Increased reactive oxygen species (ROS) cause oxidative stress and contribute to the pathophysiology of cardiovascular diseases such as hypertension and atherosclerosis [74, 75]. The implication of Ang II and ET-1 in increased generation of ROS through the activation of NAPDH oxidase has been reported in different models of hypertensive rats including DOCA-salt HRs [76, 77]. This is evidenced in studies where ET_A receptor antagonist BQ123 attenuated ET-1-induced enhanced production of superoxide anion (•O₂ ⁻) in VSMC from hypertensive rats [78] and AT₁ receptor antagonist losartan inhibited the enhanced production of O₂ ⁻ anion in VSMC from SHR [62]. In the study of Sedeek et al. [79], ET-1 induced a significant and dose-dependent augmentation of O₂ ⁻ in VSMC from ET-1-induced hypertensive rats. In our studies, AT₁, as ET_A and ET_B receptors, has been implicated in enhanced oxidative stress exhibited by VSMC from SHR as evidenced by attenuation of increased production of O₂ ⁻ and increased activity of NADPH oxidase by losartan, BQ123 and BQ788 [67]. A role of enhanced oxidative stress in Ang II-induced enhanced levels of Giα proteins has also been reported [80]. In SHR, treatment with antioxidants such as diphenyleneiodonium (DPI) and N-acetyl cysteine (NAC) restored the enhanced levels of Giα proteins, implicating oxidative stress as the causative factor in the enhanced expression of Giα protein in SHR [62]. A similar antagonism to Ang II-induced increased Giα proteins and superoxide anion (O₂ ⁻) by antioxidants in A10 VSMC was reported by us [80]. It is further strengthened by our recent findings that H₂O₂, an oxidant that induces oxidative stress, enhanced the expression of Giα proteins in aortic VSMC [81].

Mitogen-activated protein (MAP) kinase is a serine/threonine-specific kinase that plays an important role in cellular differentiation, growth, and apoptosis and in the regulation of various transcription factors and gene expression [82]. Several studies have demonstrated the implication of Ang II and ET-1 in the modulation of ERK1/2 and physiological responses in different cell types including A10 VSMC [83], cardiomyocytes [84], and fibroblasts [85]. The implication of ERK1/2 in the enhanced expression of Giα-2 and Giα-3 protein in VSMC from SHR has also been reported [62]. Furthermore, the role of endogenous Ang II in enhanced phosphorylation of ERK1/2 and enhanced expression of Giα proteins in VSMC from SHR was shown by the fact that the treatment of VSMC from SHR with captopril and losartan attenuated the hyperphosphorylation of ERK1/2 and enhanced expression of Giα proteins [61]. Furthermore, Iwasaki and collaborators [86] have shown that the ET_A receptor-induced transactivation of EGFR results in the activation of ERK1/2 in aortic VSMC. We have also shown the implication of both ET_A and ET_B receptors in ET-1-induced enhanced ERK1/2 phosphorylation in A10 VSMC and aortic VSMC from SHR, because ET_A and ET_B receptor antagonists, BQ123 and BQ788, respectively, attenuated the ET-1-induced enhanced phosphorylation of ERK1/2 [63, 67]. In addition, we also demonstrated that ET-1-induced increased phosphorylation of ERK1/2 was attributed to transactivation of EGFR, because AG1478, an inhibitor of EGFR, inhibited the ET-1-evoked increased ERK1/2 phosphorylation [63, 67] (Chap.​ 31). In addition, the enhanced phosphorylation of ERK1/2 in SHR was restored to WKY levels with the treatments of antioxidants [67] and inhibitor of EGFR [63, 67] which suggest that the enhanced oxidative stress and EGFR through MAP kinase signaling may contribute to the enhanced expression of Giα protein in SHR. Taken together, it is suggested that the enhanced levels of endogenous vasoactive peptides, through the transactivation of EGFR, increase the phosphorylation of ERK1/2 in VSMC from SHR, which contributes to the enhanced expression of Giα proteins in SHR (Fig. 34.1).

Vasoactive peptides including Ang II and ET-1 modulate the expression of Giα proteins that regulate cardiovascular functions, including vascular tone and reactivity and cell proliferation. The levels of Giα proteins and mRNA are increased in the heart and aorta from genetic and experimentally induced hypertensive rats, whereas the levels of Gsα are unaltered in genetic and decreased in experimentally induced hypertensive rats with established cardiac hypertrophy. The increased levels of Giα proteins might contribute to the pathogenesis of hypertension because the enhanced expression of Giα proteins and mRNA precede the development of blood pressure while inactivation of Giα proteins by PT treatment in prehypertensive SHR prevented the development of high blood pressure. The concentrations of vasoactive peptides, including Ang II and ET-1, and growth factors and ROS are increased in hypertension. The increased levels of endogenous Ang II and ET-1 increase oxidative stress which through the transactivation of growth factor receptors and MAP kinase signaling contribute to the enhanced expression of Giα proteins implicated in the pathogenesis of hypertension. On the other hand, natriuretic peptide C-ANP₄₋₂₃ attenuates the high blood pressure in SHR through the inhibition of enhanced expression of Giα proteins and nitroxidative stress.