Effect of melatonin on baroreflex control of heart rate in rabbits

ABSTRACT

The effect of intravenous injection of melatonin [0.1 mg/kg, 0.5 mg/Kg and 1 mg/Kg] on arterial blood pressure and heart rate in adult male rabbits was investigated in this study. Baroreflex function was assessed using phenylephrine [the pressor test] to induce moderate changes in arterial blood pressure and to alter the stimulation of baroreceptor sites. In addition, the effect of melatonin on sympathetic and parasympathetic efferent nerve activity was also investigated. Melatonin resulted in a lowering effect on arterial blood -pressure with a decrease in heart rate. The significant decrease of arterial blood pressure was observed at a dose as low as 0.5 mg/Kg. The lowering effect of melatonin on heart rate was detected at 1 mg/Kg. Baroreflex slope was not depressed significantly with a significant increase in pulse interval, until I mg/Kg melatonin. Sympathetic and vagal nerve activities were attenuated significantly by melatonin. This study indicated that baroreflex control of heart rate was depressed by intravenous administration of melatonin. There was a dissociation between the effect of melatonin on baroreflex sensitivity versus the effect of low blood pressure. Melatonin was found to alter the baroreflex heart rate response through its direct depressant effect on baroreceptors and on both sympathetic and parasympathetic efferent nerve pathways