Physiological consequences of NaCl ingestion by Na(+)-depleted rats

We investigated the temporal relationships between NaCl intake, gastrointestinal Na+ content, and plasma concentrations of Na+, aldosterone, and plasma renin activity (angiotensin). Rats depleted of approximately 2.2 mmol Na+ by combined dietary Na+ restriction and furosemide injection (10 mg sc) drank a preload of 1, 2, or 3 mmol 0.5 M NaCl. Intake of a test 0.5 M NaCl solution given 15, 30, 60, or 120 min later was reduced by approximately 50% of the preload content, irrespective of the interval between preload and test. At 15 min after starting to drink, 61-71% of ingested Na+ remained in the stomach, 15-18% in the small intestine, approximately 5% was calculated to be in extracellular fluid, and less than 1% was excreted. Rates of gastric and gastrointestinal clearance of Na+ were related to the quantity of NaCl ingested. Whereas gastric emptying was initially very rapid (30-61 mumol/min) and decreased with time, gastrointestinal clearance was constant (16-39 mumol/min). Drinking 3 mmol NaCl reliably reduced plasma aldosterone concentrations within 15 min and renin activity within 30 min. Drinking 1 or 2 mmol NaCl reliably reduced levels of these hormones within 30-60 min. The results describe for the first time the distribution of Na+ after Na(+)-depleted rats drink NaCl. They suggest that the rat determines the quantity of Na+ it requires within the first 15 min of ingestion and thus does not depend on signals generated by the prolonged influence of Na+ in the gastrointestinal tract or the fall in plasma concentrations of aldosterone and angiotensin.