Circadian rhythm of plasma sodium is disrupted in spontaneously hypertensive rats fed a high-NaCl diet

High-NaCl diets elevate arterial pressure in NaCl-sensitive individuals, an d increases in plasma sodium may trigger this effect. The present study tes ts the hypotheses that 1) plasma sodium displays a circadian rhythm in rats , 2) the plasma sodium rhythm is disturbed in spontaneously hypertensive ra ts (SHR), and 3) excess dietary NaCl elevates plasma sodium concentration i n SHR. The results demonstrate that plasma sodium has a circadian rhythm th at is inversely related to the circadian rhythm of arterial pressure. Altho ugh the plasma sodium rhythms of SHR and control rats are nearly identical, the plasma sodium concentrations are significantly higher in SHR throughou t the 24-h cycle. Maintenance on a high-NaCl diet increases plasma sodium c oncentration similarly in both SHR and control rats, but it blunts the plas ma sodium rhythm only in SHR. These results demonstrate that in rats, plasm a sodium has a circadian rhythm and that high-NaCl diets increase plasma so dium concentration.