Renal handling of calcium and phosphate

Effect of renal insufficiency on stone recurrence in patients with urolithiasis

The study was designed to assess the relationship between glomerular filtration rate (GFR) and urinary stone-forming constituents, and to assess the effect of renal insufficiency on stone recurrence risk in first stone formers (SF). Baseline serum creatinine levels were obtained, and renal insufficiency was defined as creatinine clearance ≤60 mL/min (Cockroft-Gault). This retrospective case-control study consists of 342 first SF; 171 SF with normal renal function were selected with 1:1 propensity scores matched to 171 SF with renal insufficiency. Urinary metabolic evaluation was compared to renal function. GFR was positively correlated with urinary calcium, uric acid, and citrate excretion. Subjects with renal insufficiency had significantly lower urinary calcium, uric acid, and citrate excretion than those with normal renal function, but not urine volume. With regard to urinary metabolic abnormalities, similar results were obtained. SF with renal insufficiency had lower calcium oxalate supersaturation indexes and stone recurrence rates than SF with normal renal function. Kaplan-Meier curves showed similar results. In conclusion, GFR correlates positively with urinary excretion of stone-forming constituents in SF. This finding implies that renal insufficiency is not a risk factor for stone recurrence.

Inorganic phosphate induces mammalian growth plate chondrocyte apoptosis in a mitochondrial pathway involving nitric oxide and JNK MAP kinase

Chondrocytes in the hypertrophic zone of the growth plate undergo apoptosis during endochondral bone development via mechanisms that involve inorganic phosphate (Pi) and nitric oxide (NO). Recent evidence suggests that Pi-dependent NO production plays a role in apoptosis of cells in the resting zone as well. This study examined the mechanism by which Pi induces NO production and the signaling pathways by which NO mediates its effects on apoptosis in these cells. Pi decreased the number of viable cells based on MTT activity; the number of TUNEL-positive cells and the level of DNA fragmentation were increased, indicating an increase in apoptosis. Blocking NO production using the NO synthase (NOS) inhibitor L: -NAME or cells from eNOS(-/-) mice blocked Pi-induced chondrocyte apoptosis, indicating that NO production is necessary. NO donors NOC-18 and SNOG both induced chondrocyte apoptosis. SNOG also upregulated p53 expression, the Bax/Bcl-2 expression ratio, and cytochrome c release from mitochondria, as well as caspase-3 activity, indicating that NO induces apoptosis via a mitochondrial pathway. Inhibition of JNK, but not of p38 or ERK1/2, MAP kinase was able to block NO-induced apoptosis, indicating that JNK is necessary in this pathway. Pi elevates NO production via eNOS in resting zone chondrocytes, which leads to a mitochondrial apoptosis pathway dependent on JNK.

Environmental salinity regulates the in vitro production of [3H]-1,25-dihydroxyvitamin D3 and [3H]-24,25 dihydroxyvitamin D3 in rainbow trout (Oncorhynchus mykiss)

Previous studies have shown that specific binding of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and 24,25-dihydroxyvitamin D3 (24,25(OH)2D3) to enterocyte basolateral membranes (BLM), as well as circulating concentrations, is affected in response to changes in environmental salinity. It is not known if the production of 1,25(OH)2D3 and 24,25(OH)2D3 is affected by environmental salinity. The aim of the present study was to measure the in vitro production of [3H]-1,25(OH)2D3 and [3H]-24,25(OH)2D3 in fresh water (FW) and after 1, 2, 3, and 7 days after transfer to seawater (SW). Pooled sub-cellular fractions (mitochondria and microsomes) from liver or kidney was incubated with [3H]-25(OH)D3 and the produced metabolites were separated using HPLC. Hepatic production of [3H]-1,25(OH)2D3 was decreased after 24h in SW. This was followed by an up-regulation after 48h and a second, slower decrease in production rate which leveled out after 7 days in SW. The production rate in SW was lower than the original rate in FW-adapted fish. For hepatic [3H]-24,25(OH)2D3 production the pattern was reversed. Renal production of [3H]-24,25(OH)2D3 increased significantly during the period of SW acclimation. These results suggest that environmental salinity regulates the production rate of the two antagonizing calcium regulatory hormones; 1,25(OH)2D3 and 24,25(OH)2D3. This gives further evidence to the hypothesis that there is a physiological regulation and a differentiated importance of 1,25(OH)2D3 and 24,25(OH)2D3 in relation to environmental calcium concentrations.

