**3.1.3 Bone volume**

Bone volume contributes to bone fragility and is separate from the other parameters. The bone volume is the end result of changes in bone-formation and resorption rates: if the overall bone formation rate is higher than the overall bone resorption rate, the bone is in positive balance and the bone volume will increase. If mineralization remains constant, an increase in bone volume would also result in an increase in BMD and should be detectable by dual-energy X-ray absorptiometry (DXA). Although both cortical and cancellous bone volumes decrease in typical idiopathic osteoporosis, these compartments are frequently different in patients with CKD. In dialysis patients with high PTH levels, the cortical bone volume is decreased but the cancellous volume is increased. (Lindergard et al., 1985).

#### **3.2 Bone markers**

Generally, two different types of bone markers are used to determine the bone patophysiology:

The New Kidney and Bone Disease:

**4. CKD – MBD – and vascular calcification** 

**4.1 Different types of vascular calcification** 

renal transplantation (Amann K., 2008;).

**4.2 Promoters and inhibitors of calcification** 

K et al. 2004).

Vliegenthart et al. 2002).

Chronic Kidney Disease – Mineral and Bone Disorder (CKD–MBD) 33

compensation to stabilize serum phosphate levels as the number of intact nephrons declines. As a result, FGF-23 increases urinary phosphate excretion and decreases gastrointestinal phosphate absorption directly and through inhibition of 1a-hydroxylase and reduction of circulating calcitriol levels indirectly. Oversecretion of FGF-23 allows the body to maintain phosphate levels within a 'physiological' range until very advanced CKD stages (Miyamoto

Tissue calcification is a complex and highly regulated process in bone and teeth, and also at extraosseous sites. The most threatening localization of unwanted calcification is at vascular sites, where it may manifest as both medial and intimal calcification of arteries. Studies in the general population have identified calcification in most of atherosclerotic plaques. Calcification seems to be a part of the natural development of atherosclerotic plaques, with extensive calcification associated with late-stage atherosclerosis. In the general population, atherosclerotic plaque calcification is associated with cardiovascular events such as myocardial infarction, symptomatic angina pectoris, and stroke. Medial calcification causes arterial stiffness, resulting in an elevated pulse pressure and increased pulse wave velocity, thereby contributing to left ventricular hypertrophy, dysfunction, and heart failure. Furthermore, an advanced calcification of the heart valves may lead to dysfunction contributing to heart failure and a risk of endocarditis development (Vliegenthart et al. 2002;

It is generally well recognized that the prevalence of calcification increases with progressively decreasing kidney function and is greater than that in the general population. Cardiovascular calcification is associated with increased frequency of major cardiovascular diseases, and could be of predictive importance for adverse clinical outcomes, including cardiovascular events and death (Foley RN et al., 1998). There is an increased prevalence of cardiovascular calcification in patients even at early stages of CKD. Thus, an important percentage of CKD patients are at high risk of cardiovascular events from vascular calcification. Two patterns of vascular calcification have been described: namely intimal and medial calcification. In the general population, an elevated coronary artery calcium (CAC) score almost exclusively reflects the atherosclerotic disease burden. In two small autopsy studies, it became apparent that, in dialysis patients, CAC is also predominantly localized in the coronary intima, whereas the medial calcifications observed in a minority of such patients seemed to be adjacent to plaque areas just beneath the internal elastic lamina. Although the coronary vascular bed may differ considerably from other arteries with regard to the calcification process and its manifestations, the same group observed a 'pure' medial calcification in the coronary arteries during the early stages of CKD (Schwarz et al., 2000). A 'pure' medial calcification, in the absence of intimal disease, was also observed in epigastric arteries obtained from dialysis patients at the time of

Vascular calcification is the result of passive and active processes, as is bone mineralization. It has been shown that that normal extracellular phosphate concentration is required for

#### **3.2.1 Collagen based bone markers**

Active osteoblasts secrete pro-collagen type I, and the pro-peptides at both C-terminal and N-terminal ends are immediately cleaved and can be measured in the circulation. The collagen molecules are then covalently bonded through pyridinoline cross-linking. The fragments containing these pyridinoline links (at both the C-terminal and N-terminal ends of the peptides) are released during bone resorption: carboxyterminal (CTX) and aminoterminal (NTX) cross-linking telopeptide of bone collagen, respectively. These collagen-based markers have been studied in normal populations, where there are significant but moderate correlations with bone-formation/resorption rates. These markers are usually increased after bone fracture (Ureña and De Vernejoul, 1999; Ivaska et al., 2007).

#### **3.2.2 Non collagen type of bone markers**

Osteoblasts secrete other proteins that have been used to assess their function, including b-ALP, osteocalcin, osteoprotegerin, and receptor activator for nuclear factor kB ligand. Osteoclasts secrete tartrate-resistant acid phosphatase. Osteocytes secrete FGF-23 in response to phosphate and calcitriol. High levels of FGF-23 are seen in patients with CKD, but this is a new measurement, and clinical significance remains to be determined. Some of these markers are excreted by the kidneys, so in CKD, the serum concentrations may merely represent accumulation instead of bone turnover (Rogers and Eastell, 2005).

