High Dietary Salt and Angiotensin II Chronically Increase Renal Sympathetic Nerve Activity

Overactivity of the sympathetic nervous system has long been implicated in the hypertensive response to elevated angiotensin II (Ang II) levels. Although recent studies suggest that high dietary salt may alter cardiovascular responses to Ang II, direct evidence demonstrating chronic activation of sympathetic nerve activity is lacking. The objective of this study was to determine whether a low dose of Ang II, on a background of high salt intake, would result in a chronic increase in renal sympathetic nerve activity (RSNA). Arterial pressure and RSNA were recorded via telemetry. Two groups of rabbits were studied: 1 group drank a 0.9% NaCl solution and received Ang II (20 ng/kg per minute for 21 days, Salt+Ang), and the other drank tap water throughout and was not infused with Ang II (Control). In the Salt+Ang group, mean arterial pressure increased over the first week and remain elevated by 18.5±4.1 mm Hg at day 21. RSNA was not significantly different between groups on day 7 but was significantly elevated in the Salt+Ang group on day 21 (13.5±3.2% compared with 6.8±0.8% in the Control group; P<0.05). Baroreflex control of RSNA showed a rightward shift on day 21, but not day 7, and baroreflex responses indicated that RSNA could not be completely suppressed when arterial pressure was increased. No changes were observed in either mean arterial pressure or RSNA variables in the Control group. Our results support the hypothesis that elevated Ang II levels, in conjunction with a high salt diet, have the ability to chronically increase RSNA and, thus, potentially contribute to the maintenance of hypertension.