Possible involvement of inositol phosphates and calmodulin in calcitonin-induced stimulation of phosphate transport in LLC-PK1 cells

The present study investigated the possible involvement of phosphatidylinositol breakdown and Ca2+-calmodulin complex in the calcitonin-induced stimulation of phosphate transport in LLC-PK1 cells. This cell line with calcitonin receptors possesses Na+-dependent phosphate transport and has been employed as a model for studying the mechanism of renal tubular phosphate transport. (Asu1,7) eel calcitonin stimulated the phosphate transport in LLC-PK1 cells in a dose-dependent fashion with accompanying increase of inositol triphosphate (IP3) production. When the cells were preincubated with the potent calmodulin antagonist, w-7 or w-13, the stimulatory effect of calcitonin on phosphate transport was significantly inhibited. These findings indicate that Ca2+-calmodulin complex formed by increased cytosolic Ca2+, which is mobilized from intracellular pools by IP3, may be involved in the signal transduction of calcitonin in these cells.

Arachidonic acid inhibits phosphate transport by cultured renal cells

Because arachidonic acid and its metabolites are reported to be intracellular messengers of various exogenous stimuli, we studied whether arachidonic acid influences phosphate transport by cultured mouse renal epithelial cells. Arachidonic acid, at 10(-7)-10(-4)M, inhibited phosphate transport without influencing cyclic adenosine 3':5'-monophosphate production. Nordihydroguaiaretic acid and indomethacin, inhibitors of arachidonic acid metabolism, did not cancel the arachidonic acid-induced inhibition of phosphate transport. Furthermore, unsaturated fatty acids other than arachidonic acid also inhibited phosphate transport and their inhibitory effect increased as the number of double bond increased. These data demonstrate that arachidonic acid inhibits the phosphate transport by the cultured renal epithelial cells, probably not via conversion to its metabolites.

[Peroral calcium administration test with free diet in idiopathic calcium nephrolithiasis--possibilities and limits]

Seventeen patients who recurrently formed idiopathic calcium kidney stones (SF) and 25 age- and sex-matched healthy blood donors (H) were challenged by an oral calcium load (1 g) after an overnight fast. Their usual diet was not changed before the test. Urine samples were taken before, 2 1/2, and 4 h after the calcium load. A blood sample was drawn 3 3/4 h after calcium loading. Before and 2 1/2 h after calcium dosage urinary measurements of calcium, magnesium, phosphate, oxalate, uric acid, and creatinine did not reveal any differences between SF and H. According to the calciuria after 4 h SF were separated in normocalciurics (NCSF) and hypercalciurics (HCSF). Nine-tenths of the NCSF had higher serum ionic calcium levels than H after calcium load (P less than 0.001), whereas HCSF were not different from H. Serum phosphate in SF was lower than in H (P less than 0.001). Carboxy-terminal parathormone, measured in 3 NCSF and 2 HCSF, was normal. Depending on the calciuria or calcemia 4 h after an oral calcium load, 16 of 17 SF showed a metabolic abnormality (hypercalcemia or hypercalciuria). It is concluded that intestinal calcium absorption in SF might be increased to variable rates.

Electrical properties of Madin-Darby-canine-kidney cells. Effects of extracellular sodium and calcium

In incompletely confluent Madin Darby canine kidney cells continuous measurements of the potential difference across the cell membrane (PD) were made with conventional microelectrodes during rapid changes of extracellular sodium and/or calcium concentration. During control conditions PD averages -50.6 +/- 0.7 mV. Reduction of extracellular sodium concentration from 131.8 to 17.8 mmol/l leads to a reversible hyperpolarization of the cell membrane to -65.3 +/- 1.1 mV. This hyperpolarization is not significantly reduced by omission of glucose or presence of amiloride (1 mmol/l) in the perfusates. Instead, 1 mmol/l amiloride depolarizes the cell membrane by +5.2 +/- 0.4 mV. 1 mmol/l barium depolarizes the cell membrane to -31.3 +/- 1.1 mV. Step increases of extracellular potassium concentration from 5.4 to 10 and 20 mmol/l depolarize the cell membrane by +5.5 +/- 0.5 mV and +16.5 +/- 1.8 mV respectively. In the presence of barium, the depolarizing effect of increasing extracellular potassium concentration and of amiloride is almost abolished. Reduction of extracellular sodium concentration in the presence of barium, however, leads to a transient hyperpolarization of the cell membrane. During this transient hyperpolarization, increasing extracellular potassium concentration depolarizes the cell membrane despite the continued presence of barium. Omission of extracellular calcium (EDTA) depolarizes the cell membrane by +36.7 +/- 3.2 mV. In the absence of extracellular calcium, the hyperpolarizing effect of reduced extracellular sodium concentration is markedly reduced (-4.5 +/- 1.2 mV). 2 mumol/l A23187 in the presence of extracellular calcium hyperpolarizes the cell membrane to -72.5 +/- 0.6 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of Russell's viper (Vipera russelli siamensis) venom on renal hemodynamics in dogs

