Phosphate homeostasis

Treatment of phosphorus balance disorders

Phosphorus (P) homeostasis in ruminants has received increased attention over the past decades. Although environmental concerns associated with excessive P excretion in cattle manure have led to incentives to lower dietary P intake, hypophosphatemia-particularly in the periparturient dairy cow-has been associated with conditions, such as the downer cow syndrome or postparturient hemoglobinuria. The objective of this article is to revisit current understanding of P homeostasis in ruminants, to discuss the pathophysiology and clinical presentation of P balance disorders, and to review different treatment approaches to correct imbalances of the body's P equilibrium.

Inhibition by cyclic AMP and phorbol esters of sodium-dependent uptake of phosphate by rat hepatocytes

Na(+)-dependent phosphate uptake by rat hepatocytes in primary culture is inhibited in a time-dependent fashion by cyclic AMP and by the myristate, acetate ester of phorbol. After incubation for 15 min at 37 degrees C with 10(-7) M dibutyryl cAMP, the Vmax of transport is decreased from 0.52 to 0.23 nmol Pi/min per mg protein but the Km value of approximately 1 mM is hardly affected by the treatment. Thus, physiological control of Pi uptake by liver cells probably involves protein phosphorylation(s) catalysed by protein kinases. Protein kinase C may be important but the relatively high concentration of phorbol ester needed to cause inhibition of transport is not convincing evidence for protein kinase C involvent. In the presence of fructose, the rate of Pi uptake is decreased by 50%. This effect is probably secondary to a depletion of cellular ATP.

Sodium-dependent transport of phosphate by rat liver plasma membrane vesicles

Plasma membrane fractions were prepared from homogenates of rat liver by density gradient centrifugation and then used for the formation of right-side-out vesicles. Uptake of Pi into the vesicles is rapid when an inwardly directed sodium gradient is present and an overshoot of uptake occurs indicative of accumulation against a Pi concentration gradient. Initial Pi uptake rate in the presence of a K+ gradient is approx half that seen with Na+, but uptake in the presence of a choline chloride gradient is very slow. An overshoot does not occur with either K+ or choline gradients. The Km(Pi) for the Na-dependent component of Pi uptake is approx. 1 mM and Vmax at 20 degrees C is 0.8 nmol/min per mg protein. The relationship between initial uptake rate and Na+ concentration is sigmoid, with a Hill coefficient of 2.6. It is concluded that the cotransporter resembles that of kidney and intestine in possessing at least two interacting sites for Na+ and that in intact cells the Na+ gradient maintained by the sodium pump ATPase provides the energy for accumulation of Pi against the unfavourable membrane potential.

Uptake of phosphate by rat hepatocytes in primary culture: a sodium-dependent system that is stimulated by insulin

Primary cultures of rat hepatocytes take up phosphate by a saturable Na(+)-dependent process. Thus the plasma membrane possesses an N(+)-Pi cotransporter of the type described for many cell types, e.g., kidney proximal tubular cells and enterocytes. Coupling to Na+ overcomes the barrier to anion entry represented by the membrane potential. At 0.12 mM Pi, the effect of Na+ is cooperative with a Hill coefficient of 1.7 suggesting two sodium sites per molecule of carrier. At 37 degrees C, the Km (for Pi) and Vmax for the sodium-dependent fraction of Pi uptake are approx. 1 mM and 0.35 nmol Pi/min per mg cell protein, respectively. Insulin stimulates Vmax four-fold with no significant effect on Km. Pi uptake in the absence of sodium is not affected by insulin. The stimulation by insulin could be of metabolic significance. Glucose phosphorylation at the expense of ATP is raised in liver following insulin stimulation, and thus, initially there may be an increased demand for Pi for oxidative phosphorylation until new steady-state conditions of hexose phosphate concentrations and of ATP turnover become established.

Studies of the regulation of renal gluconeogenesis in normal and Pi depleted proximal tubule cells

Pi depletion of proximal tubule cells isolated from mouse kidney results in a decrease in the cell content of fructose-2,6-bisphosphate and an increase in the rate of gluconeogenesis from pyruvate, malate and succinate. Gluconeogenesis from glycerol is unaffected by Pi depletion. Introduction of fructose-2,6-bisphosphate into the cytosol of ATP-permeabilized cells is accompanied by a fall in gluconeogenesis. The presence of external Ca2+ stimulates gluconeogenesis. When cytosolic Ca2+ is raised to 1.8 microM by permeabilization, the resealed cells still require 2.5 mM Ca2+ in the bathing medium in order to perform gluconeogenesis at the maximum rate. Cells permeabilized in the presence of cAMP show a decreased rate of glucose production. Phorbol ester stimulates gluconeogenesis provided that the phorbol treatment is performed in the absence of Ca2+ ions. It is suggested that Pi depletion may stimulate pyruvate carboxylase activity and facilitate the entry of certain gluconeogenic substrates into mitochondria. It is also proposed that important aspects of the control of renal gluconeogenesis by parathyroid hormone are mediated by protein kinase C.

