Angiotensin II Modulates Calcium/Phosphate Excretion in Experimental Model of Hypertension: Focus on Bone

Abstract

A link between hypertension and long-term bone health has been suggested. The aim of this study was to investigate the effects of chronic angiotensin II administration on urinary calcium/phosphate excretion, bone mineral density, bone remodeling and osteoblast population in a well-established experimental model of hypertension, in the absence of possible confounding factors that could affect bone metabolism. Male Sprague-Dawley rats, divided in the following groups: (a) Angiotensin II (Ang II, 200 ng/kg/min, osmotic minipumps, sub cutis, n = 8); (b) Ang II+losartan (Los, 50 mg/kg/day, per os, n = 6); (c) control group (physiological saline, sub cutis, n = 9); and (d) control+losartan (n = 6) were treated for four weeks. During the experimental period, 24-hour diuresis, urinary calcium, phosphate and sodium excretion were measured prior to the treatment, at two weeks of treatment, and at the end of the treatment. Systolic blood pressure was measured by plethysmography technique (tail cuff method). At the end of the experimental protocol, the rats were euthanized and peripheral quantitative computed tomography at the proximal metaphysis and at the diaphysis of the tibiae and quantitative bone histomorphometry on distal femora were performed. Angiotensin II-dependent hypertension is associated with increased calcium and phosphate excretion. AT1 receptor blockade prevented the increase of blood pressure and phosphate excretion but did not affect the increase of calcium excretion. These changes took place without significantly affecting bone density, bone histology or osteoblast population. In conclusion, in our experimental conditions, angiotensin II-dependent hypertension gave rise to an increased urinary excretion of calcium and phosphate without affecting bone density.

Keywords: angiotensin II; bone; experimental hypertension; rats.

Figure 1

Effects of Ang II and losartan administration on 24 h diuresis and urinary sodium excretion before and after two and four weeks of different treatments. Data are means ± SEM. * = p < 0.05; § = p < 0.01.

Figure 2

Effects of Ang II and losartan administration on 24 h urinary calcium and phosphate excretion before and after two and four weeks of different treatments. Data are means ± SEM. * = p < 0.05; § = p < 0.01.

Figure 3

Representative image of the metaphyseal site of the rat tibiae analyzed by pQCT. The bar on the left is the colour scale bar of the density (mg/cm³): white > 700 mg/cm³; grey ≤ 100 mg/cm³.

Figure 4

Effects of Ang II and Losartan administration on bone tissue. (a) Representative images from H&E-stained sections showing osteoblasts (arrow) and (b) quantification of osteoblast parameters in the different groups of rats. Data are means ± SEM. N.Ob: Osteoblast number; BS: bone surface; Ob.S: osteoblast surface; BS: bone surface. Ang II: Angiotensin II; Los: losartan.

Figure 5

Effects of Ang II and losartan administration on bone tissue. (a) Representative histological pictures from Sirius red-stained bone tissue sections (trabecular bone stained in red) and (b) quantification of femoral trabecular bone in the different groups of rats. BV: Trabecular bone volume; TV: tissue volume. Data are means ± SEM. Ang II: Angiotensin II; Los: losartan.