Excess dietary salt alters angiotensinergic regulation of neurons in the rostral ventrolateral medulla

Abstract

Excess dietary salt intake contributes to or exacerbates some forms of hypertension by increasing sympathetic nerve activity (SNA) and arterial blood pressure (ABP) through angiotensin II (Ang II) type 1 receptor activation in the rostral ventrolateral medulla (RVLM). Despite this interaction among dietary salt, Ang II, and the RVLM, no studies have directly examined whether dietary salt by itself alters Ang II-dependent responses and regulation of RVLM neurons, SNA, and ABP. Therefore, the present study directly tested this hypothesis. Male Sprague-Dawley rats were fed normal chow and given access to water or 0.9% NaCl solution for 14 days. Unilateral injection of Ang II (0.6, 6, and 60 pmol) into the RVLM produced a significantly greater increase in renal SNA and mean ABP of rats drinking 0.9% NaCl versus water. However, dietary salt did not alter mRNA levels of RVLM Ang II type 1a receptors or the SNA and ABP responses to stimulation of the dorsolateral funinculus. Additional experiments demonstrate that blockade of RVLM Ang II type 1 receptors significantly reduced renal SNA, splanchnic SNA, and mean ABP of rats drinking 0.9% NaCl but not water. Blockade of iontotropic glutamate receptors had no effect. Altogether, these findings suggest that elevated dietary salt enhances the sympathoexcitatory actions of Ang II in the RVLM via changes in the intrinsic properties of RVLM neurons. Moreover, elevated dietary salt intake differentially affects the tonic activity of the peripheral versus brain RVLM Ang II type 1 receptors to regulate baseline SNA and ABP.

Figure 1

A, Peak increase in mean ABP (n=8 to 13 per group per dose) and renal SNA (n=5 to 6 per dose per group) after injection of Ang II (0.6 and 6.0 pmol) into the RVLM was significantly greater in rats drinking 0.9% NaCl (^) vs water (○). SNA values are reported only for injections performed ipsilateral to the nerve recording. B, Individual example of ABP, mean ABP, and renal SNA during unilateral injection of Ang II (6 pmol) into the RVLM (Figure S1). * indicates significant difference between rats drinking water and 0.9% NaCl (P<0.01);▾, microinjection of Ang II.

Figure 2

Top, Peak increase in mean ABP of rats drinking water (open; n=5) or 0.9% NaCl (filled; n=4) during electric stimulation (5-second train, 1-ms pulse) of the dorsolateral funiculus. Bottom, Peak increase in renal SNA of rats drinking water or 0.9% NaCl during electric stimulation of the dorsolateral funiculus (1 Hz; 1-ms pulse). There were no significant differences between groups.

Figure 3

Quantitative RT-PCR of AT₁a and AT₂ receptor expression in the RVLM and AT1a receptor expression in the renal cortex of rats drinking water (open; n=5) or 0.9% NaCl (filled; n=5) for 14 days. Values were calculated using the 2^−(ΔΔCt) method and normalized to β-actin. * indicates significant difference from rats drinking water (P<0.01).

Figure 4

Mean ABP, renal SNA, and splanchnic SNA response to bilateral injection of losartan (n=11 per group; A) or kynurenic acid (n=5 per group; B) into the RVLM of rats drinking water (○) or 0.9% NaCl (^) for 14 days. Injections were performed at 0 minutes and separated by 2 to 3 minutes. SNA values are based on a smaller number of animals (losartan, n≥7 per group; kynurenic acid, n≥4). * indicates significant difference between groups (P<0.05).