Glomerular Calcification Induced by Bolus Injection with Dibasic Sodium Phosphate Solution in Sprague—Dawley Rats

Association between Serum Phosphorus Levels and Diabetic Retinopathy: A Cross-Sectional Study

Background and Aims

The aim of this study was to investigate the association between serum phosphate levels and diabetic retinopathy (DR) in patients with type 2 diabetes mellitus (T2DM).

Methods and Results

The study sample consisted of 1657 T2DM patients hospitalized between 2017 and 2019. Patients were categorized into quartiles based on their serum phosphate levels (Q1-Q4). An increasing trend in the prevalence of DR was observed across these quartiles. Subsequently, logistic regression analysis was employed to adjust for potential confounders, such as gender, age, BMI, and duration of diabetes, and to evaluate the odds ratios (ORs) associated with these quartiles. The prevalence of DR showed an increasing trend with elevated serum phosphate levels. Logistic regression further confirmed that serum phosphate levels remain an independent risk factor for DR.

Conclusion

Elevated serum phosphate levels are closely associated with the prevalence of DR in hospitalized T2DM patients. Further studies are needed to establish causality. This trial is registered with chiCTR2000032374.

Hyperglycemic environment disrupts phosphate transporter function and promotes calcification processes in podocytes and isolated glomeruli

Soft tissue calcification is a pathological phenomenon that often occurs in end-stage chronic kidney disease (CKD), which is caused by diabetic nephropathy, among other factors. Hyperphosphatemia present during course of CKD contributes to impairments in kidney function, particularly damages in the glomerular filtration barrier (GFB). Essential elements of the GFB include glomerular epithelial cells, called podocytes. In the present study, we found that human immortalized podocytes express messenger RNA and protein of phosphate transporters, including NaPi 2c (SLC34A3), Pit 1 (SLC20A1), and Pit 2 (SLC20A2), which are sodium-dependent and mediate intracellular phosphate (Pi) transport, and XPR1, which is responsible for extracellular Pi transport. We found that cells that were grown in a medium with a high glucose (HG) concentration (30 mM) expressed less Pit 1 and Pit 2 protein than podocytes that were cultured in a standard glucose medium (11 mM). We found that exposure of the analyzed transporters in the cell membrane of the podocyte is altered by HG conditions. We also found that the activity of tissue nonspecific alkaline phosphatase increased in HG, causing a rise in Pi generation. Additionally, HG led to a reduction of the amount of ectonucleotide pyrophosphatase/phosphodiesterase 1 in the cell membrane of podocytes. The extracellular concentration of pyrophosphate also decreased under HG conditions. These data suggest that a hyperglycemic environment enhances the production of Pi in podocytes and its retention in the extracellular space, which may induce glomerular calcification.

The impact of type III sodium-dependent phosphate transporters (Pit 1 and Pit 2) on podocyte and kidney function

The sodium-dependent phosphate transporters Pit 1 and Pit 2 belong to the solute carrier 20 (SLC20) family of membrane proteins. They are ubiquitously distributed in the human body. Their crucial function is the intracellular transport of inorganic phosphate (Pi) in the form of H₂ PO₄ ^- . They are one of the main elements in maintaining physiological phosphate homeostasis. Recent data have emerged that indicate novel roles of Pit 1 and Pit 2 proteins besides the well-known function of Pi transporters. These membrane proteins are believed to be precise phosphate sensors that mediate Pi-dependent intracellular signaling. They are also involved in insulin signaling and influence cellular insulin sensitivity. In diseases that are associated with hyperphosphatemia, such as diabetes and chronic kidney disease (CKD), disturbances in the function of Pit 1 and Pit 2 are observed. Phosphate transporters from the SLC20 family participate in the calcification of soft tissues, mainly blood vessels, during the course of CKD. The glomerulus and podocytes therein can also be a target of pathological calcification that damages these structures. A few studies have demonstrated the development of Pi-dependent podocyte injury that is mediated by Pit 1 and Pit 2. This paper discusses the role of Pit 1 and Pit 2 proteins in podocyte function, mainly in the context of the development of pathological calcification that disrupts permeability of the renal filtration barrier. We also describe the mechanisms that may contribute to podocyte damage by Pit 1 and Pit 2.

Diabetes, Diabetic Complications, and Phosphate Toxicity: A Scoping Review

This article presents a scoping review and synthesis of research findings investigating the toxic cellular accumulation of dysregulated inorganic phosphate-phosphate toxicity-as a pathophysiological determinant of diabetes and diabetic complications. Phosphorus, an essential micronutrient, is closely linked to the cellular metabolism of glucose for energy production, and serum inorganic phosphate is often transported into cells along with glucose during insulin therapy. Mitochondrial dysfunction and apoptosis, endoplasmic reticulum stress, neuronal degeneration, and pancreatic cancer are associated with dysregulated levels of phosphate in diabetes. Ectopic calcification involving deposition of calcium-phosphate crystals is prevalent throughout diabetic complications, including vascular calcification, nephropathy, retinopathy, and bone disorders. A low-glycemic, low-phosphate dietary intervention is proposed for further investigations in the treatment and prevention of diabetes and related diabetic pathologies.

