Effect of salt loading on aldosterone response to long-term infusion of angiotensin II in rats

We examined changes in plasma aldosterone level and blood pressure (BP) after long-term (12 days) intravenous infusion of angiotensin II (Ang II; 15 and 60 ng/min) with two different sodium intakes (0.66% and 8% salt-containing diet) in Wistar rats. Without salt loading, 15 ng/min of Ang II did not increase mean BP (112 +/- 1 vs. 112 +/- 2 mm Hg), but 60 ng/min of Ang II increased mean BP (144 +/- 3 mm Hg, p less than 0.01). However, the 8% salt diet not only enhanced the pressor effect of 60 ng/min Ang II (168 +/- 9 mm Hg, p less than 0.05), but also elevated mean BP with 15 ng/min of Ang II (159 +/- 3 mm Hg, p less than 0.01). On a normal-salt diet (0.66%), plasma aldosterone was increased by Ang II infusion from 11.3 +/- 0.3 ng/dl to 12.7 +/- 0.5 ng/dl (NS) at a rate of 15 ng/min Ang II, and to 15.7 +/- 0.4 ng/dl (p less than 0.01) at 60 ng/min Ang II. However, salt loading reduced Ang II-induced elevation of aldosterone: 11.3 +/- 0.5 ng/dl (less than 0.01) at a rate of 60 ng/min Ang II. Thus, salt loading diminished aldosterone response to Ang II, which might compensate salt-induced enhancement of the pressor effect of Ang II.