Vascular α1D-adrenoceptors are overexpressed in aorta of the aryl hydrocarbon receptor null mouse: role of increased angiotensin II

The hypothesis that α1D-adrenoceptors may mediate the pro-hypertensive actions of angiotensin II (Ang II) was tested in isolated aorta (α1D-adrenoceptor bearing tissue) of the aryl hydrocarbon receptor null mouse (AhR−/−), which shows increased levels of Ang II, cardiac hypertrophy and hypertension. The effect of captopril (an angiotensin converting enzyme inhibitor) on both blood pressure and aortic α1D-adrenoceptor expression and function in mice were determined. Basal blood pressure was higher in AhR−/− mice, while captopril therapy decreased it to wild-type (WT) values. Aortas of adult WT and AhR−/− mice were stimulated by phenylephrine or noradrenaline to induce contraction; the maximal effect was higher in AhR−/− mice, without a significant change in pEC50. PA2 values for the selective α1D-adrenoceptor antagonist BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazynil]ethyl]-8-azaspiro [4.5]decane-7,9-dione) were 9.19 and 8.94 for WT and AhR−/−, respectively; while Schild slopes were not different from 1. PCR experiments showed c. 77% increase in AhR−/− α1D-adrenoceptors cDNA compared with WT mice; while western blot analysis demonstrated c. 88% increase in α1D-adrenoceptor protein in AhR−/− mice. Captopril therapy decreased α1D-adrenoceptor-induced contraction and protein in AhR−/− mice to WT levels. These data support the hypothesis that under conditions where Ang II is elevated, vascular α1D-adrenoceptors are increased, and further suggest that both Ang II and vascular α1D-adrenoceptors could be related in the onset of hypertension. Keywords: angiotensin II, α1D-adrenoceptors, aryl hydrocarbon receptor null mouse, hypertension, captopril Introduction Angiotensin II (Ang II) plays an important role in regulating systemic arterial pressure through its synthesis by the renin–angiotensin system (RAS). Ang II is a potent vasoconstrictor that activates Ang II type 1 (AT1) receptors on vascular smooth muscle and affects vascular and cardiac remodelling cardiac contractility and heart rate through increased sympathetic nervous system tone by promoting noradrenaline release (Weir & Dzau, 1999). Several physiological parameters regulated by the RAS, such as plasma renin activity, plasma angiotensinogen concentration (Ruiz et al., 1990; Zicha & Kune, 1999) and kidney renin release (Henrich & Levi, 1991; Zicha & Kune, 1999) are known to be elevated in young spontaneously hypertensive rats (SHR), suggesting that these components might contribute to the pathogenesis of genetic hypertension. Recent evidence suggests that the AT1 receptor signalling pathway mediates both the physiological and pathogenic pleiotropic actions of Ang II (reviewed in Hunyady & Catt, 2006). On the other hand, a decade ago it was demonstrated that Ang II induces α1-adrenoceptor expression, mainly the α1D-subtype, in isolated rat aorta smooth muscle cells (VSMC) (Hu et al.,1995). Activation of the α1D-adrenoceptor subtype increases protein synthesis in VSMC (Xin et al., 1997). These data suggest that Ang II may facilitate aorta smooth muscle hypersensitivity and hypertrophy through α1D-adrenoceptors expression. Increasing evidence shows that vascular α1D-adrenoceptors are functionally related with the genesis and/or maintenance of hypertension, they seem to be present prior to the establishment of hypertension, and it has been suggested that an increase in the population of constitutively active α1D-adrenoceptor could be involved in the pathology of elevated sympathetic tone found in SHR (Villalobos-Molina & Ibarra, 1996, 1999; Villalobos-Molina et al., 1999; Guimaraes & Moura, 2001; Gisbert et al., 2002; Garcia-Sainz & Villalobos-Molina, 2004). We have found that inhibiting Ang II production with captopril decreased both the expression and function of α1D-adrenoceptors in the aorta of pre-hypertensive SHR rats (Godinez-Hernandez et al., 2006). Taken together, these data suggest that the RAS and α1D-adrenoceptors might interact with each other at the onset of hypertension. Thus, we hypothesize that under conditions of elevated levels of Ang II, vascular α1D-adrenoceptors will also be increased, and may mediate some of the pro-hypertensive actions of the hormone (Villalobos-Molina & Ibarra, 2005; Godinez-Hernandez et al., 2006). One condition where elevated levels of AngII are observed is the aryl hydrocarbon receptor null (AhR−/−) mouse, which exhibits cardiac hypertrophy, increased levels of endothelin-1 and high arterial blood pressure (Lund et al., 2003). However, so far no explanation exists as to why the absence of the aryl hydrocarbon receptor leads to the pathology observed in these mice. As it is known that mouse aorta express α1D-adrenoceptors that mediate contraction (Yamamoto & Koike, 2001; Lazaro-Suarez et al., 2005), we chose this model to test our hypothesis.