oxide, nitrite, and nitrate were measured before and after a renal vasodilatory stimulus of a mixed amino acid intravenous infusion. There were no significant differences in age, duration of diabetes, and baseline glomerular filtration (57.1 [14.1] versus 55.8 [10.1] yr; P = 0.82, 14.5 [10.2] versus 9.1 [7.0] yr; P = 0.19 and 125.9 [30.9] versus 127.2 [44.6] ml/min per 1.73 m(2); P = 0.94) between the African-Asian and white groups. Functional renal reserve, change in renal plasma flow, and percentage change in nitrate and nitrite clearance was significantly higher in the white compared with the African-Asian group (21.9 [45.7] versus -2.5 [28.2] ml/min per 1.73 m(2); P = 0.043, 155.8 [205.9] versus -90.1 [146.0]; P = 0.03 ml/min per 1.73 m(2) and 26.7 [85.1] versus -44.7 [16.9] %; P = 0.013, respectively). The differences in functional reserve were not confounded after adjustment for diabetes duration (P = 0.034). The data suggest that these patients with type 2 diabetes of African and Asian origin lose functional renal reserve earlier in the evolution of nephropathy than whites. The differences appear to be due to defective nitric oxide production or bioavailability and might explain some of the propensity to develop end-stage renal disease.