Pancreatic insulin release and peripheral insulin sensitivity in German black headed mutton and Finish Landrace ewes: evaluation of the role of insulin resistance in the susceptibility to ovine pregnancy toxemia.

German black headed mutton (GBM) ewes are recognized as being highly susceptible to ovine pregnancy toxemia (OPT). The present trial was performed to evaluate whether a breed-dependent gestational diabetes mellitus-like insulin resistance during late pregnancy might be responsible for the high incidence of OPT in the GBM breed. Modified frequently sampled intravenous glucose tolerance tests (300 mg glucose and 0.03 IU insulin per kg of BW) were performed during mid and late pregnancy, the periparturient, and the dry period in polytocous 3.5-yr-old GBM and Finnish Landrace (FL) ewes fed according to their requirements. The corresponding blood samples were analyzed for plasma concentrations of glucose, insulin, nonesterified fatty acids (NEFAs) and β-hydroxybutyrate (β-HB). In addition, the baseline plasma cortisol concentrations were determined during late pregnancy. The BW gain during pregnancy and the rearing success did not differ between the GBM and FL ewes. In both breeds, late pregnancy was associated with decreased basal plasma glucose concentrations and enhanced glucose disposal, as well as elevated baseline β-HB values. Only in the GBM ewes did the plasma NEFA concentrations increase significantly during advancing pregnancy. Moreover, significantly higher baseline plasma NEFA concentrations as well as lower (P < 0.05) basal plasma glucose values were recorded during late pregnancy in the GBM than in the FL ewes. The first-phase insulin secretion, the peripheral insulin sensitivity, and the baseline plasma cortisol values did not differ between both breeds during late pregnancy. It is concluded that increased lipolysis during late pregnancy is a characteristic of the GBM breed. Moreover, elevated plasma NEFA concentrations may contribute to impaired pancreatic insulin response and peripheral insulin resistance in GBM ewes and thus promote OPT.