OR-48 BNP ENHANCES RENAL ACTIONS OF LASIX AND SUPPRESSES LASIX-INDUCED ALDOSTERONE ACTIVATION IN EXPERIMENTAL HEART FAILURE Alessandro Cataliotti, Guido Boerrigter, Lisa C Costello-Boerrigter, Toshihiro Tsuruda, John A Schirger, Horng H Chen, Lorenzo S Malatino, John C Burnett Jr. Cardiorenal Research Laboratory, Mayo Clinic and Foundation, Rochester, MN. Congestive heart failure (CHF) is characterized by sodium and water retention. While loop diuretics are powerful natriuretic agents, they can reduce glomerular filtration rate (GFR) and activate the renin-angioten- sin-aldosterone system (RAAS). Studies have reported that GFR is a key predictor of mortality in CHF. Therefore, new therapeutic strategies are warranted to both minimize side effects of conventional diuretics and to potentiate their renal actions. Brain natriuretic peptide (BNP) has di- uretic, natriuretic and renin and aldosterone inhibiting properties. The renal actions of BNP in CHF are associated with preserved GFR without activation of RAAS. The objective of this study was to test the hypothesis that BNP potentiates the renal actions of Lasix (L) in a model of CHF. We hypothesized that BNP in combination with L maintains GFR and suppresses the RAAS. CHF was produced in two groups of dogs by rapid ventricular pacing (10 days at 240 bpm). At day 11 one group received continuous (60 min) iv administered L (1mg/Kg/hr). A second group (BNP+L) received 30 min iv co-infusion of L (1mg/Kg/hr) and low dose (2 pmol/kg/min) of BNP followed by 30 min co-infusion of L (1mg/Kg/ hr) and high dose (10 pmol/kg/min) of BNP. * indicates p0.05 vs baseline. There were no differences in cardiovascular hemodynamics between groups. L increased urinary flow (from baseline 0.78 0.05 to 5.01.06* mL/min), but the effect of BNP+L was significantly (p0.0021) higher (from 0.46 0.03 to 8.041.21* low and 7.390.94* high dose mL/min). Similarly, urinary sodium excretion (L: from 74.89141.71 to 614.0323.13* mEq/min; BNP+L: from 39.4175.9 to 988.16282.12* low and 950.93299.37* high dose mEq/min) was significantly higher in BNP+L group (p0.0025). While GFR tended to decrease in L group, it increased in BNP+L group (p=0.006). Plasma aldosterone increased with L but remained preserved in BNP+L group(p0.05). L and the BNP+L had similar beneficial cardiovascular hemodynamic actions. Importantly, with a profound na- triuretic and diuretic response with BNP+L, GFR was increased with no activation of aldosterone. Co-administration of BNP and loop diuretic may be beneficial in preserving renal function and inhibiting activation of aldosterone while maximizing natriuresis and diuresis. Key Words: BNP, Lasix, Heart Failure OR-49 INCREASED PLASMA ENDOTHELIN-1 AND IMPAIRED VASORELAXATION IN MICE FEATURING ENDOTHELIAL CELL SPECIFIC KNOCKOUT OF THE ENDOTHELIN B RECEPTOR Alan J Bagnall, Fiona Gulliver-Sloan, Kelland F Nick, Yanagisawa Masashi, David J Webb, Kotelevtsev Yuri. Centre for Cardiovascular Science, University of Edinburgh, Edinburgh, Mid Lothian, United Kingdom; Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas. The role of the murine endothelial cell (EC) endothelin B receptor (ETBR) in vascular function and BP regulation was investigated. Floxed ETBR mice with loxP sites flanking exons 2 and 3 were generated through homologous recombination in embryonic stem cells. Flox/Wild (F/W) mice were crossed with Tie2-Cre transgenic mice to produce EC-specific ETBR knockout mice (KO) (F/F Tie2). Wild type (W/W -/-) and single transgenic (W/W Tie2 and F/F -/-) littermates acted as controls. In vitro vascular function was assessed by wire myography and in vivo BP is being measured in conscious animals using i.a. catheters. 125I ET-1 binding was measured in pulmonary EC. Plasma ET-1 was measured by RIA. Aortic rings of F/F Tie2 mice exhibit impaired vasodilation to the selective ETBR agonist Sarafotoxin 6c (Sx6c; see fig) and to acetylcho- line (ACh; logEC50  SEM: F/F Tie2, 6.76  0.08; W/W -/- 7.06  0.10; W/W Tie2 7.09  0.07; F/F -/- 7.02  0.12; p0.0001 for all groups; n = 16 in each group). There was no difference in Sx6c-mediated contraction of tracheal rings (non-EC control) from F/F Tie2 animals compared to controls. 125I ET-1 binding in pulmonary EC was down by 85% and plasma ET-1 increased in EC ETBR knockout mice (see fig). Preliminary data shows that BP is not elevated in the absence of sodium loading. We have successfully achieved EC-specific knockout of the ETBR in the mouse. This model has an impaired vasodilator response to Sx6c and ACh but maintains normal tracheal smooth muscle ETBR-mediated contraction. The mechanism underlying the impaired ACh response awaits further investigation. The EC ETBR is likely to represent the clearance receptor for circulating ET-1. The floxed ETBR KO mouse will be a useful tool for the study of the role of EC ETBR in cardiovascular function. Key Words: Endothelin, ETB receptor, Cre recombinase OR-50 DIETARY SALT REGULATES RENAL AMINOPEPTIDASE N ABUNDANCE: GENETIC AND TRANSCRIPTIONAL EVIDENCE FOR AMINOPEPTIDASE N AS A CANDIDATE GENE IN THE DAHL SALT-SENSITIVE RAT Theressia L. Washington, Mariam Farjah, Bryan P. Roxas, Robert S. Danziger. Cardiology/Internal Medicine, University of Illinois, Chicago, IL; West Side Veterans Administration, Chicago, IL. Purpose: Aminopeptidase N (APN), known also as microsomal amino- peptidase, is a major constitutent of the brush border membranes of kidney proximal tubule cells. The enzyme releases neutral amino acids from oligopeptides and converts angiotensin III to angiotensin IV. The present study was undertaken to determine whether it plays a role in the regulation of renal salt-handling and the pathogenesis of salt-sensitive hyperension. Methods: Renal APN transcript abundance was measured, using SYBR Green reverse transcription coupled with the polymerase chain reaction (RT/PCR), in Sprague Dawley (SD) and Dahl salt-sensitive (SS) rats on either 8% (high) or 0.3% (basal) NaCl diets for 10 days. The APN gene was mapped with respect to known quantitative trait loci (QTL) using radiation-hybrid mapping and available mapping data. Meprin cDNA sequences were determined in both Sprague Dawley and Dahl SS/Jr rat strains using dedeoxy sequencing. 22A AJH–May 2003–VOL. 16, NO. 5, PART 2 ORALS: Vasoactive Hormones/Endothelial Factors 0895-7061/03/$30.00 © 2003 by the American Journal of Hypertension, Ltd. Published by Elsevier Inc. 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