Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Clinical Disturbances of Phosphate Homeostasis
In the kidney, filtered inorganic phosphate ions are reabsorbed along the proximal tubules. This transepithelial process involves sodium-dependent phosphate transporters that are localized at the apical (brush border) membrane. Currently, three Na/Pi-cotransporters that belong to the SLC 20 (PiT-2) and SLC 34 (NaPi-lla and NaPi-llc) families have been assigned to proximal tubular phosphate reabsorption, whereby SLC34 proteins play the major role. The primary functional difference between SLC34 and SLC20 proteins is that the former preferentially transport divalent P i whereas the latter prefer monovalent Pi.
Renal excretion of phosphate is controlled by the number of Na/Pi-cotransporters residing in the apical membrane. The abundance of Na/Pi-cotransporters is controlled by a multidue of hormones and metabolic factors. Genetic diseases associated with disturbed phosphate homeostasis affect either Na/Pi-cotransporters directly or the stability and production of regulatory factors.
Keywords
proximal tubule, phosphate, NaPi-cotransport, NaPi-lla, regulation, NHERF1
The Importance of Phosphorus in Biological Systems
Phosphorus is the sixth most abundant element in the body and is essential for energy-consuming metabolic processes of cells. Approximately 85% of phosphate in the body is present in bones, 14% in cells from soft tissues, and 1% in extracellular fluids. Phosphorus plays an important role in a number of biological processes, and is an exceptionally important component of hy droxyapatite, the major component of bone mineral. In addition, phosphorus is present in nucleic acids, bioactive signaling proteins, phosphorylated enzymes, and cell membranes. A prolonged deficiency of phosphorus and inorganic phosphate results in serious biological problems, including bone mineralization resulting in osteomalacia or rickets, abnormal erythrocyte, leukocyte and platelet function, impaired cell membrane integrity that can result in rhabdomyolysis, and impaired cardiac function. Therefore, the maintenance of appropriate phosphorus homeostasis is critical for the well-being of the organism.
The Regulation of Phosphate Balance
Phosphorus exists in plasma almost entirely as inorganic phosphate. The adult animal or human maintains phosphate balance through a series of complex hormonally and locally regulated metabolic adjustments. In states of neutral phosphate balance, net accretion equals net output. The major organs involved in the absorption, excretion, and reabsorption of phosphate are the intestine and the kidney ( Fig. 69.1 ). Furthermore, the movement of phosphorus between the extracellular fluid and bone and soft tissue also plays an important role in the maintenance of normal serum phosphate concentrations. A normal diet adequate in phosphorus normally contains ≈1500 mg of phosphorus. Approximately 1100 mg of ingested dietary phosphate is absorbed in the proximal intestine, predominantly in the jejunum. About 200 mg of phosphorus is secreted into the intestine via pancreatic and intestinal secretions, giving a net phosphorus absorption of approximately 900 mg/24 hours. Phosphorus that is not absorbed in the intestine or is secreted into the intestinal lumen eventually appears in the feces. Absorbed phosphorus enters the extracellular fluid pool and moves in and out of bone (and to a smaller extent in and out of soft tissues) as needed (≈200 mg). Approximately 900 mg of phosphorus (equivalent to the amount absorbed in the intestine) is excreted in thse urine.
A number of hormones such as parathyroid hormone (PTH) and 1α,25(OH) 2 D 3 are involved in the control of phosphorus metabolism. Concentrations of these hormones are regulated by phosphorus in a manner that is conducive to the maintenance of normal phosphate. Other peptide factors and hormones, such as growth hormone and insulin-like growth factor 1, alter phosphorus balance, although their circulating concentrations are not directly controlled by ambient phosphorus concentrations. The “phosphatonins,” fibroblast growth factor 23 (FGF-23), and secreted frizzled related protein-4 (sFRP-4) that induce a state of negative phosphate balance directly by inhibiting renal phosphate reabsorption in the proximal tubule and indirectly by inhibiting the synthesis of 1α,25(OH) 2 D 3 and reducing the intestinal absorption of phosphorus, also play a key role in the regulation of phosphate balance. Two factors, fibroblast growth factor 7 (FGF-7) and matrix extracellular phosphoglycoprotein (MEPE), have been shown to inhibit phosphate transport in renal epithelial cells in culture and, in the case of matrix extracellular phosphorus glycoprotein, to induce phosphaturia in mice. FGF7 and MEPE, however, have not been demonstrated to prevent compensatory increases in serum 1α,25(OH) 2 D 3 concentrations seen in hypophosphatemic states or to directly inhibit 25-hydroxyvitamin D 1α-hydroxylase activity.
