Glucose-dependent insulinotropic polypeptide (GIP) and the enteroinsular axis in equines (Equus caballus).

This research undertook an investigation into the role of the enteroinsular axis (EIA) – the relationship between the gut and insulin secretion – in horses and ponies, particularly in response to oral and intravenous glucose administration. The data shows the EIA functions similarly for both large horses and ponies, with a notably higher insulin glucose ratio (IGR) following oral glucose intake. Additionally, the EIA may contribute to hyperinsulinaemia, a condition of excess insulin levels, in horses.

Research Methodology

• This experiment involved the participation of healthy, normal weight, large horses, and Shetland ponies.
• Both oral glucose tolerance tests (oGTT) and intravenous glucose tolerance tests (i.v.GTT) were performed on the animals. Here, 5.6mmol glucose per kg of body weight was given orally, and 1mmol glucose/kg body weight was administered intravenously.
• Scientists analysed the animals’ plasma for glucose, glucose-dependent insulinotropic polypeptide (GIP), and insulin. GIP is a hormone that stimulates the insulin secretion response.

Findings

• Regardless of species, a significant rise in plasma GIP concentrations was observed only when glucose was taken orally.
• The insulin-glucose ratio (IGR) was significantly higher during oGTT compared to i.v.GTT, indicating more insulin was produced in response to orally ingested glucose, across both species.
• Baseline plasma glucose levels were higher in large horses as compared to ponies, in both varieties of experiments.
• During the oGTT, the maximum glucose values were found to be significantly higher in ponies.
• Ponies showed a trend towards higher insulin secretion, but the IGRs remained the same in both species after oral and intravenous glucose administration.

Special Case Study

• One pony, which appeared clinically normal, exhibited hyperinsulinaemia – significantly elevated insulin levels in its bloodstream. Furthermore, in the oGTT scenario, the animal showed signs of insulin resistance, glucose intolerance, and GIP over-secretion.

Conclusions

• The study findings suggest the existence of an EIA in horses, due to the higher IGRs observed during the oGTT.
• The similarities observed in plasma GIP levels and IGRs in both ponies and large horses suggest that the activity of the EIA is comparable in both species.
• In relation to the pony that showed symptoms of insulin resistance, the EIA seems to have a role in hyperinsulinaemia in horses.