Hypertension is a common disease characterized by increased vasomotor tone, smooth muscle remodeling and vessel inflammation. These aspects of vascular smooth muscle (VSM) biology are extensively regulated by G-protein-coupled signal transduction, which can be abnormal in the hypertensive state. In this regard, G1i2 has generated interest because (a) G1i2 expression and activity are increased in VSM and other cells from hypertensive animals and humans, (b) inhibition of G1i2 with pertussis toxin has an antihypertensive effect in rodents with genetic and non-genetic forms of hypertension, and (c) polymorphisms in the G23 subunit of the Gi heterotrimer are associated with increased G1i2 activity and hypertension. However, while these findings indicate that G1i2-dependent signaling can influence blood pressure control, it is unclear whether an increase in G1i2 expression/activity in VSM contributes to the pathogenesis or maintenance of the hypertensive state. In this proposal, we will interrogate the pathophysiological significance of altered G1i2 signaling in VSM by taking advantage of recently generated transgenic mice that selectively overexpress either G1i2 or a G1i2 inhibitory 'minigene'in smooth muscle. Preliminary experiments show that arterial rings from the G1i2 overexpressing mice generate significantly greater contractile forces than arterial rings from their nontransgenic littermates. This is a significant observation because it provides the first evidence that directly links the level of G1i2 expression in VSM to vasomotor responsiveness. Moreover, the finding that increased G1i2 expression alone is sufficient to increase the vasoconstrictor response of VSM provides supporting rationale for the overall hypothesis: the induction of G1i2 expression/activity in VSM is a central event in the pathogenesis and/or maintenance of hypertension. To examine this hypothesis, Aim 1 will investigate the significance of G1i2 induction in VSM in an in vivo model. Blood pressure and cardiac function in transgenic mice with augmented expression of G1i2 in VSM will be assessed following acute vasopressor/vasodilator challenges, and during experimentally induced hypertension.
Aim 2 will determine the mechanism(s) by which G1i2 regulates VSM responsiveness. Because preliminary data show that the magnitude of agonist-induced intracellular calcium fluxes in cultured VSM cells from G1i2 overexpressors and nontransgenic mice do not differ, this aim will focus on calcium sensitization as the primary mechanism by which G1i2 enhances vasoconstriction.
Aim 3 will use transgenic mice that express a G1i2 inhibitory 'minigene'in VSM to determine whether inhibition of G1i2 signaling in VSM can prevent or attenuate the development of vasomotor hyperresponsiveness and hypertension. The successful completion of these aims will directly substantiate or refute a role for altered G1i2 signaling in hypertension, and thereby could lead to novel antihypertensive therapies that target G1i2 rather than individual G-protein-coupled receptors.
Hypertension is a highly prevalent disease that is a major risk factor for cardiac disease and stroke. G-protein receptor signaling abnormalities in hypertension may contribute to elevated blood pressure by enhancing contraction and impairing relaxation of arteries. Successful completion of these studies may improve our understanding of these abnormalities, and thereby could ultimately lead to novel strategies to treat hypertension.