Kidney Pathophysiology, Toxicology, and Drug-Induced Injury in Drug Development

Anatomically, the kidneys are paired, bean-shaped (in most mammals), excretory organs that lie in the retroperitoneum. High blood flow to the kidneys, together with high oxygen consumption, makes them more vulnerable to exposure, via the circulation, and subsequent injury related to high concentrations of xenobiotics and chemicals. In preclinical drug development and safety assessment of new investigational drugs, changes in kidney structure and/or function following drug administration in experimental laboratory animals need to be put in context with interspecies differences in kidney functional anatomy, physiology, spontaneous pathologies, and toxicopathological responses to injury. In addition, translation to human relevance to avoid premature drug termination from development is vital. Thus, detection and characterization of kidney toxicity in preclinical species and human relevance will depend on the preclinical safety testing strategy and collective weight-of-evidence approach including new investigational drug mechanism of action (MOA), preclinical and clinical interspecies differences, and MOA relevance to humans. This review describes kidney macroscopic and microscopic functional anatomy, physiology, pathophysiology, toxicology, and drug-induced kidney toxicities in safety risk assessment and drug development.

Effect of a high phosphorus diet on indicators of renal health in cats

Objectives High phosphorus intake may further impair renal health in cats with chronic kidney disease (CKD). The hypothesis that a high phosphorus (HP) diet might be nephrotoxic for healthy animals was tested in cats, a species with a high incidence of naturally occurring CKD. Methods Thirteen healthy adult cats were fed a phosphorus excess diet (about five times maintenance requirements), and this HP group was compared with cats on a balanced control diet (CON). The trial lasted for 29 days (10 days of faeces and urine collection). Endogenous creatinine clearance was determined towards the end of the trial. Fresh urine was tested for glucose and proteins. Results Glucosuria and microalbuminuria were observed exclusively in the HP group in 9/13 cats. Creatinine clearance was significantly decreased after feeding HP. In the HP group phosphorus was highly available (apparent digestibility around 60%). Renal phosphorus excretion was significantly increased in the HP group (115 mg/kg body weight/d vs 16 mg/kg body weight/d in the CON group). Conclusions and relevance The intake of a diet with an excessive content of highly available phosphorus may have adverse effects on parameters of kidney function in healthy cats.

High-Dose Menaquinone-7 Supplementation Reduces Cardiovascular Calcification in a Murine Model of Extraosseous Calcification

Cardiovascular calcification is prevalent in the aging population and in patients with chronic kidney disease (CKD) and diabetes mellitus, giving rise to substantial morbidity and mortality. Vitamin K-dependent matrix Gla-protein (MGP) is an important inhibitor of calcification. The aim of this study was to evaluate the impact of high-dose menaquinone-7 (MK-7) supplementation (100 µg/g diet) on the development of extraosseous calcification in a murine model. Calcification was induced by 5/6 nephrectomy combined with high phosphate diet in rats. Sham operated animals served as controls. Animals received high or low MK-7 diets for 12 weeks. We assessed vital parameters, serum chemistry, creatinine clearance, and cardiac function. CKD provoked increased aortic (1.3 fold; p < 0.05) and myocardial (2.4 fold; p < 0.05) calcification in line with increased alkaline phosphatase levels (2.2 fold; p < 0.01). MK-7 supplementation inhibited cardiovascular calcification and decreased aortic alkaline phosphatase tissue concentrations. Furthermore, MK-7 supplementation increased aortic MGP messenger ribonucleic acid (mRNA) expression (10-fold; p < 0.05). CKD-induced arterial hypertension with secondary myocardial hypertrophy and increased elastic fiber breaking points in the arterial tunica media did not change with MK-7 supplementation. Our results show that high-dose MK-7 supplementation inhibits the development of cardiovascular calcification. The protective effect of MK-7 may be related to the inhibition of secondary mineralization of damaged vascular structures.

Nephrotic Syndrome Induced by Dibasic Sodium Phosphate Injections for Twenty-eight Days in Rats

Sprague-Dawley rats received once daily tail-vein injections of 360 mM dibasic sodium phosphate solution at 8 mL/kg for fourteen or twenty-eight days. Clinical examination revealed persistent proteinuria from three days after the first dosing and thereafter severe proteinuria from eight days or later in the phosphate-treated groups. Proteinuria developed without remission even after fourteen-day withdrawal in the fourteen-day dosed group. Phosphate-treated animals developed lipemia, hypercholesterolemia, anemia, higher serum fibrinogen levels, and lower serum albumin/globulin ratios on day 29. Renal weight increased significantly compared with control animals, and the kidneys appeared pale and enlarged with a rough surface. Histopathologically, glomerular changes consisted of mineralization in whole glomeruli, glomerular capillary dilatation, partial adhesion of glomerular tufts to Bowman’s capsule, and mesangiolysis. Ultrastructural lesions such as an increased number of microvilli, effacement of foot processes, and thickening of the glomerular basement membrane, and immunocytochemical changes in podocytes, mainly decreased podoplanin-positive cells and increased desmin expression, were also conspicuous in the phosphate-treated rats for twenty-eight days. Marked tubulointerstitial lesions were tubular regeneration and dilatation, protein casts, mineralization in the basement membrane, focal interstitial inflammation, and fibrosis in the cortex. These clinical and morphological changes were similar to features of human nephrotic syndrome.