**3.2 Vascular calcification**

Phosphate is probably the predominant inducer of vascular calcification, and elevated serum levels are strongly associated with increased vascular calcification and mortality (Goodman et al., 2000). Elevated phosphate triggers a concentration-dependent precipitation of calcium in vascular smooth muscle cells, and phosphate is also a potent stimulus for the differentiation of vascular smooth muscle cells. In vitro studies demonstrate that high phosphate levels in incubation media enhance calcification with associated extracellular matrix synthesis(Jono et al., 2000). Phosphate and sodium dependent phosphate transporter seem to play a very important role in vascular smooth muscle cells mineralization. Type III sodium-dependent phosphate transporter presents two discrete subtypes, Pit-1 and Pit-2. In human vascular smooth muscle cells, Pit-1 is mainly expressed (London et al., 2000). Apatite formation by smooth muscle cells, as a response to increased phosphate levels, is fully inhibited by phosphonoformic acid (PFA), a sodium dependent phosphate transporter inhibitor, a finding supporting the notion that vascular calcification is an active rather than passive cellular process (Giachelli et al., 2001; Ketteler et al., 2003).

Hyperphosphataemia induces osteocalcin and Cbfa-1 in vascular smooth muscle cells and promotes vascular calcification. Animals deficient in Cbfa-1 fail to mineralize bone(Komori et al., 1997), and there is also increased expression of Cbfa-1 when vascular smooth muscle cells are incubated in uremic serum compared with pooled human serum (Moe et al., 2003). There is now considerable evidence that hyperphosphataemia regulates several signalling pathways of cell functions. Of great interest is the recent identification of a novel phosphateregulating gene, klotho (Kuro-o et al., 1997; Yoshida et al., 2002), which in mice is involved in the development of a syndrome resembling human ageing. The klotho mutant mice show abnormal calcium phosphate vitamin D metabolism and develop hyperphosphataemia and vascular calcification (Kuro-o et al., 1997; Yoshida et al., 2002). Hyperphosphataemia also down-regulates klotho gene expression (Fig. 1).

### **3.3 Endothelial dysfunction**

Endothelial dysfunction is the principal cause of atherosclerosis resulting in cardio vascular disease (Ross, 1999). High phosphate loading on endothelial cells inhibited nitrogen oxide (NO) production through increased reactive oxygen species (ROS) production and endothelial NO syntase (eNOS) inactivation via conventional protein kinase C, resulting in impaired endothelium-dependent vasodilation (Shuto et al., 2009). Furthermore, dietary phosphate loading can deteriorate flow-mediated vasodilation in healthy men, suggesting

sudden death. After percutaneous and surgical coronary revascularization, dialysis patients are still remaining at a high risk for sudden cardiac death (Furgeson, 2008). Hyperphosphatemia is a known factor contributing to the increased risk of cardiac death both in patients with end-stage renal disease and in those under renal replacement treatment with dialysis (Goodman et al., 2000). In patients with renal disease, in fact, the well-known relationship between hyperphosphataemia, secondary hyperparathyroidism, bone turnover and extra osseous calcifications has recently been followed by the recognition of a major role played by elevated serum phosphorus levels in the induction of vascular calcification, cardiac interstitial fibrosis and arterial thickening which highly increase the risk of cardiac death (Goodman et al., 2000; Block & Port, 2000; Amann et al., 2003;

Phosphate is probably the predominant inducer of vascular calcification, and elevated serum levels are strongly associated with increased vascular calcification and mortality (Goodman et al., 2000). Elevated phosphate triggers a concentration-dependent precipitation of calcium in vascular smooth muscle cells, and phosphate is also a potent stimulus for the differentiation of vascular smooth muscle cells. In vitro studies demonstrate that high phosphate levels in incubation media enhance calcification with associated extracellular matrix synthesis(Jono et al., 2000). Phosphate and sodium dependent phosphate transporter seem to play a very important role in vascular smooth muscle cells mineralization. Type III sodium-dependent phosphate transporter presents two discrete subtypes, Pit-1 and Pit-2. In human vascular smooth muscle cells, Pit-1 is mainly expressed (London et al., 2000). Apatite formation by smooth muscle cells, as a response to increased phosphate levels, is fully inhibited by phosphonoformic acid (PFA), a sodium dependent phosphate transporter inhibitor, a finding supporting the notion that vascular calcification is an active rather than

Hyperphosphataemia induces osteocalcin and Cbfa-1 in vascular smooth muscle cells and promotes vascular calcification. Animals deficient in Cbfa-1 fail to mineralize bone(Komori et al., 1997), and there is also increased expression of Cbfa-1 when vascular smooth muscle cells are incubated in uremic serum compared with pooled human serum (Moe et al., 2003). There is now considerable evidence that hyperphosphataemia regulates several signalling pathways of cell functions. Of great interest is the recent identification of a novel phosphateregulating gene, klotho (Kuro-o et al., 1997; Yoshida et al., 2002), which in mice is involved in the development of a syndrome resembling human ageing. The klotho mutant mice show abnormal calcium phosphate vitamin D metabolism and develop hyperphosphataemia and vascular calcification (Kuro-o et al., 1997; Yoshida et al., 2002). Hyperphosphataemia also

Endothelial dysfunction is the principal cause of atherosclerosis resulting in cardio vascular disease (Ross, 1999). High phosphate loading on endothelial cells inhibited nitrogen oxide (NO) production through increased reactive oxygen species (ROS) production and endothelial NO syntase (eNOS) inactivation via conventional protein kinase C, resulting in impaired endothelium-dependent vasodilation (Shuto et al., 2009). Furthermore, dietary phosphate loading can deteriorate flow-mediated vasodilation in healthy men, suggesting

Goldsmith et al., 2004; Floege & Ketteler, 2004).

passive cellular process (Giachelli et al., 2001; Ketteler et al., 2003).

down-regulates klotho gene expression (Fig. 1).

