Uremia-related vascular calcification: More than apatite deposition

Abstract

In the present study, we characterized and compared the mineral phase deposited in the aortic wall of two different frequently used chronic renal failure rat models of vascular calcification. Vascular calcification was induced in rats by either a 4-week adenine treatment followed by a 10-week high-phosphate diet or 5/6 nephrectomy followed by 6 weeks of 0.25 microg/kg/day calcitriol treatment and a high-phosphate diet. Multi-element mapping for calcium and phosphate together with mineral identification was performed on several regions of aortic sections by means of synchrotron X-ray-mu-fluorescence and diffraction. Bulk calcium and magnesium content of the aorta was assessed using flame atomic absorption spectrometry. Based on the diffraction data the Von Kossa-positive precipitate in the aortic regions (N=38) could be classified into three groups: (1) amorphous precipitate (absence of any diffraction peak pattern, N=12); (2) apatite (N=16); (3) a combination of apatite and magnesium-containing whitlockite (N=10). The occurrence of these precipitates differed significantly between the two models. Furthermore, the combination of apatite and whitlockite was exclusively found in the calcitriol-treated animals. These data indicate that in adenine/phosphate-induced uremia-related vascular calcification, apatite is the main component of the mineral phase. The presence of magnesium-containing whitlockite found in addition to apatite in the vitamin D-treated rats, has to be seen in view of the well-known vitamin D-stimulated gastrointestinal absorption of magnesium.

Ultrastructural Analysis of Vascular Calcifications in Uremia

“…With synchrotron radiation analysis, however, because of the combination of micrometer resolution and short acquisition times (a few seconds), a more systematic scanning of the sample covering a larger area can be obtained, which is a major advantage of this technique-albeit at the cost of a lower resolution when compared with high-resolution TEM techniques. To the best of our knowledge, only Verberckmoes et al 10 applied synchrotron radiation analysis to examine vascular calcifications in rats with uremia, in which they also identified whitlockite as an important component of the mineral phase, particularly in vitamin D-treated animals. No studies using synchrotron radiation for diffraction analysis of human vascular calcification have been reported so far.…”

“…These previous studies used different tissues and yielded different results: Hydroxyapatite [Ca 10 (PO 4 ) 6 OH 2 ]-as seen in the skeleton-was described as the sole mineral phase of uremic arterial calcifications in two studies, 7,8 whereas another study found both brushite and hydroxyapatite in calcifications of stenotic arteriovenous fistulas. 9 In a rodent study of uremic calcification, whitlockite, a magnesium-containing crystal [(Ca,Mg) 3 (PO 4 ) 2 ], was detected in addition to hydroxyapatite.…”

“…9 In a rodent study of uremic calcification, whitlockite, a magnesium-containing crystal [(Ca,Mg) 3 (PO 4 ) 2 ], was detected in addition to hydroxyapatite. 10 Different mineral phases arising during uremic medial calcification may potentially be the result of different pathomechanisms ultimately affecting dissolution of the calcifications and the choice of therapy. We therefore performed a systematic ultrastructural analysis of uremic media calcifications in patients with ESRD.…”

Chronic kidney disease is a risk factor for cardiovascular disease

“…In addition, the accompanying The increase in phosphate levels predisposes to the precipitation of calcium phosphate crystals in the media of the walls of the vessels. (18,19) In an elegant study on the rat model, Verberckmoes et al found calcium phosphate and whitlockite crystals in the aortas of uraemic animals.…”

Persistence of Vascular Calcification after Reversal of Uremia

“…16 However, we did not observe significant reversal of calcification after aortic transplantation in an animal model of this disorder. 17 Vascular calcifications are composed primarily of apatitic mineral with varying amounts of whitlockite, 18,19 both of which are highly insoluble under physiological conditions 20 and thus are not expected to disappear spontaneously. Apatite appears not to form spontaneously from calcium and phosphate ions, whereas initially brushite (CaHPO 4 2H 2 O) and/or other intermediates (including amorphous calcium phosphate) that are more soluble may be present in variable amounts in calcified vessels.…”