**1. Introduction**

Epidemiologic data suggest a possible link between arterial hypertension, characterized by cardiovascular and renal complications, and a degenerative disease of the bone, known as osteoporosis [1–4]. Both arterial hypertension and osteoporosis show increased frequency in aging, especially in post-menopausal women, and share the same risk factors, such as low physical activity or smoking. These two diseases have similar pathophysiological mechanisms, such as increased oxidative stress, inflammation, and dysfunctions of calcium homeostasis [5–9].

**Citation:** Castoldi, G.; Carletti, R.; Ippolito, S.; Villa, I.; Palmisano, B.; Bolamperti, S.; Rubinacci, A.; Zerbini, G.; Meani, M.; Zatti, G.; et al. Angiotensin II Modulates Calcium/Phosphate Excretion in Experimental Model of Hypertension: Focus on Bone. *Biomedicines* **2022**, *10*, 2928. https://doi.org/10.3390/ biomedicines10112928

Academic Editors: Ivana Vanˇeˇcková and Josef Zicha

Received: 2 October 2022 Accepted: 8 November 2022 Published: 14 November 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Systemic renin-angiotensin system (RAS) has a fundamental role in the regulation of blood pressure and electrolyte balance [10,11], and Angiotensin II (Ang II) has been historically considered the main effector of RAS. Besides the 'classical' systemic RAS, it has been demonstrated that the components of RAS are expressed in multiple tissues [12]. At the same time, Ang II, in addition to its well-known hemodynamic action, has many cellular effects in different tissues promoting inflammation [13–15], fibrosis [13,16,17], and hypertrophy [17–19], mainly mediated by AT1 receptors.

Bone tissue expresses the components of RAS which are involved in bone remodeling [20–22]. In fact, experimental studies have demonstrated that Ang II increases the activity of osteoclasts, enhancing bone loss in rats affected by osteoporosis caused by estrogen deficiency [23]. A role for bone RAS has been described in age-related osteoporosis in mice [24] and in the reduction of bone formation and in the increase of resorption in obstructive nephropathy [25].

AT1 receptor knockout mice showed an increase in bone mass [26], possibly suggesting a protective role of RAS inhibition in bone. However, the inhibitory effects of captopril on bone RAS components did not improve bone damage caused by type II diabetes in db/db mice [27]. In type 1 diabetes mice (streptozotocin induced) the increased activity of bone RAS was shown to be involved in osteoporosis processes, which was not improved by ACE inhibitors [28]. Finally, in ovariectomized rats, the stimulation of the protective arm of RAS (ACE-2/Ang 1-7/Mas) was shown to mediate the osteo-protective effects of RAS inhibition [21].

These data strongly suggest that the role of RAS in bone remodeling might differ depending on the underlying pathological conditions, such as estrogen deficiency, hypertension, diabetes and osteoporosis.

Nevertheless, the direct effects of angiotensin II dependent hypertension on bone tissue are not completely understood.

In the present study we investigated the effects of chronic angiotensin II administration on urinary calcium/phosphate excretion, bone mineral density, bone remodeling and on osteoblast population—the cells responsible for the synthesis of bone matrix—in a wellestablished experimental model of hypertension, in the absence of possible confounding factors that could affect bone metabolism.

In this study we demonstrate that angiotensin II-dependent hypertension is associated with a modulation of urinary calcium/phosphate excretion over time leading to an increase of 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, and osteoblast population.