The effect on renal hemodynamics of Russell's viper (Vipera russelli siamensis) venom was studied in 8 mongrel dogs. The venom (0.10 mg/kg) was injected i.v. Measurements of general circulation and renal function were carried out over 48 hr. During the initial post-injection period, mean arterial blood pressure, pulse pressure and heart rate decreased. Total peripheral vascular resistance and renal vascular resistance showed a tendency to increase. There was no change in cardiac output. Thereafter the blood pressure and heart rate returned to the control level 2 hr after injection and remained stable throughout the experiment. The cardiac output remained unchanged, but the pulse pressure later increased. Renal blood flow, glomerular filtration rate, renal fraction and the rate of urine flow were decreased 24 hr after venom injection and rose to the control levels at 48 hr. Renal vascular resistance remained relatively increased, while peripheral resistance decreased, at 24 hr. Blood volume was unchanged throughout the 48 hr. Disseminated intravascular coagulation was not observed, although the clotting time was prolonged. Renal histological studies showed no remarkable changes; tubular necrosis was not seen. The renal hemodynamic changes may initially be due to catecholamine release and later to renin - angiotensin activation with renal vasoconstriction.

Sodium-dependent transport of phosphate in LLC-PK1 cells

Transport of phosphate has been studied in subconfluent monolayers of LLC-PK1 cells. It was found that this transport system shows similar characteristics to those observed in the kidney. Uptake of phosphate is mediated by a Na+-dependent, substrate-saturable process with an apparent Km value for phosphate of 96 +/- 15 mumol/l. Kinetic analysis of the effect of Na+ indicated that at (pH 7.4) two sodium ions are cotransported with one HOP4(2-) ion (Hill coefficient 1.5) with an apparent Km value for sodium of 56 mmol/l. Pi uptake is inhibited by metabolic inhibitors (ouabain and FCCP). In the pH range of 6.6 of 7.4 Pi uptake rate does not change significantly, indicating that both the monovalent and the divalent form of phosphate are accepted by the transport system. It is suggested that phosphate is transported by LLC-PK1 cells together with sodium (2 Na+:1 HPO4(2-) in an electroneutral manner down a favourable sodium gradient.

The influence of renal prostaglandins on urinary calcium excretion in idiopathic urolithiasis

Hypercalciuria is well recognized as an important factor in the cause of idiopathic calcium stone disease. Identification of the exact mechanism for the renal tubular handling of calcium has proved elusive, hence, treatment methods to alter the concentration of urine calcium in hypercalciuric stone formers have hitherto been non-specific. It is now well established that renal prostaglandins influence intrarenal hemodynamics and tubular electrolyte excretion. As the renal handling of sodium and calcium is intimately related, the possibility that the mechanism underlying hypercalciuria may be prostaglandin mediated was considered. Experiments were performed in conscious Sprague-Dawley rats (n = 10) to determine the changes in calcium excretion following prostaglandin synthetase inhibition with indomethacin. Calcium excretion was significantly reduced (p less than 0.01), compared with control animals (n = 10). Further experiments were performed in anesthetized monkeys (Macaca fascicularis) to see if the inhibitory effect of indomethacin was reversible. Exogenous prostaglandin (PGE2) infusion resulted in a marked calciuretic response without producing changes in glomerular filtration rate or blood pressure. Forty-three hypercalciuric patients were treated with a prostaglandin inhibitor for periods ranging from 2 to 4 weeks, and all showed a significant fall in urinary calcium excretion to within the normal range. This clinical and experimental study suggests that prostaglandin (PGE2) is a hormone which determines the renal handling of calcium by influencing renal tubular function.

Effects of whole body UV-irradiation on oxygen delivery from the erythrocyte

In 16 healthy caucasian volunteers (mean age: 22.2 years) the influence of whole body UV-irradiation on the oxygen transport properties of erythrocytes was investigated. Four hours after irradiation with UV (using the minimal erythema dose, MED) no variation of haemoglobin concentration, hematocrit, mean corpuscular haemoglobin concentration, pH or standard bicarbonate could be found, whereas inorganic plasma phosphate (Pi), calcium, the intraerythrocytic 2,3-diphosphoglycerate (2,3-DPG), the activity of erythrocytic phosphofructokinase (PFK) and pyruvatekinase (PK) increased significantly. The half saturation tension of oxygen (P50-value) tended to increase. The increase of Pi causes--via a stimulation of the glycolytic pathway--an increase in 2,3-DPG concentration and thus results in a shift of the oxygen dissociation curve. It is therefore possible to enhance tissue oxygenation by whole body UV-irradiation.