High Water Intake Combined with Low Sodium Intake Abolishes the Antidiuretic Effect of Angiotensin II in Conscious Dogs

Abstract

1. We studied post-prandial changes in renal function in dogs adapted to either low or high sodium intake with and without concomitant post-prandial infusion of angiotensin II. Six trained dogs were exposed to diets containing either 0.5 or 14.5 mmol Na+ day−1 kg−1 body weight (low or high sodium respectively). They were studied from 20 min before to 4 h after food intake. In half of the experiments a physiological dose of angiotensin II (4 ng min−1 kg−1 body weight) was administered after food intake for four post-prandial hours. The water intake was high and equal on both diets (91 ml day−1 kg−1 body weight). 2. On a high-salt diet post-prandial sodium excretion and urine volume increased considerably above fasting values. This post-prandial increase was attenuated when angiotensin II was infused (post-prandial sodium excretion was 31% ± 3% of intake without versus 10% ± 1% with angiotensin II, post-prandial urine volume was 22% ± 2% without versus 8% ± 1% with angiotensin II, P < 0.05). Post-prandial increases in glomerular filtration rate and fractional sodium excretion were attenuated during angiotensin II infusion in dogs on a high-salt diet. 3. On a low-salt diet post-prandial sodium excretion remained low with or without angiotensin II infusion, whereas urine volume increased post-prandially, and this increase was greater when angiotensin II was administered (40% ± 3% versus 34% ± 2% of intake, P < 0.05). 4. Angiotensin II infusion increased mean arterial pressure by an average of 12 mmHg in dogs on a high-salt diet (P < 0.05) and by 7 mmHg in dogs on a low-salt diet. In dogs on a high-salt diet, right atrial pressure was on an average 3 cmH2O higher with than without angiotensin II (P < 0.05). In animals on a low salt intake post-prandial right atrial pressure decreased slightly, but remained in the range of fasting values, during angiotensin II infusion. 5. Neither plasma osmolality nor plasma sodium concentration changed in any of the four protocols. 6. We conclude that the post-prandial effects of angiotensin II in dogs on a high water intake depend on the amount of concomitant sodium intake. Angiotensin II reduces the post-prandial diuresis and natriuresis when given to sodium-loaded dogs, whereas angiotensin II does not reduce post-prandial urine flow and sodium excretion rates in dogs on a low sodium intake and may even augment water excretion in this condition.