GlomSpheres as a 3D co-culture spheroid model of the kidney glomerulus for rapid drug-screening.

The glomerulus is the filtration unit of the kidney. Injury to any component of this specialised structure leads to impaired filtration and eventually fibrosis and chronic kidney disease. Current two and three dimensional (2D and 3D) models that attempt to recreate structure and interplay between glomerular cells are imperfect. Most 2D models are simplistic and unrepresentative, and 3D organoid approaches are currently difficult to reproduce at scale and do not fit well with current industrial drug-screening approaches. Here we report a rapidly generated and highly reproducible 3D co-culture spheroid model (GlomSpheres), better demonstrating the specialised physical and molecular structure of a glomerulus. Co-cultured using a magnetic spheroid formation approach, conditionally immortalised (CI) human podocytes and glomerular endothelial cells (GEnCs) deposited mature, organized isoforms of collagen IV and Laminin. We demonstrate a dramatic upregulation of key podocyte (podocin, nephrin and podocalyxin) and GEnC (pecam-1) markers. Electron microscopy revealed podocyte foot process interdigitation and endothelial vessel formation. Incubation with pro-fibrotic agents (TGF-ß1, Adriamycin) induced extracellular matrix (ECM) dysregulation and podocyte loss, which were attenuated by the anti-fibrotic agent Nintedanib. Incubation with plasma from patients with kidney disease induced acute podocyte loss and ECM dysregulation relative to patient matched remission plasma, and Nintedanib reduced podocyte loss. Finally, we developed a rapid imaging approach to demonstrate the model's usefulness in higher throughput pharmaceutical screening. GlomSpheres therefore represent a robust, scalable, replacement for 2D in vitro glomerular disease models.

Impact of passive heat acclimation on markers of kidney function during heat stress.

NEW FINDINGS: What is the central question of this study? Does passive heat acclimation alter glomerular filtration rate and urine-concentrating ability in response to passive heat stress? What is the main finding and its importance? Glomerular filtration rate remained unchanged after passive heat stress, and heat acclimation did not alter this response. However, heat acclimation mitigated the reduction in urine-concentrating ability and reduced the incidence of albuminuria in young healthy adults after passive heat stress. Collectively, these results suggest that passive heat acclimation might improve structural integrity and reduce glomerular permeability during passive heat stress. ABSTRACT: Little is known about the effect of heat acclimation on kidney function during heat stress. The purpose of this study was to determine the impact of passive heat stress and subsequent passive heat acclimation on markers of kidney function. Twelve healthy adults (seven men and five women; 26 ± 5 years of age; 72.7 ± 8.6 kg; 172.4 ± 7.5 cm) underwent passive heat stress before and after a 7 day controlled hyperthermia heat acclimation protocol. The impact of passive heat exposure on urine and serum markers of kidney function was evaluated before and after heat acclimation. Glomerular filtration rate, determined from creatinine clearance, was unchanged with passive heat stress before (pre, 133 ± 41 ml min-1 ; post, 127 ± 51 ml min-1 ; P = 0.99) and after (pre, 129 ± 46 ml min-1 ; post, 130 ± 36 ml min-1 ; P = 0.99) heat acclimation. The urine-to-serum osmolality ratio was reduced after passive heating (P < 0.01), but heat acclimation did not alter this response. In comparison to baseline, free water clearance was greater after passive heating before (pre, -0.86 ± 0.67 ml min-1 ; post, 0.40 ± 1.01 ml min-1 ; P < 0.01) but not after (pre, -0.16 ± 0.57 ml min-1 ; post, 0.76 ± 1.2 ml min-1 ; P = 0.11) heat acclimation. Furthermore, passive heating increased the fractional excretion rate of potassium (P < 0.03) but not sodium (P = 0.13) or chloride (P = 0.20). Lastly, heat acclimation reduced the fractional incidence of albuminuria after passive heating (before, 58 ± 51%; after, 8 ± 29%; P = 0.03). Collectively, these results demonstrate that passive heat stress does not alter the glomerular filtration rate. However, heat acclimation might improve urine-concentrating ability and filtration within the glomerulus.

Combination of vitamin E and vitamin C alleviates renal function in hyperoxaluric rats via antioxidant activity.

