\title{Does Brain Natriuretic Peptide Have a Direct Renal Effect in Human Hypertensives?}

\author{Kim van der Zander, Alphons J.H.M. Houben, Abraham A. Kroon, Thomas K.A. Wierema, Monique J.M.J. Fuss-Lejeune, Derk Koster, Peter W. de Leeuw}

\begin{abstract}
Systemic infusion of brain natriuretic peptide (BNP) stimulates natriuresis and diuresis but has variable effects on the renal vasculature. In this study, we investigated whether BNP has any direct effects on the kidney in hypertensive patients. Three stepwise increasing doses of BNP (60, 120, and 180 pmol/min) or placebo were infused into the renal artery of 26 hypertensive patients. Renal blood flow was determined with the $^{133}$Xenon washout technique. Before and after infusion of BNP, arterial and venous blood samples were taken for cGMP, renin, and creatinine concentration. Intra-arterial blood pressure and heart rate were monitored continuously. Intrarenal BNP infusion did not induce significant changes in renal blood flow despite increases in circulating levels of cGMP. The latter, however, was not associated with changes in the cGMP gradient across the kidney. In addition, we did not find any BNP-related changes in the secretion of active renin and in creatinine extraction. At the highest dose, heart rate increased after BNP infusion without a change in mean intra-arterial blood pressure. In conclusion, this study suggests that at least in hypertensive subjects, BNP has no direct intrarenal hemodynamic effects and that the rise in circulating cGMP without changes in net renal extraction of this second messenger is related to a primary extrarenal target of BNP.
\end{abstract}

\section{Introduction}
Key Words: natriuretic peptides; vasodilation; hypertension, renal; human; kidney; hemodynamics; vasoconstriction; hormones

Systemic infusion of brain natriuretic peptide (BNP) stimulates natriuresis and diuresis\cite{1,2,3,4,5,6} and inhibits plasma renin activity\cite{1,2,3,7,8}, but has variable effects on the renal vasculature. Although most but not all studies in healthy humans reported that BNP infusion increases glomerular filtration rate\cite{1,5,6,7}, renal plasma flow has been found to decrease\cite{1,7}, to increase\cite{5}, or to remain unchanged\cite{3,8}. Variations in the renovascular effects of BNP could be related to differences in BNP levels reached during the experiments, but it is equally possible that the renal changes are, in part, secondary to systemic effects. Indeed, BNP not only acts on the kidney but also affects blood pressure, heart rate, cardiac output, and systemic vascular resistance\cite{2,3,5,6,8,9}. In those studies in which changes in renal plasma flow were reported, it is important to keep in mind that such alterations may simply be due to concurrent changes in cardiac output. If, for instance, renal fraction (that is, the proportion of cardiac output perfusing the kidneys) remains unaltered during systemic BNP infusion, it is unlikely the peptide has exerted a direct effect on the renal vasculature. If, on the other hand, renal fraction increases, relative renal vasodilation must have occurred. In most studies, this has not been taken into account.

Local administration of BNP in a regional vascular bed in amounts that will not have systemic effects allows for assessing whether BNP has any direct effects on certain parts of the circulation. With this approach, we and others have demonstrated previously that BNP induces a dose-dependent vasodilation in forearm vasculature\cite{10,11}. In the present study, we investigated whether BNP has any direct effects on the renal circulation. To this end, we infused BNP into the renal artery of hypertensive patients who were scheduled for renal angiography. Before and during the infusion, we measured renal blood flow, renin release, and the extraction of creatinine, the latter being taken as a marker of glomerular filtration. We also measured concentrations of cGMP, since this second messenger may reflect BNP activity.

\section{Methods}

\subsection{Subjects}
This study was performed in 26 hypertensive patients in whom renal artery stenosis was suspected on the basis of one or more of the following criteria: treatment-resistant hypertension despite the use of at least two adequately dosed antihypertensive agents, overt peripheral vascular disease, the presence of an abdominal bruit or an increase in serum creatinine during ACE inhibitor treatment. Antihypertensive medication, if any, was discontinued for 3 weeks before