Predominance of nocturnal sympathetic nervous activity in salt-sensitive normotensive subjects

To assess the relation between salt sensitivity and autonomic nervous function by power spectral analysis of heart rate variability in normotensive subjects, low and high salt diets were given to 13 normotensive men (aged 25 to 39 years) for 4 days each. Autonomic function was assessed by power spectral analysis of R-R intervals based on an autoregressive algorithm from 24-h Holter electrocardiogram. Subjects whose mean blood pressure was increased more than 3 mm Hg by high salt diet were defined as salt sensitive (SS, N = 5), and the remainder as salt resistant (SR, N = 8). Using the low frequency (LF, 0.1 Hz) and high frequency (HF, 0.25 Hz) components, the LF to total power ratio (%LF) was used as a marker of sympathetic activity, and the HF to total power ratio (%HF) as a marker of parasympathetic activity. Compared to the daytime, SR revealed a decrease in %LF and an increase in %HF during the night on both diets. In SS, these circadian changes were observed only during low-salt diet. During the night, SS showed a higher %LF and a lower %HF than SR. Plasma catecholamines tended to be decreased by the high sodium diet in SR but not in SS subjects. These results suggest that the persistent nocturnal predominance of sympathetic nervous activity in salt-sensitive men may contribute to the subsequent increase of blood pressure in these subjects.