The Effects Of High Salt Diet On The Circadian Rhythm Of Renin-angiotensin System In Rat

Objective: In humanbeings, various kinds of vital phenomena show some degree of biological rhythms and the most frequent was the carcadian rhythm with period of approximately 24 hours. The cardiovascular system is organized according to an oscillatory pattern, and most cardiovascular functions, such as blood pressure, exhibit typical circadian rhythm. Recently, it has been found that the abnormal rhythms of blood pressure not only increase the risks of target organs damage but also increase the cardiovascular morbility and mortality. It would be very important that to get well known of the chronobiology characters of blood pressure and relative influential factors.The regulation of blood pressure is a complicated process, which the renin-angiotensin system (RAS), may plays important roles. Studies have confirmed that the levels of plasma renin activity (PRA), angiotensin -II (Ang II) and angiotensin converting enzyme (ACE) show circadian rhythm, and their chronobiology characters have closely correlation to the circadian rhythm of cardiovascular diseases. In addition, it is well known that salt was one of the most important risk factors of hypertension, and high salt intake is not only contribute to the elevation of the blood pressure but also increases the mass of the left ventricular wall, stiffens conduit arteries, and thickens the wall of and narrows the lumen of resistance arteries independent of the blood pressure. Therefore, in this study, we investigated the circadian rhythm of RAS in the plasma and myocardium and the effects of high salt intake on the circadian rhythm of RAS.Methods: The experiment was carried out in sixty-four age-matched male Sprague-Dawlay rats (with body weight 250-275g). After one week's free-running period in natural light-dark cycles, the rats were housed at 22±2°C and free access to food and water. They were divided randomly into two groups: rats that drank the deionized water were designated as "N group"; rats that drank the sodium chloride solution (the concentration was 0.9%) were designated as "T group". There were 32 rats in each group, which was divided into 4 sub-groups (with 8 rats in each one). Each group was maintained in same condition for 8 weeks before the experiment. Animal experiments were conducted in accordance with guidelines of Hebei Geriatric Institute.For the determination of the diurnal pattern of PRA, ACE, Ang II in serum and myocardial tissue, as well as AT-1mRNA expressions in rats myocardial tissue, the rats were sacrificed every 6 hours (with one sub-group at 02:00, 08:00, 14:00 and 20:00, respectively) by operating on chest, and the indices above were measured. Cosinor fitting analysis and zero amplitude test were introduced to analyze the chronobiological features of RAS in every group. The compare of circadian values between two groups was managed based on student's t test, and p<0.05 was defined as statistically different borderline.Results: 1. The levels of plasma sodium were significantly higher in T group than those in N group based on t-test (p<0.01). The level of ACE in serum was also higher in T group than that in N group (p<0.05). The levels of PRA and Ang II in serum were significantly lower in T group than those in N group (p<0.01) .2. The levels of Ang II and AT-1mRNA in myocardial tissue were higher in T group than those in N group by using t-test (p<0.05).3. Based on cosinor fitting analysis and zero amplitude test, circadian rhythms were found in both groups, and the curves and characteristic values of serum level of Na, PRA, Ang II and ACE were obtained respectively. The level of plasma sodium showed significantly diurnal variation (P<0.05), with the peak time at 16:06 and 12:32 in N group and T group, respectively. The amplitudes of vibration in the two groups were similar. The phases of peak value of PRA, Ang II and ACE in serum of N group occurred in turn were 13:10, 18:48, 13:32, of T group occurred in turn were 14:37, 19:36, 14:30, which were somewhat later than those in N group. Furthermore, the amplitudes of vibration of PRA, Ang II and ACE in serum were augmented in T group.4. Based on cosinor fitting analysis and zero amplitude test, Ang II and AT-1mRNA in myocardial tissue of the two groups showed typical circadian rhythms. The phase of the peak value of Ang II and AT-1mRNA in myocardial tissue of N group occurred at 13:43 and 21:19, respectively, and of T group occurred at 11:12 and 2:30, which were earlier than those in N group. Furthermore, the amplitudes of vibration of Ang II and AT-1mRNA in myocardial tissue were also augmented in T group.Conclusion: 1. The levels of Na, PRA, Ang II, ACE and AT-1mRNA in serum and/or myocardial tissue of the two groups show typical circadian rhythms.2. High salt diet might increase the levels of plasma sodium and ACE, while the levels of PRA and Ang II in the serum were significantly decreased. In addition, The expression of Ang II and AT-1mRNA in myocardial tissue were up-regulated with high salt intake.3. High salt diet could change the circadian features of the PRA, Ang II, ACE and AT-1mRNA in serum and/or myocardial tissue, including the alteration of the peak phase and the amplitudes of vibration.