**3.3 Endothelial dysfunction** 

**3.2 Vascular calcification** 

that dietary phosphate loading or elevation of serum phosphorus level may be a risk factor for cardiovascular disease in healthy persons as well as CKD patients (Takeda et al., 2006; Shuto et al., 2009). Di Marco et al. also reported that high phosphate loading increased ROS production via phosphate influx and induced apoptosis in endothelial cells (Di Marco et al., 2008). Association of serum phosphorus level and vascular dysfunction has been well investigated, because fasting serum phosphorus level could not increase in healthy persons, even if dietary phosphate was overloaded. However, postprandial phosphorus elevation was associated with %FMD in young healthy men (Shuto et al., 2009). Thus, dietary phosphate loading can cause endothelial dysfunction within a short time. Oxidative stress and decreased NO production in endothelial cells are possible mechanisms for the impaired endothelial function mediated by phosphate loading (Fig. 2).

Fig. 2. Dual pathways for vascular dysfunction caused by hyperphosphatemia

### **3.4 Arterial stiffness**

Arterial disease observed in end-stage kidney disease patients is characterized by extensive intimal as well as medial calcification. Histological changes in coronary arteries from dialysis patients, compared with age matched controls, reveal a similar magnitude of atherosclerotic plaque burden and intimal thickness but markedly increased medial calcification (arteriolosclerosis) (Schwarz et al., 2000). Medial calcification has been shown to affect vascular elasticity and leads to increased arterial wall stiffness of large capacity, elastic-type arteries like the aorta and the common carotid artery, increased pulse pressure

Complications and Managements of Hyperphosphatemia in Dialysis 321

mg/day. Net phosphorus absorption averages 60% to 70% of intake (Delmez & Slatopolsky, 1992; Sheikh et al., 1989), however, this percentage can rise as high as 86% of ingested phosphate with calcitriol use and decrease to 30% to 40% of ingested phosphate with

Other foods that are high in phosphate are processed foods such as processed meats which have phosphate based additives to improve the consistency and appearance of the food. Since 1990, intake of phosphate from additives has doubled and has been 1,000 mg in USA (Calvo & Park, 1996). This is the amount that some renal patients are advised for the whole day from all food groups (James & Jackson, 2002). As people are becoming more reliant on processed and packaged meals due to convenience, phosphate from these sources needs to be considered when advising on diet. Fresh meat is considered suitable for someone following a phosphate restriction, however processed foods may in fact be providing much more phosphate than realised (Sullivan et al., 2007). Beverages such as sodas, juices and sport drinks also contain phosphate additives (Murphy-Gutekunst, 2007). It has been estimated that for a person on hemodialysis the average phosphate removal per day is 300 mg (Vaithilingham et al., 2004). This leaves the patient with a positive balance for

The clearance of phosphate varies among the different modalities of dialysis. Ideally, adequate dialysis in any form would remove adequate amounts of all uremic toxins, including phosphate. Unfortunately, conventional thrice-weekly hemodialysis (4 h duration) removes approximately 900 mg of phosphorus each treatment (an average of only 300 mg/day) (Gotch et al., 2003). Increasing the dosage of dialysis, preferably to lengthy three times per week dialysis, hemodiafiltration, or, even better, daily/nightly dialysis may prevent phosphorus retention and even require no dietary phosphate restriction or the withdrawal of phosphate binders (Maduell et al., 2003; Benaroia et al., 2008). However, regular dialysis treatment is not able to remove all the phosphorus ingested with a diet

Isakova et al analyzed a prospective cohort study of 10,044 incident hemodialysis patients at Fresenius Medical Care facilities in 2004 and 2005 comparing 1-year all-cause mortality among patients who were treated with phosphate binders (Isakova et al., 2009). In an intention-to-treat analysis, they compared patients who began treatment with any phosphate binder during the first 90 days after initiating hemodialysis, with those who remained untreated during that period. Treatment with phosphate binders was independently associated with decreased mortality compared with no treatment. In the unmatched cohort, the phosphate binder-treated group had a relative risk reduction of 42%, while in the intention-to-treat and as-treated analyses, the magnitude of the survival benefit ranged between 18% and 30% in multivariate models. The association between use of phosphorus binders and survival was observed within each quartile of baseline serum phosphorus except the lowest. Results from human data suggest that lowering of phosphorus levels by intake of phosphate binders will substantially reduce serum FGF-23 levels (Koiwa et al., 2005; Pande et al., 2006). In this prospective observational study, treatment with phosphate binders was associated with a reduced 1-year mortality among

optimal binder usage.(Sheikh et al., 1989; Delmez & Slatopolsky, 1992).

containing protein of 1.0 - 1.2 g/kg/day (Mallick & Gokal, 1999).

incident hemodialysis patients (Isakova et al., 2009).

phosphate.

**4.3 Hemodialysis** 

**4.4 Phosphate binders** 

and decreased perfusion of coronary arteries during diastole (Blacher et al., 1998; London, 2003; Speer & Giachelli, 2004). Recent studies also demonstrated that elevated FGF23 levels were associated with arterial stiffness, increased left ventricular mass index and increased prevalence of left ventricular hypertrophy in patients with CKD (Hsu & Wu, 2009; Mirza et al., 2009; Gutierrez et al., 2009).
