Effects of sex, pregnancy and season on insulin secretion and carbohydrate metabolism in horses.

In pregnant mares, peripheral insulin antagonism channels glucose preferentially to the foetus. On the other hand, horses reduce their metabolic activity in winter. Taking these aspects of equine pregnancy and metabolism together, we hypothesized that glucose clearance from blood and the insulin response to glucose do not only change throughout gestation but also with season. To test this hypothesis, the glucose and insulin response to an oral glucose test and relative insulin release were analysed in pregnant mares (n=12) and in geldings (n=10) as controls. Animals were tested in June, September, December, and in March (geldings) and on day 320 of gestation (mares). Furthermore, the 6 mares foaling early and 6 foaling later in the year were compared. In mares and geldings, plasma glucose concentration increased after glucose feeding (p<0.001). The increase was more pronounced in mares (p<0.05) and increased from June to December in mares (p<0.001) but not geldings (month x group p<0.05). This indicates constant glucose clearance in geldings but reduced clearance in pregnant mares. A partial insulin resistance is thus induced by pregnancy independent from season. Insulin release increased after glucose feeding (p<0.001) similarly in geldings and mares. The insulin response to glucose and relative insuslin release increased from June to December (p<0.001) indicating seasonal changes in β-cell sensitivity. Glucose and insulin concentration did not differ between early and late foaling mares. In conclusion, in horses, β-cell sensitivity to glucose is affected by season while insulin sensitivity during pregnancy decreases independent from season.