Diminished insulinotropic effects of gastrin-releasing peptide in pregnant sheep are reproduced by progesterone treatment of nonpregnant animals.

Abstract

Gastrin-releasing peptide (GRP) is insulinotropic in several species, but possible alterations in this action during pregnancy have not been explored. Therefore, changes in plasma insulin and glucagon concentrations were examined in response to exogenous GRP in nonpregnant and pregnant sheep that were feed restricted, fed ad libitum, or infused with glucose. Administered GRP provoked insulin and glucagon release in pregnant and nonpregnant fed animals. This effect was reduced with feed restriction and potentiated in the presence of glucose. The responses were less in pregnant than in nonpregnant animals. Interpretation of this result, however, was confounded by lower plasma immunoreactive concentrations of GRP achieved in pregnant than in nonpregnant sheep in response to the same infusion rate (picomoles per kg BW) of exogenous GRP. Therefore, nonpregnant ovariectomized sheep were treated with estradiol (E2) or E2 plus progesterone (P4), given to reproduce circulating steroid levels in pregnancy, in order to examine insulinotropic responses to GRP in the absence or presence of concurrent glucose administration. Similar plasma GRP and glucose concentrations were achieved by exogenous infusions in the different groups of nonpregnant animals treated with steroids and in control animals not receiving steroids. E2 alone did not alter the insulin response to GRP compared to control, but E2 plus P4 treatment attenuated these responses to values similar to those in pregnant animals. We conclude that GRP is insulinotropic in sheep, and this action is modulated by the circulating glucose concentration. The response to GRP is less in pregnant than in nonpregnant animals, and this attenuation is mimicked in nonpregnant animals treated with E2 plus P4. We suggest that in species such as sheep, GRP is a potentiator of glucose-stimulated insulin release. This action is diminished in response to the altered endocrine environment of pregnancy and may contribute to the metabolic changes that occur at this time.