The overall goal of the proposed research is to determine the molecular and physiological mechanisms by which vascular NCX1 (Na+/Ca2+ exchanger type-1) influences arterial blood pressure (BP) normally and in salt- dependent hypertension. Two key determinants of arterial diameter, and hence of arterial BP in vivo, are sympathetic nerve activity (SNA) and myogenic tone (MT). My overall hypothesis is that VSM NCX1 increases both SNA-mediated vasoconstriction and MT, via net Ca2+ influx that increases VSM [Ca2+], and thus regulates BP. Transgenic mice that express altered levels of smooth muscle NCX1 (NCX1smTg/Tg, overexpressors;NCX1sm-/-, knockouts) as well as an exogenous (transgenic) FRET based, Ca2+/calmodulin-myosin light chain kinase (MLCK) activity biosensor molecule will be used. The NCX1 overexpressors will reveal 'gain of function'and knockouts will reveal loss of NCX1 function.
Three Specific Aims will be tested:
Aim 1 : test the hypothesis that vascular NCX1-mediated Ca2+ entry increases cytoplasmic [Ca2+] and activation of MLCK in both SNA-mediated vasoconstriction and in MT in isolated arteries;
Aim 2 : test the hypothesis that Ca2+ entry via VSM NCX1 also contributes to SNA- and MT-mediated arterial tone in vivo, hence regulates BP;
Aim 3 : test the hypothesis that salt-dependent hypertension involves further increases in NCX1 activity. The putative cellular mechanism for NCX1 in SNA-mediated contractions is Ca2+ influx via 'reverse mode'NCX1, as a result of local Na+ accumulation after G protein-coupled receptor (GPCR)-induced activation of Na+ permeant transient receptor potential canonical channels, TRPC6. This would increase sarcoplasmic reticulum-dependent Ca2+ waves that activate MLCK. The putative mechanism of NCX1 in MT is Ca2+ influx after stretch activation of Na+ channels (TRPC6 and/or TRPM4). This would increase cytoplasmic [Ca2+]. Confocal '4-D'imaging will be used to observe cytoplasmic Ca2+ waves in isolated pressurized mesenteric small arteries during SNA-mediated contraction. A key technique to be used is intra-vital FRET imaging of exteriorized small arteries of anesthetized living mice to observe [Ca2+], MLCK activation, artery diameter, and carotid artery BP. Salt-dependent hypertension will be induced in biosensor/NCX1 transgenic mice by high salt intake. Blood flow (to determine cardiac output, CO) and telemetric arterial BP will be measured in conscious, freely moving mice. Total peripheral resistance (TPR) will be calculated since BP H CO x TPR. Pharmacological GPCR blockers and a specific NCX blocker (SEA0400) will be administered systemically or locally (to the artery being studied) in the in vivo experiments. The research will elucidate the molecular mechanisms of the established association of BP with vascular NCX1. This knowledge should increase our understanding of salt-dependent hypertension, an increasingly urgent health problem.
The proposed research will elucidate the molecular and physiological mechanisms by which arterial NCX1 (Na+/Ca2+ exchanger type-1) influences intracellular Ca, vascular tone, and blood pressure normally and in salt-dependent hypertension. The sympathetic nervous system and myogenic constriction are two key determinants of vascular tone in vivo, and NCX1 is proposed to be a specific molecular component in both mechanisms. This research will utilize transgenic mouse models with intrinsic fluorescence that enable Ca measurement in living animal to provide new information about the role of arterial NCX1 in he control of blood pressure.
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