Parathyroid hormone, by virtue of its phosphaturic effect in the kidney, decreases overall phosphate retention, whereas 1α,25(OH) 2 D 3 increases phosphate retention by enhancing the efficiency of phosphorus absorption in the intestine and in the kidney. It should be noted that parathyroid hormone has two opposing effects. As noted previously, parathyroid hormone increases urinary phosphate excretion. At the same time, it also increases the synthesis of 1α,25(OH) 2 D 3 by stimulating the activity of the 1α-hydroxylase enzyme in the kidney. In turn, 1α,25(OH) 2 D 3 increases the efficiency of phosphorus absorption in the intestine and kidney. The phosphatonins, in contrast, increase renal phosphate excretion and inhibit 25-hydroxyvitamin D 1α-hydroxylase activity, thereby further decreasing the retention of phosphorus.
Figure 69.2 shows the physiological changes known to occur with low or high dietary intakes of phosphate. A decrease in serum phosphate concentrations results in increased ionized calcium concentrations, decreased parathyroid hormone secretion, and a subsequent decrease in renal phosphate excretion. At the same time, by parathyroid hormone-independent mechanisms, there is an increase in renal 25-hydroxyvitamin D 1α-hydroxylase activity, increased 1α,25(OH) 2 D 3 synthesis, and increased phosphorus absorption in the intestine and reabsorption in the kidney. Conversely, with elevated phosphate intake, there are decreased calcium concentrations, increased parathyroid hormone release from the parathyroid gland, and increased renal phosphate excretion. Increased serum phosphate concentrations inhibit renal 25-hydroxyvitamin D 1α-hydroxylase and decrease 1α,25(OH) 2 D 3 synthesis. Reduced 1α,25(OH) 2 D 3 concentrations decrease intestinal phosphorus absorption as well as renal phosphate reabsorption. All of these factors tend to bring serum phosphate concentrations back into the normal range.
The Physiology of Phosphate in the Kidney
In conditions of phosphate deprivation, the kidney rapidly increases tubular phosphate reabsorption and reduces urinary phosphate excretion to negligible levels in order to preserve phosphate balance. In infants and children, phosphate reabsorption is high so as to maintain a positive phosphate balance required for growth. Conversely, decreased phosphate reabsorption has been demonstrated in the elderly. Phosphate is freely filtered at the glomerulus. Under conditions of normal dietary phosphate intake, and in the presence of intact parathyroid glands, approximately 20% of the filtered phosphate load is excreted. The other 80% of the filtered load of phosphate is reabsorbed by the renal tubules.
The proximal tubules are the major sites of phosphate reabsorption along the nephron. There is little phosphate reabsorption between the late proximal tubule and the early distal tubule in animals with intact parathyroid glands. However, in the absence of parathyroid hormone, phosphate is avidly reabsorbed between the late proximal tubule and early distal tubule, reflecting phosphate reabsorption by the proximal straight tubule ( Fig. 69.3 ). Phosphate transport rates are approximately three times higher in the proximal convoluted than in the proximal straight tubules. Renal phosphate handling is characterized by intranephronal heterogeneity, reflecting segmental differences in phosphate handling within an individual nephron as well as internephronal heterogeneity.
The uptake of phosphate is mediated by sodium-phosphate cotransporters that are located at the apical border of proximal tubule cells (NaPi IIa and NaPi IIc). The structure and physiology of these phosphate transport molecules have been extensively reviewed, and the reader is directed to other publications in this regard. The sodium-phosphate cotransporters are highly homologous and are predicted to have similar structures. Mice with ablation of the NaPi IIa gene exhibit renal phosphate wasting, and it is estimated that the NaPi IIa transporter is responsible for approximately 85% of proximal tubular phosphate transport which contributes to the adaptive increase in tubular phosphate transport in animals fed a low-phosphate diet ( Fig. 69.4 ).