Hyperoxaluria and oxidative stress are risk factors in calcium oxalate (CaOx) stone formation. Supplement with antioxidant could be effective in prevention of recurrent stone formation. The present study aims to evaluate the protective effects of vitamin E and vitamin C in hyperoxaluric rat. The experiment was performed in rats for 21 days. Rats were divided into 5 groups as follows: control (group 1, n=8), hyperoxaluric rats (group 2, n=8), hyperoxaluric rats with vitamin E supplement (group 3, n=7), hyperoxaluric rats with vitamin C supplement (group 4, n=7) and hyperoxaluric rats with vitamin E and C supplement (group 5, n=7). Hyperoxaluria was induced by feeding hydroxyl L-proline (HLP) 2% w/v dissolved in drinking water. Intraperitoneal 200 mg/kg of vitamin E was given in groups 3 and 5 on days 1, 6, 11 and 16, while 500 mg of vitamin C was injected intravenously in groups 4 and 5 on days 1 and 11. Renal functions and oxidative status were measured. The urinary oxalate excretion was increased in HLP supplement rats, while glomerular filtration rate, proximal water and sodium reabsorption were significantly lower in group 2 compared with a control (P<0.05). Giving antioxidants significantly lower urinary calcium oxalate crystals (P<0.05). Hyperoxaluric rats had higher plasma malondialdehyde (PMDA) and lower urinary total antioxidant status (UTAS), which were alleviated by vitamin E and/or vitamin C supplement. In conclusion, giving combination of vitamin E and vitamin C exerts a protective role against HLP-induced oxalate nephropathy.

[Renal physiology]. / Physiologie rénale.

The kidneys are responsible for the urinary excretion of uremic toxins and the regulation of several body systems such as intra and extracellular volume status, acid-base status, calcium and phosphate metabolism or erythropoiesis. They adapt quantitative and qualitative composition of the urine to keep these systems in balance. The flow of plasma is filtered in the range of 120 mL/min, and depends on the systemic and renal hemodynamics which is subject to self-regulation. The original urine will then be modified in successive segments of the nephron. The proximal nephron is to lead the massive reabsorption of water and essential elements such as sodium, bicarbonates, amino-acids and glucose. The distal nephron includes the distal convoluted tubule, the connector tube and the collecting duct. Its role is to adapt the quality composition of urine to the needs of the body.

A computer program for single-injection tracer clearance from one, two, or many blood samples

A computer program based on a Bayesian statistical model has been developed for calculating tracer clearance from any number of plasma samples drawn at arbitrary time intervals. Bayesian prior parameters were calculated from clinical data for Tc99m-MAG3, Tc99m-EC, I131-OIH, Tc99m-DTPA, and Yb169-DTPA and then used to calculate clearance from prospective data. Clearance estimates using only one or two plasma samples were found to closely approximate the results of using multiple samples. When only one or a few samples are available, the program supplements the observed data by a Bayesian prior probability distribution (based on prior clinical measurements) to achieve agreement with multisample clearance. When many points are available, the observed data overwhelm the prior probability, and results approach those of conventional curve fitting but with less sensitivity to bad data points and less risk of fitting failure.

Comparison of the effects of vitamins and/or mineral supplementation on glomerular and tubular dysfunction in type 2 diabetes.

OBJECTIVE: The present study was designed to assess the effect of magnesium plus zinc, vitamins C plus E, and a combination of these micronutrients on nephropathy indexes in type 2 diabetic patients. RESEARCH DESIGN AND METHODS: In a randomized, double-blind, placebo-controlled clinical trial, 69 type 2 diabetic patients were randomly divided into four groups, each group receiving one of the following daily supplement for 3 months: group M (n = 16), 200 mg Mg and 30 mg Zn; group V (n = 18), 200 mg vitamin C and 100 IU vitamin E; group MV (n = 17), minerals plus vitamins; and group P (n = 18), placebo. Urinary albumin excretion and N-acetyl-beta-d-glucosaminidase activity (NAG) in urine were determined at the beginning and at the end of the trial. Treatment effects were analyzed by general linear modeling. RESULTS: Results indicate that after 3 months of supplementation, levels of urinary albumin excretion decreased in the V and MV groups (P = 0.034 and P = 0.005, respectively). Urinary NAG activity did not significantly change in any treatment groups. Levels of systolic, diastolic, and mean blood pressure significantly decreased in the MV group (P = 0.008, P = 0.017, and P = 0.009, respectively). Also, combination of vitamin and mineral supplementation had significant effects in decreasing fasting serum glucose (P = 0.035) and malondialdehyde concentrations (P = 0.004) and in increasing HDL cholesterol and apolipoprotein A1 levels (P = 0.019). There was no significant change in the levels of these parameters in the other three groups. CONCLUSIONS: In conclusion, the results of the present study provide evidence for the effects of vitamins C and E and also combination of magnesium, zinc, and vitamins C and E supplementation on improvement of glomerular but not tubular renal function in type 2 diabetic patients.

