Acute and Chronic Captopril , but Not Prazosin or Nifedipine , Normalize Alterations in Adrenergic Intracellular Ca 2 Handling Observed in the Mesenteric Arterial Tree of Spontaneously Hypertensive Rats

The effect of hypertension and acute (36-h) or chronic (from age 6 to 16 weeks) antihypertensive treatment with prazosin (2 mg kg 1 per day), nifedipine (50 mg kg 1 per day), or captopril (50 mg kg 1 per day) on Ca mobilization due to 1-adrenoceptor activation was analyzed in functional studies using arterial rings [four conductance/distributing vessels: aorta, main mesenteric, iliac, and tail arteries and two resistance vessels; first and second small mesenteric artery branches obtained from spontaneously hypertensive rats (SHR, 6 and 16 weeks old) and age-matched Wistar Kyoto rats (WKY)]. Maximal response to noradrenaline in the presence of extracellular Ca is not affected by hypertension or by the antihypertensive treatment. The extracellular Ca -independent contractile responses increased with age in iliac, tail, and small mesenteric arteries (SMA) and were further increased in SHR in SMA from both young and adult animals and in the main mesenteric artery of adult SHR. In main mesenteric artery, this increased contraction in SHR was associated with a higher increase in cytosolic [Ca ] mobilized by noradrenaline without changes in the total stored Ca . Acute or chronic treatment with captopril abolished the differences observed between WKY and SHR in the noradrenaline-induced contraction in mesenteric arteries loaded in Ca -free medium. In contrast, animals acutely treated with prazosin or chronically treated with either prazosin or nifedipine exhibit the same differences in Ca handling than untreated rats. In conclusion, these differences are not a consequence of increased blood pressure but precede it and can only be normalized by inhibition of the rennin-angiotensin system. High blood pressure in hypertensive humans or animals is caused by an elevation of systemic resistance, due to structural and functional changes in the wall of resistance vessels. However, at present, the mechanisms by which the most important vasoactive systems, such as sympathetic nervous system or renin-angiotensin system, contribute to the rise of vascular resistance in hypertension remain unclear (Schlaich et al., 2004). The crucial role of the increased sympathetic nervous system tone in the pathogenesis of hypertension is mediated not only by induction of cardiac hypertrophy and vascular remodeling, which, at least in spontaneously hypertensive rats (SHR), may even precede the rise of blood pressure but also by enhanced vasoconstriction (Zicha and Kunes, 1999). In hypertension, the postsynaptic -adrenergic functions are attenuated, whereas 1-adrenergic functions become dominant, although the total number of 1-adrenoceptors remains unchanged in most vessels. The augmented vasoconstriction mediated by 1-adrenoceptors in SHR could be due not only to a higher norepinephrine release from sympathetic nerve fibers but also to the hyper-reactivity of vessels or their supersensitivity to 1-adrenoceptor agonists (Zicha and Kunes, 1999). Differences in Ca handling have been also invoked to This study was supported by a research grant from the Spanish Comisión Interministerial de Ciencia y Tecnologı́a (SAF2001-2656). Part of this work has been previously presented at the 3rd Meeting of the Federation of the European Pharmacological Societies (EPHAR), July 2001; XXV Congress of the Spanish Society of Pharmacology, October 2003; and XXVI Congress of the Spanish Society of Pharmacology, September 2004. Article, publication date, and citation information can be found at http://jpet.aspetjournals. org. doi:10.1124/jpet.104. 078725. ABBREVIATIONS: SHR, spontaneously hypertensive rat(s); WKY, Wistar Kyoto rat(s); SMA, small mesenteric arteries; SMA-1, first branch of small mesenteric artery; SMA-2, second branch of small mesenteric artery; SBP, systolic blood pressure; NA, noradrenaline. 0022-3565/05/3131-359–367$20.00 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 313, No. 1 Copyright © 2005 by The American Society for Pharmacology and Experimental Therapeutics 78725/1196230 JPET 313:359–367, 2005 Printed in U.S.A. 359 at A PE T Jornals on A ril 0, 2017 jpet.asjournals. org D ow nladed from explain the increased adrenergic tone found in hypertension. There is previous evidence indicating that cytosolic Ca is elevated in different cell lines, including platelets and smooth muscle cells in SHR in comparison with normotensive (WKY) as well as in hypertensive rats compared with normotensive human individuals (Cortes et al., 1997; Salomonsson and Arendshorst, 2001; Kisters et al., 2004). In addition, an increase in intracellular Ca mobilization by adrenergic agonists has been found in renal resistance vessels from SHR (Salomonsson and Arendshorst, 2001). However, most of the previous studies have been performed in cultured smooth muscle cells obtained from large conduit vessels of animals with established hypertension, but an extensive analysis about changes in Ca handling related to adrenoceptor activation, including young and adult spontaneously hypertensive rats as well as different vessels (conductance and resistance vessels) has not been previously performed. The aim of the present work was to analyze the Ca mobilization due to 1-adrenoceptor activation in different vessels and the possible changes due to hypertension and the antihypertensive treatment. Therefore, we tested noradrenaline-induced contractile response in a Ca -containing and a Ca -free medium. The study was performed in four conductance/distributing vessels: aorta, main mesenteric, iliac, and tail arteries and in two resistance vessels: first and second mesenteric branches. Vascular tissues were obtained from different groups of animals: young (6-week-old) SHR and WKY; adult (16-week-old) SHR, with the hypertensive syndrome; and controls (WKY). Simultaneous measurements of cytosolic Ca and contraction were also performed in main mesenteric arteries from WKY and SHR. We have also analyzed whether antihypertensive agents acting through different mechanisms of action such as prazosin, nifedipine, and captopril modify Ca handling in the SHR. Materials and Methods Male WKY and SHR aged 6 or 16 weeks were used (Harlan Interfauna Ibérica, Barcelona, Spain) and housed under a 12-h light/ dark cycle at 22°C and 60% humidity. Starting at 6 weeks, some of the rats received prazosin (2 mg kg 1 per day), nifedipine (50 mg kg 1 per day), or captopril (50 mg kg 1 per day) in drinking water, until the age of 16 weeks (chronic treatment), whereas others were given no drugs. Systolic blood pressure was measured weekly from the tail of unanesthetized rats using a pletismographic method (LE 5650/6; Letica Scientific Instruments, Barcelona, Spain). Other groups of adult animals (16 weeks old) received prazosin (2 mg kg 1 per day), nifedipine (50 mg kg 1 per day), or captopril (50 mg kg 1 per day) in drinking water during 36 h (acute treatment), whereas others were given no drugs. Systolic blood pressure was also measured before receiving the treatment and 36 h later, shortly before isolating the vessels. All experimental procedures were approved by the Institutional Animal Care and Use Committee.