Factors Regulating Renal Phosphate Excretion
Dietary Phosphate and Renal Phosphate Reabsorption
The influence of dietary phosphate intake on the urinary excretion of phosphate has been known for many years. The reabsorption of phosphate is decreased in animals fed a high-phosphate diet, whereas animals with a low intake of phosphate reabsorb almost 100% of the filtered load of phosphate. These changes in phosphate reabsorption are associated with parallel changes in the abundance of NaPi IIa and IIc transporters. The dietary intake of phosphate can differ considerably depending on the ingestion of foods containing varying amounts of phosphate.
Although dietary phosphate deprivation results in marked changes in the plasma concentrations of several hormones ( Fig. 69.2 ) that contribute to the increases in phosphate reabsorption, the enhanced tubular reabsorption can also be demonstrated independent of changes in these hormones. The mechanism of upregulation of Na/Pi cotransport in OK cells by low-Pi media involves two regulatory mechanisms: an immediate (early) increase (after two hours) in the expression of Na/Pi cotransporter, independent of mRNA synthesis or stability, and a delayed (late) effect (after 4–6 hours), resulting in an increase in NaPi-4 mRNA abundance ( Fig. 69.5 ). Although the changes in phosphate reabsorption in response to a low- or high-phosphate meal are demonstrable within two hours, there are not always concomitant alterations in plasma phosphate concentrations. Thus, the sensing mechanism that initiates the renal adaptations in phosphate reabsorption to the changes in phosphate intake is speculative. The enhanced phosphate reabsorption of short-term phosphate deprivation has been linked to decreased intrarenal synthesis of dopamine and/or stimulation of beta adrenoreceptors, since infusion of dopamine or propranolol restores the phosphaturic response to PTH in short-term (less than three days) phosphate deprivation. The concept of central control of phosphate homeostasis was suggested since decreased dietary phosphate intake upregulated the NaPi IIa expression in the brain and increased phosphate intake downregulated the expression of NaPi IIa in the brain. In this study, increasing cerebrospinal fluid phosphate concentrations in the presence of low plasma phosphate concentrations reversed the adaptations to feeding a low-phosphate diet, suggesting that the phosphate concentration in the brain regulates not only central but also renal expression of NaPi IIa transporters. Studies using cultured renal proximal tubular cells provide persuasive evidence of an intrinsic ability of these cultured cells to increase phosphate transport when exposed to a low phosphate concentration in the medium. In addition to the factors that play a role in enhancing or decreasing phosphate reabsorption in the proximal nephron in response to changes in dietary phosphate noted previously, it should be remembered that alterations in serum phosphate concentrations also alter 1α,25(OH) 2 D 3 synthesis and serum concentrations. Infusions of 1α,25(OH) 2 D 3 increase the renal reabsorption phosphate, predominantly in the proximal nephron. Since the proximal tubule is a major site of phosphate reabsorption, it is the primary site for the tubular adaptation to changes in dietary phosphate intake. The enhanced phosphate reabsorption along the nephron during phosphate deprivation in specific nephron subsegments is dependent on the length and severity of phosphate deprivation.
Parathyroid Hormone and Renal Phosphate Reabsorption
Plasma-ionized calcium levels are a critical determinant of PTH secretion. A fall in plasma-ionized calcium increases PTH secretion and an elevation of plasma-ionized calcium above normal levels decreases PTH secretion. Parathyroidectomy decreases renal phosphate excretion and, conversely, injection of PTH increases urinary phosphate excretion. Micropuncture studies show that PTH decreases and parathyroidectomy increases phosphate reabsorption along the proximal tubule ( Fig. 69.3 ). The proximal straight tubule is an important site of PTH modulation of phosphate transport and may be critical in the final regulation of phosphate excretion. Parathyroid hormone maintains phosphate homeostasis as a result of its regulation of the sodium phosphate cotransporters in the kidney. Renal sodium-phosphate cotransporters are reduced in number along the apical borders of proximal tubular cells following the administration of parathyroid hormone 1 through 34 but not by the administration of parathyroid hormone 3 through 34. The renal sodium-phosphate cotransporters NaPi IIa have been shown to be internalized and degraded within the lysosomes. Disruption of the NaPi IIa gene in mice resulted in increased excretion of phosphate compared to wildtype mice and a resistance to the phosphaturic response to PTH, although the cyclic adenosine monophosphate (cAMP) response is normal ( Fig. 69.4 ). Under conditions where the phosphaturic effect of PTH is blunted or absent, such as short-term phosphate deprivation or acute respiratory alkalosis, the inhibitory effect of PTH on phosphate reabsorption by the proximal convoluted tubule remains intact. However, this increased delivery of phosphate is blunted by enhanced reabsorption by the proximal straight tubule. These studies suggest that the regulation of phosphate reabsorption by PTH in the proximal convoluted and proximal straight tubule subsegments may be mediated by different mechanisms. It should be noted that parathyroid hormone has two opposing effects. As noted previously, parathyroid hormone increases urinary phosphate excretion. At the same time, it also increases the synthesis of 1α,25(OH) 2 D 3 by stimulating the activity of the 25-hydroxyvitamin D 3 1α-hydroxylase enzyme in the kidney.