Effect of high dose ramipril with or without indomethacin on glomerular selectivity.

BACKGROUND: Despite the accumulating evidence of their efficacy, angiotensin-converting enzyme inhibitors (ACEi) still provide imperfect renoprotection. Up-titration above conventional doses and combined therapy with other antiproteinuric agents may serve to achieve renoprotection in patients at risk of rapid disease progression. METHODS: The effect of maximum tolerated ACEi doses (ramipril 15 mg/day, range 5 to 20) alone or combined with indomethacin (75 mg x 2/day) on urinary protein excretion (UPE) and glomerular barrier size-selective function was evaluated in 19 patients with chronic non-diabetic nephropathies and persistent proteinuria. RESULTS: Maximum ramipril doses decreased UPE more effectively than non-ACEi therapy. Proteinuria reduction was associated with significant reduction (>50%) of the non-selective glomerular membrane shunt, but did not correlate with concomitant changes in arterial pressure and renal hemodynamics, nor was it influenced by treatment duration. The reduction in UPE and sieving coefficient of the largest neutral dextrans exceeded by twofold the reduction achieved by conventional ACEi doses in historical controls with similar renal dysfunction and proteinuria, previously studied under identical experimental conditions. Indomethacin did not influence renal effects of maximum ramipril doses and was prematurely withdrawn in six patients because of reversible side effects. Serum potassium significantly increased only in combination with indomethacin and never required treatment withdrawal. CONCLUSIONS: Up-titration to maximally tolerated doses safely increases ACEi antiproteinuric effect and may serve to achieve maximum renoprotection in the long-term. Combination with indomethacin is poorly tolerated and ineffective. Innovative approaches are needed to use ACEi more effectively.

Evidence for the distinct nature of F2-isoprostane receptors from those of thromboxane A2.

In rat glomeruli and mesangial cells, the thromboxane A2 (TxA2) mimetic, U-46,619, but not 8-iso-prostaglandin F2alpha (8-iso-PGF2alpha), reduced glomerular inulin space and increased inositol 1,4,5-trisphosphate production, effects abolished by SQ-29,548. In competitive binding studies using 8-iso-[3H]PGF2alpha or [3H]SQ-29,548, mesangial cells displayed TxA2 binding sites but not ones for 8-iso-PGF2alpha. In contrast, rat aortic smooth muscle cells possessed specific binding sites for both TxA2 and 8-iso-PGF2alpha and displayed functional responses to both agonists, such as time- and dose-dependent activation of mitogen-activated protein kinases. In these cells, the mean dissociation constant value for the isoprostane receptor was 31.8 +/- 5.7 nM. When human TxA2 receptor cDNA was expressed in Xenopus oocytes injected with the Ca2+-specific photoprotein, aequorin, 8-iso-PGF2alpha gave much weaker responses than U-46,619. These studies provide the first radioligand binding characteristics of the F2-isoprostane receptor and demonstrate its specific and heterologous cellular localization. These studies support the distinct nature and biological significance of isoprostane receptors and provide a tool for their further molecular characterization.

Measurement of glomerular filtration rate using inulin prepared from Vernonia herbacea, a Brazilian native species.

