The unique structural and functional characteristics of glomerular endothelial cell fenestrations and their potential as a therapeutic target in kidney disease.

Glomerular endothelial cell (GEnC) fenestrations are a critical component of the glomerular filtration barrier. Their unique nondiaphragmed structure is key to their function in glomerular hydraulic permeability, and their aberration in disease can contribute to loss of glomerular filtration function. This review provides a comprehensive update of current understanding of the regulation and biogenesis of fenestrae. We consider diseases in which GEnC fenestration loss is recognized or may play a role and discuss methods with potential to facilitate the study of these critical structures. Literature is drawn from GEnCs as well as other fenestrated cell types such as liver sinusoidal endothelial cells that most closely parallel GEnCs.

Reduced Glomerular Filtration in Diabetes Is Attributable to Loss of Density and Increased Resistance of Glomerular Endothelial Cell Fenestrations.

BACKGROUND: Glomerular endothelial cell (GEnC) fenestrations are recognized as an essential component of the glomerular filtration barrier, yet little is known about how they are regulated and their role in disease. METHODS: We comprehensively characterized GEnC fenestral and functional renal filtration changes including measurement of glomerular Kf and GFR in diabetic mice (BTBR ob-/ob- ). We also examined and compared human samples. We evaluated Eps homology domain protein-3 (EHD3) and its association with GEnC fenestrations in diabetes in disease samples and further explored its role as a potential regulator of fenestrations in an in vitro model of fenestration formation using b.End5 cells. RESULTS: Loss of GEnC fenestration density was associated with decreased filtration function in diabetic nephropathy. We identified increased diaphragmed fenestrations in diabetes, which are posited to increase resistance to filtration and further contribute to decreased GFR. We identified decreased glomerular EHD3 expression in diabetes, which was significantly correlated with decreased fenestration density. Reduced fenestrations in EHD3 knockdown b.End5 cells in vitro further suggested a mechanistic role for EHD3 in fenestration formation. CONCLUSIONS: This study demonstrates the critical role of GEnC fenestrations in renal filtration function and suggests EHD3 may be a key regulator, loss of which may contribute to declining glomerular filtration function through aberrant GEnC fenestration regulation. This points to EHD3 as a novel therapeutic target to restore filtration function in disease.

Development of an estimated glomerular filtration rate formula in cats.

BACKGROUND: Estimated glomerular filtration rate (eGFR) formulas are routinely used in human patients to provide a more accurate evaluation of GFR compared to serum creatinine concentration alone. Similar formulas do not exist for cats. OBJECTIVES: To validate a prediction formula for eGFR in cats based on adjusting serum creatinine concentration. ANIMALS: Client-owned cats with various levels of renal function. METHODS: The study was cross-sectional. Glomerular filtration rate was determined by iohexol clearance. Variables including signalment, biochemical markers, and noninvasive measurements considered to represent surrogate markers of muscle mass were evaluated with the reciprocal of serum creatinine concentration in a multivariable regression model. The derived eGFR formula was subsequently tested in another group of cats and agreement with GFR assessed. RESULTS: The formula was developed in 55 cats. Only a single morphometric measurement (pelvic circumference) along with the reciprocal of serum creatinine concentration (creatinine-1 ) independently predicted GFR in the final multivariate model. The derived eGFR formula was 0.408 + (243.11 × creatinine-1 [µmol/L]) - (0.014 × pelvic circumference [cm]). When the formula was tested in another 25 cats it was not found to offer any advantage over creatinine-1 alone in its relationship with GFR (eGFR, R2 = 0.44, P < .001 vs reciprocal of creatinine, R2 = 0.45, P < .001). Furthermore, agreement between eGFR and GFR was poor. CONCLUSIONS AND CLINICAL IMPORTANCE: An eGFR formula for cats that adjusted serum creatinine concentration for a marker of muscle mass was developed. The formula did not provide a reliable estimate of GFR, and therefore, its routine use cannot be recommended.

Repeated measurements of renal function in evaluating its decline in cats.