Vitamin D and Renal Phosphate Reabsorption
A complex interrelationship exists between vitamin D and PTH. Both hormones play important roles in calcium and phosphate regulation. Decreases in plasma ionized calcium levels increase PTH levels and PTH also stimulates the renal conversion of 25(OH) 2 D 3 to 1,25(OH) 2 D 3 by the 25-hydroxyvitamin D 3 1α-hydroxylase located in the proximal tubule of the kidney. Dietary phosphate deprivation or hypophosphatemia induces 25-hydroxyvitamin D 3 1α-hydroxylase. Mice or rats, but not pigs, fed a low-phosphate diet show a decrease in the activity of the 25-hydroxyvitamin D 3 24-hydroxylase (a renal enzyme involved in the catabolism of 1,25(OH) 2 D 3 ) compared with rats fed a normal phosphate diet within 24 hours of phosphate restriction. Vitamin D modestly decreases renal phosphate excretion, and its primary effect is to enhance phosphate transport in the intestine. Vitamin D receptor (VDR)-mutant mice exhibit decreased serum phosphate, however, phosphate transport by renal cortical brush border membranes, phosphate excretion or NaPi IIa or NaPi IIc mRNA levels were not different between VDR-null or wildtype mice, while NaPi IIa protein expression and NaPi IIa cotransporter immunoreactive signals were slightly but significantly decreased in the VDR −/− mice compared with the wildtype mice. When VDR knockout mice were fed a low-phosphate diet, serum phosphate concentrations were more markedly decreased in the VDR knockout mice than in the wildtype mice. Other studies performed in vitamin D receptor and 25-hydroxyvitamin D 1α-hydroxylase null mutant mice show that both these knockout mice adapt to phosphate deprivation with increased NaPi IIa protein in a manner similar to that found in wildtype mice. However, when these mice were fed a high-phosphate diet, phosphate excretion was less in the vitamin D receptor and 25-hydroxyvitamin D 1α-hydroxylase null mutant mice compared to the wildtype mice. In vitamin D-deprived rats, NaPi IIa transporter protein and mRNA were reported to be decreased in juxtamedullary but not superficial renal cortical tubules compared with normal rats.
Insulin, Growth Hormone, Insulin-Like Growth Factor, and Renal Phosphate Reabsorption
Insulin decreases plasma phosphate and phosphate excretion in human and animal models. This enhanced renal phosphate reabsorption can be demonstrated in the absence of changes in blood glucose, PTH, and phosphate levels or urinary sodium excretion. Initial micropuncture studies by DeFronzo et al. demonstrate enhanced phosphate reabsorption in hyperinsulinemic dogs. Conversely, somatostatin infusion decreases plasma insulin levels and increases phosphate excretion. Growth hormone decreases phosphate excretion and has been postulated to contribute to increased phosphate reabsorption and positive phosphate balance demonstrated in growing animals. Administration of a growth hormone antagonist for 4 days to immature rats suppressed growth in these rats and was associated with increased phosphate excretion and a decreased transport capacity for phosphate reabsorption. Subsequent studies performed in juvenile rats in which growth hormone was suppressed showed increase phosphate excretion to levels comparable to adult rats as a result of decreased NaPi IIa expression, demonstrating the important role for growth hormone in the enhanced phosphate reabsorption in developing animals. Hammerman et al. demonstrated that growth hormone administration increased phosphate uptake by brush border membrane vesicles prepared from kidneys of adult dogs. These effects of growth hormone on phosphate reabsorption may also be due to insulin-like growth factor-1 (IGF-1). Growth hormone stimulates the renal synthesis and release of IGF-1. The addition of IGF-1 to cultured renal opossum kidney cells stimulates sodium-dependent phosphate transport. A selective increase in sodium-dependent phosphate uptake was detectable after 15 minutes and is maximal at five hours. Chronic administration of IGF-1 infused by osmotic mini-pump for six days significantly increased the maximal tubular reabsorption of phosphate in the presence and absence of PTH and enhanced phosphate transport by renal brush border membranes.
