This subproject is one of many research subprojects utilizing theresources provided by a Center grant funded by NIH/NCRR. The subproject andinvestigator (PI) may have received primary funding from another NIH source,and thus could be represented in other CRISP entries. The institution listed isfor the Center, which is not necessarily the institution for the investigator.As of 2/07/06BackgroundBlood pressure is influenced by dietary salt intake; indeed 30% of people with normal blood pressure and 50-70% of people with hypertension are salt-sensitive. Dopamine, a well-known neurotransmitter, has been identified as an important modulator of blood pressure, sodium balance, renal function and adrenal function through an independent, paracrine effect on the kidneys. Angiotensin II is also an important paracrine regulator of sodium balance and blood pressure. The dopaminergic and renin-angiotensin systems have been shown to interact in vitro and in vivo. In this protocol, we will study the effects of inhibition of the renin-angiotensin system on the effects of stimulation of the dopaminergic system, in salt loaded and salt depleted states, in normotensive and hypertensive human subjects. We will correlate these effects with the presence or absence of recently described genetic polymorphisms in the post-receptor signaling pathway of the dopamine receptor in the proximal tubule of the kidney, specifically the G protein- coupled receptor kinase, GRK4. Together, this information will help us better understand the effects of salt and genotype on the pathogenesis of essential hypertension, a disease that affects more than a billion people worldwide.Hypothesis The angiotensin converting enzyme (ACE) inhibitor, Enalapril, augments natriuresis caused by D1 receptor stimulation, in normotensive and hypertensive human subjects, in salt loaded and salt depleted states. These effects are reduced in subjects who have polymorphisms of the G protein-coupled receptor kinase, GRK4. The augmentation of natriuresis is increased in hypertensive subjects with increased renin-angiotensin tone.Methods/Study DesignThis is a double blind, placebo-control, crossover design. 20 normotensive volunteers and 28 hypertensive subjects, recruited from an Internal Medicine clinic, of both sexes, between 18-55 years will participate in the study. They will all have 5 days each of low (50meq/day) sodium, and moderate (300meq/day) sodium diet, 4 weeks apart. Each subject will have four interventions applied in sequence, the first intervention being randomized as per a computerized plan. The four interventions are: high salt diet + placebo, high salt diet + Enalapril, low salt diet +placebo, and low salt diet + Enalapril. All subjects will undergo renal function tests, before, during and after a 3-hour infusion of Fenoldopam, which is a selective dopamine (D1) receptor agonist. We will measure urinary sodium excretion, and, by measuring creatinine , PAH and lithium clearance, we will estimate glomerular filtration rate, effective renal plasma flow, and proximal tubular sodium reabsorption of sodium respectively.All subjects will be genotyped for the common polymorphisms associated with the development of essential hypertension, specifically, GRK4, alpha adducin, aldosterone synthase, and genetic variants of the human D5 receptor.Statistical Analysis We will compare the change in the excretion of sodium from baseline between Enalapril/placebo treatments, and between low and moderate salt diet periods, in each subject. Urinary sodium excretion will be studied separately in normotensive and hypertensive groups, and then compared. We will genotype all subjects and using single locus and multi loci analyses to determine differences in the occurrence of polymorphisms between hypertensive and normotensive subjects.Anticipated ResultsIn normotensive subjects, inhibition of angiotensin II formation by ACE inhibition will augment Fenoldopam -induced natriuresis in the moderate sodium diet, and also in the low sodium diet, where normally the renin-angiotensin system has an important sodium-retaining role. In hypertensive subjects, where the natriuretic response to Fenoldopam has been reported to be impaired, we anticipate that Enalapril will enhance natriuresis to a greater extent in those subjects with increased renin-angiotensin tone. Basic Science ComponentWe study the function of dopamine, specifically dopaminergic post receptor signaling pathways in the human coronary vascular smooth muscle cells in humans, specifically its possible role in activation of big potassium channels, abnormalities in which may contribute to the increase in myogenic vascular tone seen in hypertension. This work has led to a first author abstract entitled'D5 receptor mediates Dopamine effects on Big KCa channels in human coronary artery smooth muscle cells' Aruna Natarajan, Guichun Han, Richard White and Pedro Jose, which will be presented as a poster at the Experimental Biology meeting in April 2006. This work will comprise Aruna Natarajan's thesis report for a PhD in Physiology and Biophysics at Georgetown University, currently in preparation. 3. Please provide scientific progress achieved to date (since last year's report if applicable): Initial abstractDopamine is a well-known neurotransmitter, with paracrine effects on the kidney that have recently been elucidated. These effects result in natriuresis in response to a sodium load, mediated by a G-protein coupled receptor, the D1 receptor. Post-receptor signaling pathways have been found to be impaired in people with salt sensitive hypertension, attributed to a polymorphism of a G-protein coupled receptor kinase, GRK4.This study aims to elucidate the interaction between the renal dopaminergic system and the renin-angiotensin system (RAS) in salt-loaded and depleted states, by studying the effect of angiotensin converting enzyme (ACE) inhibition on the effect of Fenoldopam, a D-1 receptor agonist on salt excretion in the urine. The study will correlate these effects with the presence or absence of genetic polymorphisms of GRK4, aldosterone synthase, alpha-adducin and other polymorphisms, all of which have been described in people with salt-sensitive hypertension. This study will help us understand the interactions of salt, dopamine, RAS and genetic makeup in the pathogenesis of essential hypertension.
SPECIFIC AIMS :This is an investigator-initiated study supported by the mentored Clinical Research Scholar Program Award, Grant RR 17613, from the NCRR, NIH, DHSS. The dopaminergic system has a major regulatory effect on fluid and electrolyte balance and blood pressure. The objective of this project is to determine the interaction of the renal dopaminergic and RAS in the regulation of sodium excretion during varying amounts of sodium intake.
The specific aims are to determine:1. The effect of sodium intake on the interaction between the RAS and D1 dopamine receptors on sodium excretion.2. The effect of GRK4 gene variants on the natriuretic response to D1 dopamine receptor agonist and its modulation by ACE inhibition.Hypothesis: The natriuretic effect of D1 dopamine receptors, which is best manifest under conditions of sodium loading is antagonized by the antinatriuretic effect of AT1 angiotensin receptors. In essential hypertension the natriuretic effect of D1 receptors is impaired because of activating variants of GRK4 while the antinatriuretic effect of AT1 receptors is enhanced. The overall hypothesis is that ACE inhibition augments the natriuresis caused by D1 receptor stimulation in normotensive subjects, only in the salt loaded state. These effects are reduced in those subjects with GRK4 gene variants, in hypertensive subjects.
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