Renal phosphate excretion is physiologically regulated mainly by proximal tubular cells, which express Na/Pi Type II cotransporters at their apical membrane that control phosphate reclamation. Renal phosphate reabsorption is mediated primarily through the Na/Pi IIa co-transporter, whereas approximately one-third of phosphate ions are reabsorbed through the Na/Pi IIc cotransporter. FGF-23 mediates its phosphaturic effect by reducing the abundance of the Na/Pi IIa cotransporter in proximal tubular cells (Baum et al., 2005). In animal studies, transgenic mice over-expressing human or mouse FGF-23 have severe renal phosphate wasting because of suppression of renal Na/Pi cotransporter activity, whereas FGF-23 inactivation leads to hyperphosphatemia (Liu et al., 2006). In addition, FGF-23 may inhibit gastrointestinal phosphate absorption by reducing intestinal Na/Pi IIb cotransporter activity in a vitamin D dependent manner (Liu et al., 2006). In CKD patients, circulating FGF-23 levels gradually increase with renal function declining. Although the increase in FGF-23 is most pronounced in patients with advanced CKD, it may begin at a very early stage. Apparently, FGF-23 and PTH stimulate phosphaturia in a similar manner by reducing phosphate reclamation through Na/Pi IIa cotransporters. Nonetheless, PTH is not indispensable for FGF-23 activity, as the phosphaturic effects of FGF-23 are maintained in animals after parathyroidectomy (Liu et al., 2006). In CKD patients, the increase in FGF-23 starts with modestly impaired estimated glomerular filtration rate, when serum phosphate levels are still within the normal range CKD (KDOQI stages 2–3), whereas FGF-23 levels increase by more than 100-fold in advanced CKD (KDOQI stage 5) compared with healthy controls (Imanishi et al., 2004). However, this is inconsistent with the observation that there is no increase in the accumulation of degraded FGF-23 in advanced CKD. These data instead favor a mechanism involving increased FGF-23 secretion as the cause of elevated FGF-23 levels. Instead of decreased renal clearance, an end organ resistance to the phosphaturic stimulus of FGF-23 may exist because of a deficiency of the necessary Klotho cofactor (Kurosu et al., 2006). Moreover, higher FGF-23 levels in CKD may reflect a physiological

Active osteoblasts secrete pro-collagen type I, and the pro-peptides at both C-terminal and N-terminal ends are immediately cleaved and can be measured in the circulation. The collagen molecules are then covalently bonded through pyridinoline cross-linking. The fragments containing these pyridinoline links (at both the C-terminal and N-terminal ends of the peptides) are released during bone resorption: carboxyterminal (CTX) and aminoterminal (NTX) cross-linking telopeptide of bone collagen, respectively. These collagen-based markers have been studied in normal populations, where there are significant but moderate correlations with bone-formation/resorption rates. These markers are usually increased after bone fracture (Ureña and De Vernejoul, 1999; Ivaska et al., 2007).

Osteoblasts secrete other proteins that have been used to assess their function, including b-ALP, osteocalcin, osteoprotegerin, and receptor activator for nuclear factor kB ligand. Osteoclasts secrete tartrate-resistant acid phosphatase. Osteocytes secrete FGF-23 in response to phosphate and calcitriol. High levels of FGF-23 are seen in patients with CKD, but this is a new measurement, and clinical significance remains to be determined. Some of these markers are excreted by the kidneys, so in CKD, the serum concentrations may merely

Renal phosphate excretion is physiologically regulated mainly by proximal tubular cells, which express Na/Pi Type II cotransporters at their apical membrane that control phosphate reclamation. Renal phosphate reabsorption is mediated primarily through the Na/Pi IIa co-transporter, whereas approximately one-third of phosphate ions are reabsorbed through the Na/Pi IIc cotransporter. FGF-23 mediates its phosphaturic effect by reducing the abundance of the Na/Pi IIa cotransporter in proximal tubular cells (Baum et al., 2005). In animal studies, transgenic mice over-expressing human or mouse FGF-23 have severe renal phosphate wasting because of suppression of renal Na/Pi cotransporter activity, whereas FGF-23 inactivation leads to hyperphosphatemia (Liu et al., 2006). In addition, FGF-23 may inhibit gastrointestinal phosphate absorption by reducing intestinal Na/Pi IIb cotransporter activity in a vitamin D dependent manner (Liu et al., 2006). In CKD patients, circulating FGF-23 levels gradually increase with renal function declining. Although the increase in FGF-23 is most pronounced in patients with advanced CKD, it may begin at a very early stage. Apparently, FGF-23 and PTH stimulate phosphaturia in a similar manner by reducing phosphate reclamation through Na/Pi IIa cotransporters. Nonetheless, PTH is not indispensable for FGF-23 activity, as the phosphaturic effects of FGF-23 are maintained in animals after parathyroidectomy (Liu et al., 2006). In CKD patients, the increase in FGF-23 starts with modestly impaired estimated glomerular filtration rate, when serum phosphate levels are still within the normal range CKD (KDOQI stages 2–3), whereas FGF-23 levels increase by more than 100-fold in advanced CKD (KDOQI stage 5) compared with healthy controls (Imanishi et al., 2004). However, this is inconsistent with the observation that there is no increase in the accumulation of degraded FGF-23 in advanced CKD. These data instead favor a mechanism involving increased FGF-23 secretion as the cause of elevated FGF-23 levels. Instead of decreased renal clearance, an end organ resistance to the phosphaturic stimulus of FGF-23 may exist because of a deficiency of the necessary Klotho cofactor (Kurosu et al., 2006). Moreover, higher FGF-23 levels in CKD may reflect a physiological

represent accumulation instead of bone turnover (Rogers and Eastell, 2005).

**3.2.1 Collagen based bone markers** 

**3.2.2 Non collagen type of bone markers** 

compensation to stabilize serum phosphate levels as the number of intact nephrons declines. As a result, FGF-23 increases urinary phosphate excretion and decreases gastrointestinal phosphate absorption directly and through inhibition of 1a-hydroxylase and reduction of circulating calcitriol levels indirectly. Oversecretion of FGF-23 allows the body to maintain phosphate levels within a 'physiological' range until very advanced CKD stages (Miyamoto K et al. 2004).