Phosphate transport across the basolateral membrane of chick kidney proximal tubule cells

Characterization of the phosphate transport system across the basolateral membrane of renal proximal tubule has been attempted using isolated proximal tubule cells prepared from chicks. The Pi efflux system is independent of Na+ ions and is not influenced by the nature of the chief anion present in the bathing medium. Pi efflux is not sensitive to DIDS and it is concluded that a generalized anion transporter of band III type is not the chief agent for facilitating Pi exit from the cell across the basolateral membrane. Inhibition of efflux by vanadate is evidence for a specific carrier protein in the membrane. The carrier probably possesses thiol group(s) that are essential for activity. The carrier may effect electroneutral transport of Pi possibly in exchange for OH- ions. The activity of the transport process is not stimulated by depleting the cells of phosphate or inhibited by rearing the chicks on a vitamin D-deficient diet. The system is unlikely to be of great importance for the expression of various regulatory mechanisms that act on the kidney to control the excretion of Pi. The activity declines as the chicks mature however.

Study of the mechanism by which the Na+-Pi co-transporter of mouse kidney proximal-tubule cells adjusts to phosphate depletion

1. Proximal-tubule cells isolated from mouse kidney after digestion with collagenase take up Pi by an Na+-dependent and saturable process mediated by the Na+-Pi co-transporter of the brush-border membrane. 2. Pi depletion of the cells is accompanied by a stimulation of Pi-transport activity. Kinetic investigations reveal that Vmax. is increased by 90% and Km decreased by 50% after Pi depletion. Transport activity returns to normal values after incubation for 30 min at 37 degrees C of Pi-depleted cells in normal medium containing 1 mM-Pi, but the fall in transport activity under these conditions is inhibited by colchicine. 3. The energy of activation of Na+-Pi co-transport activity of depleted cells differs greatly from that found for normal replete cells. 4. The results provide evidence that stimulation of transport by Pi depletion arises from an increase in the number of carrier sites in the brush-border membrane. Additionally, changes in the properties of the transporter occur which may reflect altered phospholipid-carrier-protein interaction in the Pi-depleted condition.

Net fluxes of orthophosphate across the plasma membrane in human red cells following alteration of pH and extracellular Pi concentration

Even though net fluxes of Pi (orthophosphate) across the cell membrane may be important in clinical disorders involving the abnormal extracellular Pi concentration, in acid-base disturbances, and in the responses of some cells to hormones, relatively few studies have been made of these fluxes, owing to the complexities of interpretation. Here we have studied net fluxes in response to changes in extracellular pH and Pi concentration in the simple case of the human red cell. The permeability of the cell membrane to net Pi fluxes was described in terms of a first-order rate constant, epsilon. By means of a mathematical model, it was possible to discriminate between transmembrane Pi movement, net intracellular generation or consumption of Pi by organic phosphates, and extracellular generation of Pi from the cells lysing during the experiment. We show that net Pi influx into the cell during experimental alkalosis was probably driven by net consumption of Pi by organic phosphates, and that this was reversed during acidosis. Inhibition of net Pi influx by 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonate (SITS) suggests that, like Pi self-exchange, net influx is at least partly mediated by the band 3 transport protein. Unexpectedly, epsilon increased from 2 h-1 at extracellular pH 7.4 to approx. 7 h-1 at pH 7.8. From the value of epsilon at pH 7.4, we conclude that the apparent buffering or regulation of steady-state Pi concentrations, previously reported in red cells in vitro, was not an artifact of intracellular generation of Pi from organic phosphates.

Regulation of phosphate metabolism in human red cells following prolonged incubation to steady state in vitro

Human red cells were incubated aseptically in vitro for 24 or 48 h to allow the cellular concentrations of orthophosphate (Pi) and organic phosphates to attain steady state. In plasma at pH 7.0-8.0, the transmembrane Pi concentration ratio R (cellular Pi/plasma Pi) decreased with increasing pH, with a slope which was 2.7-times greater than that predicted if Pi simply distributed passively across the cell membrane. The concentration of 2,3-bisphosphoglycerate (2,3-BPG), the most abundant cytosolic organic phosphate, decreased at acidic pH and increased at alkaline pH, but stabilised at these values after 24 h. Therefore, while net generation or consumption of Pi by 2,3-BPG may initially have contributed to the steep dependence of R on pH, some other factor must have maintained this anomaly after 24 h. In plasma in which the Pi concentration was increased from 1 to 2.5 mM, the cellular Pi concentration increased from 0.6 to only 1.0 mmol/l cells, and 2,3-BPG increased by less than 20%. Thus, cellular Pi and 2,3-BPG concentrations seemed to be buffered or regulated in the face of changes in extracellular Pi. However, this regulation failed in a Pi-free balanced salt solution, as the 2,3-BPG concentration declined to half that observed in freshly drawn blood, although cell Pi remained at about 0.3 mM. Incubation in Pi-free solution with ouabain for 24 h to decrease the transmembrane sodium gradient, or incubation for 2 h in the absence of sodium, decreased this residual cellular Pi by about 20%, but did not abolish it. In Pi-free solution, but not with 1 mM Pi, cellular Pi increased when passive transmembrane Pi leakage was inhibited with 4-acetamido-4'-iso-thiocyanatostilbene-2,2'-disulphonate (SITS). We conclude that red cell Pi concentration cannot be explained fully by passive transmembrane distribution of Pi, nor by changes in 2,3-BPG, and that part of the anomaly may arise from sodium-linked active Pi transport.