1. The relationship between plasma atrial natriuretic peptide (ANP) and body sodium was determined in rats 1 month after myocardial infarction induced by coronary artery ligation. After operation rats received a normal or a low salt diet, and total exchangeable body sodium was measured sequentially.
2. Rats with infarction receiving a normal salt intake did not retain sodium when compared with sham-operated controls. Rats receiving a low salt diet had a 10% decrease in body sodium (P < 0.01). The decrease was the same in rats with infarction as in controls.
3. Plasma ANP was similar in control rats irrespective of salt status. Plasma ANP levels were markedly elevated in rats with infarction irrespective of salt status (P < 0.01).
4. The rise in plasma ANP was correlated with cardiac hypertrophy and infarct size in animals fed both normal and low salt diets. However, there was no relationship between plasma ANP and exchangeable body sodium.
5. These results suggest that in this model of heart failure plasma ANP is raised by increased left atrial stretch in proportion to the severity of left ventricular dysfunction. In contrast, plasma ANP concentrations do not appear to be elevated as a consequence of increased right atrial pressure caused by sodium retention and expanded extracellular volume.