Vernonia herbacea (Vell.) Rusby (Asteraceae) is a perennial herb native to the cerrado vegetation of tropical areas in Brazil, which accumulates inulin in the underground reserve organs. The aim of this paper was to determine whether the inulin extracted from V. herbacea could replace commercial inulin for the measurement of glomerular filtration rate (GFR). Underground organs of vegetative plants were collected from a preserved area of the Brazilian cerrado. The inulin fraction utilized was obtained by ethanol precipitation after discarding the high molecular mass fructans in the freeze-thawing precipitate. GFR was determined in male Wistar rats anesthetized with inactin (100 mg/kg), which received intravenously commercial inulin obtained from Dahlia sp (Sigma) or Vernonia herbacea inulin (30 mg/100 g) as a priming dose and 0.05 mg min-1 100 g-1 as a sustaining dose in isotonic saline at the rate of 0/055 ml/min. Clearance was determined during 3 periods, with urine collected from the bladder and blood from the carotid artery. There was no significant difference in the GFR measured by clearance in inulin from both sources even when the plasma concentration of inulin from V. herbacea was doubled. The mean arterial pressure did not vary after the application of both inulins, indicating that they do not produce systemic side effects. The filtered load and the excreted amount of inulin from V. herbacea were equal, showing that the substance is not influenced by tubular function. These results demonstrate that the inulin from V. herbacea can substitute for imported inulin for the determination of GFR and in experiments of kidney microperfusion as a marker of tubular water reabsorption.

Divergent effects of calcium channel and angiotensin converting enzyme blockade on glomerulotubular function in cyclosporine-treated renal allograft recipients.

The effects of amlodipine and perindopril on renal hemodynamics and tubular function in cyclosporine-treated hypertensive renal allograft recipients were evaluated in a randomized, double-blind crossover fashion. Ten patients were studied after a 2-week placebo run-in and, after 8 weeks of active treatment, allowing a 2-week placebo washout between treatments. At the end of each period, glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured as 51Cr-EDTA and 123I-hippuran clearance, respectively, and tubular function evaluated by the lithium clearance technique was determined. Both drugs maintained GFR and ERPF and lowered mean arterial pressure (MAP, mm Hg) to a similar extent (time x treatment, P = 0.466): amlodipine from 126.9 +/- 2.5 to 115.9 +/- 2.2; perindopril from 126.9 +/- 2.5 to 117.9 +/- 3.9 (time effect of all treatments together, P = 0.003). Accordingly, renal vascular resistance (RVR, mm Hg/mL/min/1.73 m2) was equally reduced (time x treatment, P = 0.955): amlodipine from 0.36 +/- 0.03 to 0.30 +/- 0.02; perindopril from 0.36 +/- 0.03 to 0.32 +/- 0.01 (time effect all treatments together, P = 0.043). Sodium clearance and fractional excretion of sodium were not affected by either drug. Output of fluid from the proximal tubules measured as clearance of lithium (CLi, mL/min) and uric acid (CUr, mL/min) was higher after amlodipine than after perindopril (CLi 19.1 +/- 2.1 v 16.5 +/- 1.7, P =0.036 and CUr 7.0 +/- 0.6 v 5.9 +/- 0.4, P = 0.007). As a consequence, after amlodipine, distal absolute reabsorption of sodium was higher (DARNa 2.57 +/- 0.28 v 2.19 +/- 0.22 mEq/min, P = 0.027) and serum uric acid was lower (5.9 +/- 0.3 v 6.7 +/- 0.4 mg/dL, P = 0.001) in comparison with perindopril. In cyclosporine-treated renal allograft hypertensives, amlodipine and perindopril lower blood pressure equally and reduce RVR to the same extent. Overall sodium excretion is not affected by either agent. Urate clearance is higher and serum uric acid lower on amlodipine as compared with perindopril.

Renal effects of low dose methotrexate in rheumatoid arthritis.

OBJECTIVE: Methotrexate (MTX) treatment at high doses may cause renal failure. The effects of treatment with low MTX doses on kidney function are, however, unknown. The objective of our study was to investigate the effect of low MTX doses on kidney function. METHODS: The glomerular filtration rate was measured as the plasma clearance of 51Cr-EDTA and the clearance of 99mTc-dimercaptoacetyl-triglycine (correction of triglycerine) (99mTc-MAG3), a new renal tubular function agent, was used to measure tubular effects of MTX. RESULTS: Significant decrease of both 51Cr-EDTA clearance from 92 +/- 7 to 83 +/- 5 ml/min x 1.73 m2 (mean +/- SEM) and of the 99mTc-MAG3 clearance from 360 +/- 18 to 309 +/- 15 ml/min x 1.73 m2 (p < 0.05) was observed. CONCLUSION: Our results indicate that low dose MTX treatment (15 mg weekly) may significantly impair kidney function which has to be considered particularly in situations with combined treatment with other potentially nephrotoxic substances.

Comparative effects of captopril versus nifedipine on proteinuria and renal function of type 2 diabetic patients.

