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

Melatonin in horses shows daily fluctuations that correspond with changes in blood glucose and insulin levels, suggesting it helps regulate these metabolic processes according to the time of day.

Background and Purpose

• Researchers proposed that melatonin, a hormone known for regulating sleep cycles, may also influence glucose homeostasis (the balance of blood sugar) and insulin secretion from pancreatic β-cells.
• The study aimed to investigate whether melatonin, glucose, and insulin exhibit daily rhythms in horses and how these rhythms might be related.

Study Design and Methods

• Subjects: Ten clinically healthy Italian Saddle geldings, aged 9-10 years and weighing approximately 490 kg.
• Housing: Horses were kept individually under natural environmental conditions reflecting typical day and night cycles, including natural sunrise (06:10) and sunset (18:10), with ambient temperature around 12-15°C and relative humidity ~77%.
• Procedure: Each horse was cannulated (catheter inserted) a day before the study to facilitate repeated blood sampling.
• Sampling: Blood samples were collected every 4 hours over a 48-hour period to measure plasma levels of melatonin, glucose, and insulin.
• Analysis: Researchers examined daily rhythms (circadian patterns) and correlations between the three parameters.

Key Findings

• Melatonin Levels: Serum melatonin concentrations were higher during the dark phase (nighttime) compared to the light phase (daytime), consistent with melatonin’s known role as a nighttime hormone.
• Glucose Levels: Blood glucose concentrations were lower during the dark phase and higher during the light phase, showing an opposite pattern to melatonin.
• Insulin Levels: Insulin peaked nearest to sunset, indicating a distinct daily rhythm separate but related to glucose and melatonin rhythms.
• Rhythmicity: All three parameters—melatonin, glucose, and insulin—demonstrated significant daily rhythms across the two days of monitoring.

Relationships Between Melatonin, Glucose, and Insulin

• There was a negative correlation between glucose and melatonin levels, meaning when melatonin was high (night), glucose tended to be low.
• A positive correlation was found between insulin and both melatonin and glucose, suggesting insulin secretion is linked to the levels of these two substances and their circadian dynamics.
• This interplay suggests melatonin may serve as a signaling hormone that helps coordinate glucose regulation and insulin secretion over the daily cycle, acting as a control signal for metabolic rhythms in horses.

Implications and Significance

• The study supports the idea that melatonin is not only important for regulating sleep-wake cycles but also plays a role in metabolic regulation.
• Understanding these rhythms in horses can help improve management of equine health, particularly conditions related to glucose metabolism such as insulin dysregulation and equine metabolic syndrome.
• These findings could lead to better timing of feeding or medical interventions to align with the horse’s natural metabolic rhythms for improved efficacy and health outcomes.