Melatonin as a daily rhythm control signal for glucose homeostasis and insulin secretion in horses

An involvement of melatonin in the regulation of glucose homeostasis and insulin secretion in pancreatic β-cells has been proposed. To evaluate the features of the daily rhythms of plasma melatonin, blood glucose, and insulin in horses, ten clinically healthy Italian Saddle geldings, 9-10 years old with a mean body weight of 490 ± 25 kg, were enrolled. The animals were housed in individual boxes under natural environmental conditions (sunrise 06:10; sunset 18:10; 12-15°C, 77% RH). All horses were cannulated the day before the start of the study for sample collection, which was performed every 4 h for a 48-h period. Blood samples were analysed for melatonin, glucose, and insulin concentrations. Our results showed higher melatonin serum concentration and lower glucose concentration during the dark phase than the light phase. All investigated parameters showed a daily rhythm during the 2 d of monitoring. Melatonin acrophase was observed in the dark phase, glucose acrophase in the light phase, and insulin acrophase nearest the sunset. A negative correlation was observed between glucose and melatonin, and a positive correlation was observed between insulin and the other two parameters.