Physiologically aldosterone acts at the renal tubule to promote salt and water retention. Its production regulates vascular volume and hence blood pressure. Inappropriate secretion of aldosterone for the salt status of an individual is pathogenic, inducing structural and functional alterations in the heart, kidneys, and vasculature by advancing disease processes such as cardiac fibrosis, nephrosclerosis and arteriosclerosis. Although aldosterone induced cardiovascular and renal damage has been associated with the activation of the ligand-activated nuclear mineralocorticoid receptor (MR), pathogenic contributions of aldosterone that are independent of the MR, such as impairment in vascular reactivity, are now well-acknowledged . Therefore, there is a strong rationale for exploring therapies that are complementary to MR blockade that directly target the production of aldosterone. Aldosterone production from adrenal zona glomerulosa (ZG) cells is Ca-dependent and circulating Ang II is the predominant aldosterone secretagogue. Nevertheless, because the ZG layer of the adrenal gland is highly vascularized, every ZG cells is adjacent to an endothelial cell, and thus ZG cells are also targets for regulation by locally produced abluminally secreted endothelin (ET-1). In this proposal we test the novel hypothesis that aldosterone production is stimulated by systemic and paracrine hormones that convert the quiescent ZG cell into an electrically excitable one that serves to sensitize the production of aldosterone to low concentrations of physiological secretagogues. Using techniques of patch-clamp electrophysiology, molecular mutagenesis and radioimmunoassay we evaluate the following specific aims:
Aim 1 : Establish the ionic basis for ZG cell membrane excitability. Specifically we will: (1.1) Define the basic properties of regenerative Vm spiking in ZG cells;(1.2) Identify the role of Cav3.2 channels and IKCa in the initiation and/or propagation of pacemaking;(1.3) Characterize the control of regenerative Vm spiking elicited by Ang II and/or ET-1;(1.4) Quantify the impact of regenerative Vm spiking on net calcium entry.
Aim 2 : Determine the molecular basis for the regulation of Cav3.2 channels by PKC??PKD and its contribution to electrical excitability and the stimulation of aldosterone production elicited by Ang II and/or endothelin. Specifically we will: (1.1) Identify critical residues on the Cav3.2 channel protein that mediate stimulation of whole-cell and single channel activity by PKC??PKD;(1.2) Determine the contribution of PKC??PKD-mediated channel regulation to channel activation and electrical excitability elicited by Ang II and/orET-1;(1.3) Introduce PKC??PKD regulation resistant channels into adrenal zona glomerulosa cells to perturb Ang II and or ET-1 induced electrical excitability and evaluate the regulation of aldosterone production elicited by Ang II and/or ET-1. It is likely that the recognition of the electrical excitability of the ZG cell will revolutionize therapeutic strategies to efficiently inhibit the production of aldosterone.
In appropriate production of aldosterone induces structural and functional alterations in the heart, kidneys and vasculature by advancing disease processes such as cardiac fibrosis, nephrosclerosis and arteriosclerosis. However, because not all cardiovascular and renal damage has been associated with the activation of the aldosterone nuclear receptor (MR) there is a strong rationale for exploring therapies that are complementary to MR blockade that target directly the regulation of aldosterone production. In this application we explore important mechanisms that regulate the production of aldosterone and provide data showing that the aldosterone producing cell of the adrenal gland is electrically excitable which revolutionizes the view of this endocrine cell and in itself changes the way in which aldosterone production could be targeted for inhibition by new drug therapies.
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