Effect of sustained hypernatraemia on the renin-aldosterone system in the dog.

1. To determine if increases in plasma sodium concentration P[Na] have any sustained effects of the renin-aldosterone system, P[Na] was increased in a group of six dogs over a period of 6 days by increasing sodium intake from 10 to 200 mmol per day while a fixed 700 ml per day water intake was maintained along with a continuous i.v. infusion of antidiuretic hormone (ADH) at a rate of 2.4 units per day. 2. P[Na] rose from 137.3 +/- 2.0 to 153.6 +/- 6.5 mmol/l during the high intake period. Plasma potassium concentration, 22Na space, and mean arterial pressure all remained near control levels in response to Na loading. 3. Plasma renin activity (PRA) averaged 1.0 +/- 0.1 ngAI/ml per hour on the final low Na day and fell transiently to 0.6 +/- 0.2 ngAI/ml per hour on the first day of sodium loading. For the duration of the study it remained at the control level. Plasma aldosterone concentration fell from the low Na level of 15.4 +/- 2.4 ng/100 ml to 10.5 +/- 1.5 ng/100 ml on the final day of high Na intake. 4. We conclude that increases in P[Na] in the absence of concomitant changes in P[K], 22Na space and MAP do not have a sustained effect on control of renin release but may exert a negative effect on aldosterone secretion.