OBJECTIVES: The aim of this study was to describe the variability in renal function markers in non-azotaemic and azotaemic cats, and also the rate of change in the markers. METHODS: Plasma creatinine concentration and its reciprocal, glomerular filtration rate (GFR) and urine specific gravity (USG) were studied as markers of renal function in client-owned cats. GFR was determined using a corrected slope-intercept iohexol clearance method. Renal function testing was performed at baseline and a second time point. The within-population variability (coefficient of variation; CV%) was determined at the baseline time point. Within-individual variability (CV%) and rate of change over time were determined from the repeated measurements. RESULTS: Twenty-nine cats were included in the study, of which five had azotaemic chronic kidney disease. The within-individual variability (CV%) in creatinine concentration was lower in azotaemic cats than in non-azotaemic cats (6.81% vs 8.82%), whereas the within-individual variability in GFR was higher in azotaemic cats (28.94% vs 19.98%). The within-population variability was greatest for USG (67.86% in azotaemic cats and 38.00% in non-azotaemic cats). There was a negative rate of change in creatinine concentration in azotaemic and non-azotaemic cats (-0.0265 and -0.0344 µmol/l/day, respectively) and a positive rate of change of GFR in azotaemic and non-azotaemic cats (0.0062 and 0.0028 ml/min/day, respectively). CONCLUSIONS AND RELEVANCE: The within-individual variability data suggest creatinine concentration to be the more useful marker for serial monitoring of renal function in azotaemic cats. In contrast, in non-azotaemic cats, GFR is a more useful marker for serial monitoring of renal function. The majority of cats with azotaemic CKD did not have an appreciable decline in renal function during the study.

Vitamin D toxicity of dietary origin in cats fed a natural complementary kitten food.

CASE SERIES SUMMARY: This case series describes two young sibling cats and an additional unrelated cat, from two separate households, that developed hypercalcaemia associated with hypervitaminosis D. Excessive vitamin D concentrations were identified in a natural complementary tinned kitten food that was fed to all three cats as part of their diet. In one of the cases, there was clinical evidence of soft tissue mineralisation. The hypercalcaemia and soft tissue mineralisation resolved following withdrawal of the affected food and medical management of the hypercalcaemia. RELEVANCE AND NOVEL INFORMATION: This case series demonstrates the importance of obtaining a thorough dietary history in patients presenting with hypercalcaemia and the measurement of vitamin D metabolites when investigating such cases. Complementary foods may have the potential to induce nutritional toxicity even when fed with complete, nutritionally balanced diets.

Hypercalcaemia in cats: The complexities of calcium regulation and associated clinical challenges.

PRACTICAL RELEVANCE: Calcium is essential for many normal physiological processes within the body. Aberrations in calcium homeostasis leading to hypercalcaemia can result in clinical signs such as polyuriav and polydipsia, lethargy and weakness due to depressed excitability of muscle and nervous tissue, and gastrointestinal (GI) signs due to effects on GI smooth muscle. Hypercalcaemia in cats is mostly idiopathic, with chronic kidney disease and neoplasia also being common causes. CLINICAL CHALLENGES: Hypercalcaemia can be a diagnostic challenge and a good understanding of the regulation of calcium homeostasis can aid in interpreting results of diagnostic tests. Furthermore, the management approach may depend on the underlying cause of hypercalcaemia, and also its severity and chronicity. AUDIENCE: This review offers a comprehensive discussion of the regulation of calcium homeostasis, with a focus on the normal response to hypercalcaemia. It also discusses the diagnostic approach to, and management of, hypercalcaemia in cats, as well as specific aetiologies. This is relevant to all clinicians working with feline patients. EVIDENCE BASE: The review draws evidence from peer-reviewed publications and also the author's own clinical experience.

Evaluation of non-polynomial equations for one-compartment correction of slope-intercept GFR: theoretical prediction and experimental measurement.

BACKGROUND: Polynomial equations for one-compartment correction of slope-intercept glomerular filtration rate (GFR) will underestimate values at high clearance rates. Non-polynomial correction equations that are independent of patient size and renal function would be advantageous and may have cross-species use. MATERIALS AND METHODS: The study explored the theoretical basis of firstly the Jodal and Brochner-Mortensen one-compartment correction equation, replacing plasma volume with extracellular fluid volume, and secondly an equation described by Peters. One-compartment correction factors (a which is related to plasma volume and v which is related to extracellular fluid volume) which avoided the need for scaling to body size were developed. Both factors were determined from the biexponential clearance curve of the markers iohexol and (51)Cr-EDTA in humans and iohexol in cats and dogs. Relationships between a and v and filtration function and body size were then determined using data from humans, cats and dogs to assess their validity and compare this with theoretical predictions. RESULTS: In all species, v was higher than a, as theoretically predicted. Both were significantly higher in humans than cats and dogs, ruling out cross-species use. Significant relationships were present between v and measures of filtration function in humans, but were weak with respect to a. Neither a nor v showed significant relationships with filtration function in animals or with body size in any species. CONCLUSIONS: a and v (which are factors independent of body size) can be used interchangeably for correcting slope-intercept clearance. However values of both for humans are higher compared to cats and dogs. Therefore a single cross-species factor cannot be used.

The role of phosphorus in the pathophysiology of chronic kidney disease.