Renal Nerves, Catecholamines, Dopamine, and Serotonin
Acute renal denervation increases urinary phosphate excretion independent of parathyroid hormone. Numerous studies have demonstrated that acute renal denervation or the administration of catecholamines alters phosphate reabsorption. The increase in urinary phosphate excretion after acute renal denervation could be due to both increased production of dopamine and decreased α- or β-adrenoreceptor activity, since acute renal denervation has been shown to initially increase renal dopamine excretion and almost completely abolish norepinephrine and epinephrine levels in the kidney. Epinephrine decreases plasma phosphate, presumably by shifting phosphate from the extracellular into the intracellular space. The hypophosphatemic response to isoproteronol infusion is blocked by propranolol, suggesting involvement of the beta adrenoreceptors. Infusion of isoproteronol markedly enhances renal phosphate reabsorption in normal rats and in hypophosphatemic mice. The enhanced phosphate reabsorption and attenuated phosphaturic response to PTH observed in acute respiratory alkalosis and phosphate deprivation is blocked by infusion of propranolol, suggesting a possible role for stimulation of β-adrenoreceptors in these conditions. Stimulation of α-adrenoreceptors by the addition of epinephrine to cultured opossum kidney cells blunts the PTH-induced increase in cAMP levels and the inhibition of phosphate transport. Stimulation of α2-adrenoreceptors in vivo has also been demonstrated to attenuate the phosphaturic response to PTH. Dopamine infusion and the infusion of L-DOPA or glupopa, dopamine precursors, increase phosphate excretion in the absence of PTH. Dopamine administration has been reported to decrease phosphate transport in cultured opossum kidney cells and rabbit proximal straight tubules. Studies suggest that dopamine may be a proximal tubular paracrine substance in the regulation of phosphate reabsorption. The enzyme that converts L-DOPA to dopamine is located exclusively in the proximal convoluted and straight tubules, also the primary sites of phosphate reabsorption. Increasing dietary phosphate intake increases urinary dopamine excretion and phosphate excretion. Inhibition of endogenous dopamine synthesis by the administration of carbidopa to rats resulted in decreased dopamine and phosphate excretion, suggesting a role for endogenous dopamine in phosphate regulation. A potential paracrine role for dopamine in phosphate regulation was strengthened by studies performed in opossum kidney cells that demonstrated that the addition of dopamine or L-DOPA selectively decreased phosphate uptake ( Fig. 69.6 ). Furthermore, phosphate-replete OK cells produced more dopamine from L-DOPA than phosphate-deprived cells. Administration of dopamine to phosphate-deprived or respiratory alkalotic rats increases phosphate excretion and enhances the phosphaturic response to PTH. Subsequent studies in opossum kidney cells performed by several laboratories demonstrated that increasing dopamine synthesis inhibits phosphate transport by multiple mechanisms including activation of DA1 and DA2 receptors. More recent studies performed using mouse kidney slices, perfused proximal tubules, and opossum kidney cells examined the effect of dopamine on NaPi IIa expression and localization using DA1 and DA2 agonists. In these studies, dopamine induced the internalization of NaPi IIa by activation of luminal DA1 receptors. Renal proximal tubules also synthesize serotonin from 5-hydroxytryptophan by the same enzyme that converts L-DOPA to dopamine. Incubation of opossum kidney cells with either serotonin or 5-hydroxytryptophan enhanced phosphate transport and raises the possibility that serotonin may also be involved in the physiological regulation of renal phosphate transport.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here