AT2 Receptors in the RVLM: Regulation of Sympathetic Outflow in Heart Failure

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ABSTRACT Chronic heart failure (CHF) is a leading cause of morbidity and mortality in the United States with the characteristics of sympathetic overactivity and activation of the renin-angiotensin system. These are the primary therapeutic targets for this syndrome. In this project, we propose three aims to explore the potential benefit of over expressing angiotensin type 2 receptor (AT2R) expression in the rostral ventrolateral medulla (RVLM) in rats with CHF. Moreover we will also determine the underlying mechanisms involved in the sympatho-inhibitory effects of AT2R overexpression in the RVLM. It has been firmly established that, in contrast to the influences of the AT1R, the AT2R facilitates the neuronal potassium channel and current, which hyperpolarizes membrane potential and suppresses neuronal excitability. On the other hand, our preliminary experiments show a down regulation of AT2R protein expression in the RVLM of rats with CHF. These phenomena lead us to postulate that a decrease in AT2R signaling in the RVLM contributes to sympatho-excitation of this syndrome by elevating the excitability of presympathetic neurons. Our global hypothesis in this proposal is that over expression of the AT2R in the RVLM by gene transfer will reduce or normalize sympathetic activation in CHF, and therefore benefit this syndrome. In preliminary experiments, we have successfully produced a rat model in which the AT2R is selectively over expressed in the RVLM by direct delivery of AT2R viral vectors into this area. Employing this animal model, we will test our hypothesis by pursuing the following 3 Specific Aims. AIM 1: To determine the hemodynamic, cardiac function, and sympathetic outflow in normal and CHF rats with overexpression of AT2R in the RVLM. This Aim includes two components. First, we will determine the chronic effects of over expressing AT2R in the RVLM on arterial blood pressure (AP), heart rate (HR), cardiac function, and norepinephrine excretion in conscious normal and CHF rats. In addition, water intake, urine excretion, and body weight will be measured. Second, we will observe the acute effects of microinjecting agonists and antagonists of AT1R and AT2R into the RVLM with AT2R over expression on AP, HR, and renal sympathetic nerve activity (RSNA). We will also explore the involvement of intracellular AR2R signaling sympatho-inhibition. This includes the NO/cGMP and PLA2/AA/12-LO/PP2A pathways. AIM 2: To determine the effects of overexpressing AT2R on single presympathetic neuronal activity in the RVLM of anesthetized rats. We will directly record extracellular single unit firing of RVLM presympathetic neurons following overexpression of AT2R. AIM 3: To determine the effects of overexpressing AT2R on potassium current of presympathetic neurons in the RVLM. Employing patch clamp and brainstem slice preparations, we will directly record potassium currents of presympathetic neurons following overexpression of AT2R. These studies will lead to an enhanced understanding of angiotensin signaling in presympathetic neurons in the setting of CHF. They will highlight the importance of a balance between AT1 and AT2 receptor signaling in setting the level of sympatho-excitation and identify possible new targets for therapy in CHF.