OBJECTIVE: To review the human and veterinary literature on the role of phosphorus in the pathophysiology of chronic kidney disease (CKD) and to explore why control of plasma phosphorus concentration is an important goal in the management of patients with this disease. DATA SOURCES: Human and veterinary studies, reviews, clinical reports, textbooks, and recent research findings focused on phosphate homeostasis and CKD patient management. HUMAN DATA SYNTHESIS: Recent studies using rodent models and human patients with CKD have focused on trying to elucidate the role of the phosphatonins, predominantly fibroblast growth factor-23, in phosphate homeostasis and the pathophysiology of secondary renal hyperparathyroidism (SRHP). Fibroblast growth factor-23 is now considered to be a key regulator of plasma phosphorus concentration in people but has only recently been investigated in companion animal species. VETERINARY DATA SYNTHESIS: Cross-sectional studies of naturally occurring CKD in dogs and cats have shown hyperphosphatemia and SRHP to be highly prevalent and associated with increased morbidity and mortality in these patients. Experimental studies of surgically induced renal impairment in the dog and cat, and cases of naturally occurring CKD have emphasized the ability of renal care diets to modify plasma phosphorus and parathyroid hormone concentrations. Evidence from these studies indicates that maintaining plasma phosphorus concentrations to within the International Renal Interest Society targets for CKD patients improves survival time and reduces clinical manifestations of hyperphosphatemia and SRHP. CONCLUSIONS: The maintenance of plasma phosphorus concentrations in to within the International Renal Interest Society targets is recommended in management of CKD patients. The discovery of the phosphatonins has improved understanding of the mechanisms involved in phosphorus homeostasis and SRHP and may lead to improved ability to monitor and manage these patients.

Parathyroid hormone concentration in geriatric cats with various degrees of renal function.

OBJECTIVE: To determine whether cats in the nonazotemic stages of chronic kidney disease have increased plasma parathyroid hormone (PTH) concentrations as a compensatory physiologic mechanism to maintain plasma phosphate concentration within the reference interval. DESIGN: Prospective longitudinal study. ANIMALS: 118 client-owned geriatric cats with various degrees of renal function. PROCEDURES: For each cat, a blood sample was obtained for plasma biochemical analysis and determination of plasma PTH concentration, and a urine sample was obtained for determination of urine specific gravity at study entry (baseline) and after 12 months. For a subset of 30 cats, plasma calcitriol concentration was determined at baseline. Cats were categorized into 1 of 3 groups on the basis of kidney function at the end of 12 months. At baseline and after 12 months, plasma concentrations of variables associated with calcium homeostasis were compared between the 3 groups and also within groups over time. Multivariable linear regression was used to identify variables associated with plasma PTH concentration. RESULTS: Plasma PTH concentration was significantly increased in cats that developed azotemia, compared with PTH concentration in cats that remained nonazotemic, and PTH concentration increased before changes in plasma calcium and phosphate concentrations were detected. A moderate positive association between plasma calcitriol and PTH concentrations was identified. Plasma PTH concentration was associated with age and plasma urea, creatinine, and total calcium concentrations in the final multivariable model. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggested that renal secondary hyperparathyroidism can develop prior to azotemia in cats, even in the absence of hyperphosphatemia and hypocalcemia.

Glomerular filtration rate estimation by use of a correction formula for slope-intercept plasma iohexol clearance in cats.

OBJECTIVE: To develop a formula for correcting slope-intercept plasma iohexol clearance in cats and to compare clearance of total iohexol (TIox), endo-iohexol (EnIox), and exo-iohexol (ExIox). ANIMALS: 20 client-owned, healthy adult and geriatric cats. PROCEDURES: Plasma clearance of TIox was determined via multisample and slope-intercept methods. A multisample method was used to determine clearance for EnIox and ExIox. A second-order polynomial correction factor was derived by performing regression analysis of the multisample data with the slope-intercept data and forcing the regression line though the origin. Clearance corrected by use of the derived formula was compared with clearance corrected by use of Brochner-Mortensen human and Heiene canine formulae. Statistical testing was applied, and Bland-Altman plots were created to assess the degree of agreement between TIox, EnIox, and ExIox clearance. RESULTS: Mean ± SD iohexol clearance estimated via multisample and corrected slope-intercept methods was 2.16 ± 0.35 mL/min/kg and 2.14 ± 0.34 mL/min/kg, respectively. The derived feline correction formula was Cl(corrected) = (1.036 × Cl(uncorrected)) - (0.062 × Cl(uncorrected)(2)), in which Cl represents clearance. Results obtained by use of the 2 methods were in excellent agreement. Clearance corrected by use of the Heiene formula had a linear relationship with clearance corrected by use of the feline formula; however, the relationship of the feline formula with the Brochner-Mortensen formula was nonlinear. Agreement between TIox, EnIox, and ExIox clearance was excellent. CONCLUSIONS AND CLINICAL RELEVANCE: The derived feline correction formula applied to slope-intercept plasma iohexol clearance accurately predicted multisample clearance in cats. Use of this technique offers an important advantage by reducing stress to cats associated with repeated blood sample collection and decreasing the costs of analysis.