| Objective: Accumulated evidences show that both physiological functions (such as heart rate, blood pressure and the function of cardiac conduction) and pathological events (such as myocardial ischemia/infarction, cardiac arrhythmia and sudden death)of cardiovascular system present circadian rhythm or diurnal rhythm. The diurnal rhythm of blood pressure is followed with interest of the scholars from either domestic or abroad. We previously found that the regulation of blood pressure is correlated with sympathetic nervous system, rennin-angiotensin system and endothelium system. Therefore, the diurnal rhythm of these vasoactive substances plays an important role in the determining of the diurnal rhythm of blood pressure. It has been suggested that high-salt diet might influence the level or the diurnal rhythm of blood pressure through impairing the endothelial function or inhibiting the function of sodium-potassium pump, which can make the blood pressure tend to be non-dipper. Compared with dippers, hypertensive individuals with a nondipping nocturnal blood pressure profile have more target organ damage and a worse cardiovascular prognosis. Therefore in this study, our aim is to investigate the chronobiological features of Na, ET-1, NO, eNOS in plasma and myocardial tissue, eNOS-mRNA in myocardial tissue, and the effects of high-salt diet on them in rats.Methods: Sixty-four age-matched (10 weeks) healthy male Sprague Dawleys rats (with body weight 250-275g) housed at 22±2°C were included in this study. They were divided randomly into two groups: rats that were administrated with tap water were designated as the"group C",and those fed with 0.9 % NaCl (high-salt diet group) were designated as the"group T". There were 32 rats in each group, which was divided into 4 sub-groups (with 8 rats in each one). After 2 months, we detected the levels of Na, ET-1, NO, eNOS, and eNOS-mRNA at different time at the interval of 6 hours. Cosinor fitting analysis and zero amplitude test were introduced to analyze the chronobiological featrues of the indices above. The compare of mean values among 2 groups was managed based on analysis of variance (ANOVA), and P≤0.05 was defined as statistically different borderline.Results:1 The level of Na in plasma is statistically increased in Group T; while ET-1, eNOS significantly decreased and NO significantly increased with no significant difference between group T and C. The level of ET-1 and eNOS in myocardial tissue are significantly decreased in the rats of high-salt diet group with statistically significant; while the levels of NO and the expression of eNOS-mRNA are significantly decreased with no significant difference between the two groups. 2 Based on cosinor fitting analysis and zero amplitude test, the chronobilogical characteristic values of ET-1, NO, eNOS and eNOSmRNA, each of which showed typical circadian rhythm(P<0.05), were obtained, respectively. The peaks of ET-1, in plasma and myocardial tissue of group C and T appeared respectively at 00:05, 12:08, and 12:09, 12:12, that of NO at 12:10, 12:22, and 00:04, 00:08, of eNOS at 02:55,12:08, and 00:10, 12:11, of eNOSmRNA in myocardial tissue at 00:08, 00:14, of Na in plasma at 12:12, 00:50. The peak-values of ET-1 and eNOS in plasma of group T were lower than those of group C, the opposite were with the amplitudes; the peak-values of Na and NO of group T were higher than those of group C, the opposite were with the amplitudes; while both the peak-values and the amplitudes of ET-1, eNOS, and eNOSmRNA in myocardial tissue of group T were lower than those of group C, but the peak-value of NO was higher in group T than that of group C, the opposite was with the amplitudes. The peak-phases of ET-1 and eNOS in plasma of group T were earlier than those in group C, the peak-phases of Na and NO in plama were delayed, while those in myocardial tissue were delayed either except eNOS.Conclusion: 1 The levels of NO, eNOS and ET-1 in plasma and myocardial tissue of the rats in all sub-groups show typical circadian rhythm, so do the expression of eNOS-mRNA and the level of Na in plasma serum of them. 2 The level of Na in plasma is significantly increased after high-salt diet with significant difference; while ET-1, eNOS significantly decreased and NO significantly increased, with no significant difference between group T and C. The levels of ET-1 and eNOS in myocardial tissue are significantly decreased after high-salt diet with significant difference; while the levels of NO and the expression of eNOS-mRNA are significantly decreased, with no difference between the two groups. 3 The peak-phases of NO both in plasma and myocardial tissue, the expression of eNOSmRNA and the level of ET-1 in myocardial tissue are delayed without resume the circadian rhythm of them. Adversely, Na, eNOS and ET-1 in plasma accompanied with eNOS in myocardial tissue converse the circadian rhythm significantly.
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