Our study compared the effects of an angiotensin-converting enzyme inhibitor (captopril) versus a calcium antagonist (nifedipine) on proteinuria and renal function in patients with diabetic nephropathy. A randomized follow-up study was designed. Type 2 diabetic patients, with established diabetic nephropathy (proteinuria greater than 0.5 g/24 h), were treated with nifedipine (10 patients, group A) or captopril (10 patients, group B) for 6 months. Arterial blood pressure, metabolic parameters, proteinuria and renal function were measured and compared. Mean percentage differences for glomerular filtration rate, renal plasma flow and filtration fraction between the two groups were calculated. No significant differences were observed in serum glucose, glycosylated hemoglobin (hemoglobin A1c), Na+, K+ or albumin in either group or between groups. Blood pressure decreased significantly with both treatments and mean blood pressure was significantly lower in group A compared with group B at 6 months (Mann-Whitney U-test, P = 0.03). Proteinuria was similar in both groups at randomization, but after 3 and 6 months of treatment significant reductions were observed only in the group treated with captopril (P less than 0.01). A significant decrease in filtration fraction was observed in group B with an increase in group A (Mann-Whitney U-test, P = 0.03). Multiple regression analysis identified the therapeutic agent administered as an independent variable for decrease in proteinuria. It is concluded that antihypertensive treatment with captopril, but not with nifedipine, reduced proteinuria in patients with diabetic nephropathy, although a better mean blood pressure was obtained with nifedipine.(ABSTRACT TRUNCATED AT 250 WORDS)

Free amino acids during chronic cyclosporine A toxicity in intact and partially nephrectomized rats.

The effects of 40 days of treatment with Cyclosporine A (CSA) on plasma and urine free amino acids were investigated in sham-operated (C) and partially nephrectomized (Pnx) female Fischer 344 rats. High Dose CSA (30 mg/kg/day ip) was associated with reduced weight gain, increased plasma urea nitrogen, and hypoproteinemia in C and Pnx animals. These animals also demonstrated increased plasma levels of alanine, markedly reduced levels of tryptophan, and an increase in urinary excretion of methylhistidines. C but not Pnx animals also showed a significant increase in plasma serine and a decrease in plasma taurine. CSA treatment of group C resulted in a progressive aminoaciduria involving substrates of the neutral and acidic renal amino acid transport systems; however, the renal excretion of taurine and beta-alanine by these animals was markedly reduced as compared to vehicle treated controls. High dose CSA exacerbated aminoaciduria in Pnx animals, but in this group, the excretion of beta amino acids was also increased. Our findings demonstrate that chronic CSA toxicity in rodents with normal renal function is characterized by increased muscle protein catabolism, significant reductions in plasma tryptophan, and an apparent decrease in whole body taurine pools. With the exception of the taurine abnormalities. CSA treatment had similar effects on Pnx animals; however, in this group, CSA-induced pathological changes were superimposed on the changes due to renal insufficiency per se. CSA toxicity as identified by the parameters investigated in this study was no more severe in Pnx animals with moderate chronic renal insufficiency than in controls with intact renal function.

Postnatal development of renal function.

Renal function in the newborn is both qualitatively and quantitatively different from older infants, children and adults. Moreover, gestational age is of great importance in the interpretation of renal functional differences. These differences in renal function should not be viewed as solely immaturity per se but rather as functional adaptations to the needs of growth and development in many instances. Although the neonatal kidney is operating at a level appropriate for a given age, the decreased functional reserve makes it more vulnerable to stressful conditions. However, the low GFR and some of the decreased transport properties may actually be of benefit to the neonate. With respect to medications, one benefit of these neonatal differences appears to be less nephrotoxicity for certain drugs in the newborn compared with the adult.

Glomerular permeability in the isolated perfused rat kidney.

Increased glomerular permeability to protein occurs during isolated rat kidney perfusion in the absence of ultrastructural changes in the glomerular capillary wall. The increase is greater with age, varies with strain and is reduced by amino acid supplementation of the perfusate. Glomerular permeability to protein is thus perceptibly influenced by non-pathogenic stimuli. Such phenomena, in turn, may influence glomerular function in disease or after experimental damage.

A network thermodynamic model of glomerular dynamics: application in the rat.

