ENaC and N‐methyl‐D‐aspartate (NMDA) receptors interact in the connecting tubule inducing renal vasodilation and blood pressure changes

Abstract

Introduction N-methyl-D-aspartate (NMDA) receptors are expressed throughout the nephron, including distal nephron principal cells (PC), and induce renal vasodilation through an unknown mechanism. Connecting tubule-glomerular feedback (CNTGF) is a vasodilator feedback mechanism mediated by PC's ENaC producing afferent arteriole (AfA) vasodilatation and promote sodium excretion. We hypothesized that NMDA receptors activation stimulate ENaC and consequently CNTGF, and that the inhibition of NMDA receptors decrease CNTGF inducing hypertension. Methods To determine the effect of NMDA receptor on CNTGF-mediated AfA dilation, we measured CNTGF-mediated AfA dilation in the presence or absence of NMDA receptor antagonists with and without NMDA antagonist (MK801), using a double-perfusion method of the AfA and CNT ex vivo in rabbits and stop-flow pressure (SFP) technique in vivo in rats. To investigate NMDAr-ENsC interaction we measured ENaC open probability (NPo) on split open tubules (CCD) from mice at baseline and with the NMDA agonist. To evaluate the effect of the NMDA receptor in blood pressure regulation, we use the NMDA inhibitor MK801 infused by osmotic minipumps for 7 days in liddle syndrome mouse (β ENaC mutated allele on129/Sv background) on normal salt diet. Under normal salt diet, blood pressure (BP) is not different from wild type despite evidence for increased sodium reabsorption in distal colon and low plasma aldosterone, suggesting chronic hypervolemia. BP was measure using tail cuff methods. Results In pre-constricted AfA, in vitro NMDA agonist application to the connecting tubule lumen increased AfA dilation (p<0.001). This amino-acid-induced dilation was blocked with the CNTGF inhibitor benzamil and blunted with the NMDA receptor blocker MK-801. In vivo, NMDA agonist increase CNTGF (P<0.01). In split open tubules, NMDA activation increased ENaC NPo (p=0.04), suggesting an interaction between those channels. Basal BP was similar in both mice group (p=0.4). The infusion of NMDA inhibitor (Mk-801) during 7 days induces hypertension (SBP 146±8 mmHg) while the vehicle infused mice remained normotensives (110.7±7.7 mmHg; p<0.01). Conclusions NMDA receptor activation induced renal vasodilation by increasing the ENaC dependent CNTGF responses . NMDA inhibition decrease ENaC activation and consequently inhibit CNTGF induced-vasodilation and promote hypertension. These results provide evidence on the role of NMDA receptors on renal hemodynamic and blood pressure regulation.