Angiotensin II (AII) acts at specific neuronal receptors in the hypothalamus and brainstem to stimulated numerous effects such as increased blood pressure, fluid intake and vasopressin secretion. There is much evidence to suggest that an overactivity (eg. increased levels or response) of these neuronal receptors contributes to certain pathological states such as hypertension. However, there is a lack of fundamental information concerning the cellular mechanisms (both signal transduction and membrane ionic currents), which are involved in AII receptor-mediated effects on neurons. The overall aims of this proposal are to investigate and compare the receptor-mediate effects of AII on membrane ionic current in neurons cultured from normotensive (Wistar Kyotom, WKY: Sprague Dawley, SD) and spontaneously hypertensive (SH) rat hypothalamus and brainstem. Further, we hope to discern the intracellular signal transduction pathways which are responsible for mediating the effects of AII on membrane ionic currents in neurons. Our published studies, preliminary data and also the background literature are consistent with the following hypotheses: (a) Neurons form SD or WKY rats contain AII type-1 (AT) receptors. AII acts at these AT receptors to stimulated phosphoinositide (PI) hydrolysis, with subsequent Ca2+ mobilization and protein kinase C (PKC) activation. Increased intracellular Ca2+ and/or PKC activation result in an inhibition of new outward membrane ionic current (Sum of all currents minus Na+; I) (b) Neurons from SD or WKY rats also contain AII type-2 (AT) receptors. AII act at these sites to reduce cellular cyclic GMP (cGMP) levels, by activation of a phosphodiesterase or inhibition of a particulate guanylate cyclase. The reduction in cellular cGMP results in a stimulation of I in particular the delayed rectifier (IK) and transient (IA) K+ components currents. (c) Neurons from SH rats exhibits increased levels of AT but studies proposed here we will investigate all of these hypotheses, using whole cell voltage clamp methods.
Specific aims are modulated following AT and AT receptor activation in normotensive rat neurons. (2) Determine the intracellular mechanisms which mediate the AT and AT receptor-induced changes in membrane ionic currents, delineated in Aim (1). (3) Determine if the AT and At receptor-induced changes in membrane ionic currents, delineated in Aim from SH rats. The significance of these studies is that we will identify basic intracellular and ionic mechanisms that are involved in the physiological actions mediated by neuronal AII receptors, and also determine whether these mechanisms are altered in SH neurons. This information is essential for a full understanding of the CNS-mediated actions of AII. In the long term, this information may be valuable for determining the mechanisms that underlie spontaneous hypertension.
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