**1. Introduction**

Arterial hypertension is the main risk factor for morbidity and mortality of cerebrovascular and cardiovascular diseases. The number of hypertensive adults aged 30–79 years has almost doubled in the last 30 years to more than 1.2 billion people worldwide, but only 59% of women and 49% of men with hypertension are diagnosed, and another 11–12% are diagnosed but not treated [1]. The problem of inadequate antihypertensive therapy is exacerbated by the patients' inconsistent drug intake and inconsequent blood pressure control by the patients and their therapists. In less than 40% of diagnosed patients, hypertension has been controlled to below 140/90 mmHg [1]. Untreated or insufficiently treated arterial hypertension leads to cardiac hypertrophy and may progress to cardiac remodeling, fibrosis, dilatation, and in the final stages, to heart failure. A long history of undetected and untreated hypertension is a risk factor as high as advanced age, particularly if hypertension has persisted for some time and the first cardiac sequelae may have already occurred. The earlier in the course of pathophysiological development an efficient antihypertensive therapy is started, the more successful it can be in preventing complications [2,3].

**Citation:** Hawlitschek, C.; Brendel, J.; Gabriel, P.; Schierle, K.; Salameh, A.; Zimmer, H.-G.; Rassler, B. How Effective Is a Late-Onset Antihypertensive Treatment? Studies with Captopril as Monotherapy and in Combination with Nifedipine in Old Spontaneously Hypertensive Rats. *Biomedicines* **2022**, *10*, 1964. https://doi.org/10.3390/ biomedicines10081964

Academic Editors: Josef Zicha and Ivana Vanˇeˇcková

Received: 29 June 2022 Accepted: 9 August 2022 Published: 12 August 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/).

Spontaneously hypertensive rats (SHR) are an established animal model for studying the causes and development of hypertension. This is due to their prehypertensive stage and their clinical complication pattern, which is similar to that of humans including LV hypertrophy and dilatation and often resulting in heart failure [4]. Upregulation of renin-angiotensin-aldosterone-system (RAAS)-related key genes and increased intracellular calcium concentration belong to the main factors responsible for the increased blood pressure (BP) in adult SHR [5,6]. Correspondingly, in numerous studies in SHR, profound antihypertensive and cardioprotective effects were achieved by antagonizing the RAAS and reducing intracellular calcium concentration [2,7–9]. The antagonization of angiotensin II (AT II) and the blockade of calcium channels have also proved to be effective antihypertensive therapies in humans and are recommended by the guidelines of the European Societies of Cardiology and Hypertension as preferred treatments of human hypertension [10]. In particular, the combination of two antihypertensive drugs is highly recommended, as better antihypertensive effects can be achieved with lower drug doses, and thus with fewer side effects and better tolerability [10,11].

One of the major problems in human hypertension and its therapy is the fact that hypertension is often diagnosed late in life when the BP has been elevated for many years. Many of those patients present an advanced degree of hypertension, and the first cardiovascular complications have already developed. The therapy has to start immediately with strict control of patients' compliance and BP development. However, even if antihypertensive therapy is well-adjusted and controlled, a late-onset treatment may be less effective than a treatment started in the very early stages of hypertension. Studies in SHR showed that antihypertensive treatment starting in a very early phase of life (between four and 10 weeks of age) reduced blood pressure (BP) even to normotension [3,12] and prevented left ventricular hypertrophy and fibrosis [7,13,14]. In contrast, the same treatment starting at a later stage of life (24 to 30 weeks of age) had a much lower antihypertensive effect [3,15]. For treatment of human hypertension, it is important to clarify whether and to what extent a late-onset antihypertensive therapy is able to significantly reduce BP and, even more importantly, to attenuate cardiac hypertrophy and fibrosis and to delay transition into cardiac failure.

We investigated these questions in an animal model of old SHR. Rats aged 24–30 weeks can be considered to be adult. At this age, normotensive rats are still in the first half of their regular life span, which is estimated to be 2.5–3.5 years [16]. Our interest was focused on an antihypertensive therapy starting in the period of senescence. The present study was designed to investigate the efficacy of such a late-onset antihypertensive therapy on BP development and on cardiac remodeling in 60-week-old SHR. In this context, two therapeutic regimens, a monotherapy and a combination therapy, were compared. In a preceding study on seven-week-old SHR, we systematically tested the antihypertensive and cardioprotective effects of several classes of antihypertensive drugs as single-drugs and as combination treatments. Specifically, captopril (CAP) as an antagonist of the RAAS and nifedipine (NIF) as a calcium channel blocker were applied. CAP as monotherapy and a combination of CAP and NIF proved to be the most effective treatments with respect to BP lowering and to preventing cardiac hypertrophy and fibrosis [14]. These two therapeutic regimens were chosen for the present study to treat 60-week-old SHR over a period of 22 weeks. We investigated BP, heart weight (HW), biochemical markers of cardiac hypertrophy and cardiac fibrosis as well as cardiac histology.

We hypothesized that these treatments would decrease BP as well as biochemical and histological markers of cardiac hypertrophy and fibrosis in old SHR as well. The results of this study might have importance for antihypertensive treatment in humans. In particular, one of the main reasons for patients' poor adherence to antihypertensive therapy is that they do not understand the serious consequences of chronically elevated BP. The results of studies like this may help to improve the education of patients and the strict control of antihypertensive therapy.

#### **2. Materials and Methods**

#### *2.1. Animal Model*

All experiments were performed on 21 male SHR supplied by Charles River, Sulzfeld, Germany. The animals were fed a standard pellet diet (Altromin C100, Altromin GmbH, Lage, Germany) and had free access to tap water. All animal protocols were approved by the state agency (Landesdirektion Sachsen, number and date of approval: TVV 36/08; 13 May 2009) in accordance with the Guide for the Care and Use of Laboratory Animals published by the National Institutes of Health and with the "European Convention for the Protection of Vertebrate Animals used for Experimental and other Scientific Purposes" [17].
