Objectives: Plasma melatonin concentrations in non-dipping patients show a blunted daily rhythm. Melatonin has a capacity to improve disturbances in biological rhythms. Hypertensive TGR(mRen2)27 (TGR) rats with an upregulated renin-angiotensin system and inverted blood pressure profile were used to elucidate whether melatonin is able to influence the control of blood pressure.
Design: Melatonin was administered in drinking water to normotensive Sprague-Dawley (SD) and hypertensive TGR rats during the dark phase of the light: dark cycle 12: 12 for 4 weeks.
Methods: The effect of melatonin on blood pressure was monitored, and the expression of clock genes per2 and bmal1 and melatonin receptor MT1 in the suprachiasmatic nucleus (SCN) and the heart was measured by real time polymerase chain reaction during a 24-h cycle.
Results and conclusion: The administration of melatonin did not influence clock gene expression in the SCN but its effect on clock gene expression in the heart was phase dependent in both SD and TGR rats. Melatonin administration did not decrease the expression of melatonin receptors in the SCN and the heart. Melatonin did not decrease blood pressure in TGR rats but influenced the peripheral oscillator in the heart independently of the SCN. A modified function of molecular circadian oscillators in the heart can interfere with anticipation and disturb the adaptation of this organ to pressure overload.