A model of glomerular dynamics has been developed by using network thermodynamics and the SPICE 2 computer program to further explore the determinants of glomerular filtration. The model is designed to be holistic and self-adjusting, taking cognizance of and permitting quantitation of the secondary alterations in individual effective glomerular resistances, glomerular blood and plasma flow, capillary oncotic pressure and glomerular capillary pressure, which inevitably result when any parameter affecting glomerular dynamics changes. Such automatic adjustment adds to the precision of computation and is unique to the present model. Few assumptions are introduced, independent variables (arterial pressure, individual resistances, hydraulic conductivity, hematocrit, and serum protein concentration) being entered whereas values for the dependent variables are determined by the computer. In rats, filtration pressure equilibrium is seen not to obtain either under physiologic conditions or with reasonably large changes in any of the independent variables. Capillary pressure is shown to be affected by any maneuver that modulates single nephron GFR (SNGFR) and flow across the efferent arteriole (for example, tubule pressure, serum protein concentration) even when arteriolar caliber is held constant. The axial rise in colloid oncotic pressure and serum protein concentration along the capillary is found to be neither linear nor semilogarithmic, a characteristic that reflects on equations used to determine capillary hydraulic conductivity. Isolated change in afferent arteriolar resistance is shown by the model to produce a linear relationship between glomerular plasma flow and capillary pressure, and thus between the former parameter and filtration. Large solitary increases in efferent arteriolar resistance raise SNGFR and a 60% fall in resistance virtually abolishes filtration while exerting little change in blood flow. Concomitant and equal alterations of afferent and efferent arteriolar resistances cause filtration to rise linearly with blood flow but to produce minor change in glomerular capillary pressure, an example of true plasma flow dependence. Plasma flow dependence is, however, found to be unique to this particular circumstance under physiologic conditions. Adding an optional element that automatically adjusts effective efferent arteriolar resistance as a function of Hct2 has but modest effects on glomerular dynamics except when systemic hematocrit is substantially altered. The data and conclusions derived in this study are based on typical values for resistances, hydraulic conductivity, systemic protein concentration, hematocrit, and arterial and tubular pressures reported for normal hydropenic rats. They will not necessarily hold in other species in which these values may be distinctly different.

Multiple changes in distal stop-flow electrolyte patterns and reduction of acid excretion induced in rabbits by angiotensin.

1. Angiotensin has previously been shown to inhibit distal renal tubular sodium reabsorption. As a consequence of this, or independently, it might influence the distal handling of other electrolytes. We have therefore examined the effects of angiotensin on the distal reabsorption or secretion of a spectrum of electrolytes. 2. Standard bilateral stop-flow studies were done on anaesthetized, adrenalectomized rabbits, in which the effects of intravenous infusions of either 0-02-0-05 mug min-1 kg-1 or 1 mug min-1 kg-1 of angiotensin were compared with control stop-flow results. 3. The lower dose of angiotensin inhibited distal sodium, chloride, water and magnesium reabsorption, inhibited distal hydrogen secretion and stimulated distal potassium secretion. The higher dose of angiotensin produced these changes and additionally inhibited distal calcium reabsorption. Most of the observed changes were dose-related. The low dose of angiotensin did not significantly raise blood pressure but the high dose was pressor. 4. Changes in the stop-flow patterns induced by the higher dose of angiotensin were compatible with, and may help to explain, the changes it produced in urinary excretion of sodium, chloride, potassium, magnesium and calcium in clearance studies before stop-flow. Suppression of hydrogen secretion caused by both doses of angiotensin in the stop-flow studies was also reflected by reductions in acid excretion produced by these infusion rates in additional experiments performed by clearance methods in acid-loaded, conscious rabbits. 5. The results support the view that angiotensin may have an important intrarenal role, at least in rabbits.

Renal regulation of phosphate excretion in the pig.

Young pigs of about 25 kg in weight were given supplements of Na2HPO4 either in their food or by intravenous infusion and the effects on urinary phosphate excretion were studied. The results obtained showed that the amount of inorganic phosphate excreted in the urine increased with increase in the amount of phosphate filtered at the glomerulus. A threshold value for phosphate excretion in these pigs occurred at a plasma concentration of 2-03 mmol/l. while over a wide range of filtered phosphate load (0-21-1-14 mmol/min) phosphate was reabsorbed at a mean maximum rate of 0-19 mmol/min. Parathyroidectomy did not alter the relationship between phosphate excretion and filtered phosphate load which suggests that in the pig adjustment of renal phosphate excretion in relation to filtered load is not dependent upon parathormone.