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

To investigate the enteroinsular axis (EIA) in equines oral (oGTT) and intravenous (i.v.GTT) glucose tolerance tests (5.6 and 1 mmol glucose/kg BW, respectively) were performed with healthy, normal weight large horses and Shetland ponies. Plasma was analysed for concentrations of glucose, glucose-dependent insulinotropic polypeptide (GIP) and insulin. In all equines plasma GIP concentrations only increased significantly when glucose was administered orally. The insulin glucose ratio (IGR) was significantly higher during the oGTT than during the i.v.GTT in both races. Basal plasma glucose levels were significantly higher in large horses than in ponies in both experiments. During the oGTT maximum glucose values were significantly higher in ponies. Ponies tended to a higher insulin secretion but the IGRs were identical in both races after oral and intravenous glucose administration. One clinically inconspicuous pony showed hyperinsulinaemia and, in case of the oGTT, insulin resistance, glucose intolerance, and GIP hypersecretion. The results of this study indicate the existence of an EIA in equines due to the higher IGRs during the oGTT. Furthermore, the similarity of plasma GIP levels and IGRs in ponies and large horses suggest a comparable activity of the EIA in both races. Regarding the elevated plasma GIP concentrations of the insulin resistant pony the EIA appears to participate in equine hyperinsulinaemia.