Angiotensin-converting enzyme inhibition and Na appetite: microbehavioral analysis and nycthemeral physiology

The present experiments describe a marked nycthemeral rhythm in both the appetite for 0.3 M NaCl solution and components of the renin-angiotensin-aldosterone axis stimulated in Sprague-Dawley rats by chronic administration of enalapril, an angiotensin I-converting enzyme inhibitor. Continuous recording of water, NaCl, and food intakes shows that the sodium appetite is manifest as discrete bouts of salt ingestion in temporal proximity to meals and is partially independent of water bouts. In particular, salt bouts occur without water bouts in the late afternoon of a 12:12-h light-dark cycle and continue periprandially with water bouts during the night. Intake of all three commodities is minimal in the morning. In a second experiment, it was determined that plasma renin activity (PRA) was maximally elevated by chronic enalapril in the daytime and that plasma aldosterone was reduced by enalapril but continued to show nycthemeral rhythm peaking in the afternoon. The concurrent maxima in PRA and aldosterone in the afternoon in enalapril-treated rats thus coincides with NaCl intake in the absence of water intake.