Insulin secretion and intestinal peptides during lactation in sheep

Intravenous infusions of glucose and/or glucagon-like peptide-1(7-36)-amide (GLP) or somatostatin-28 (S28) were administered to dry and lactating sheep and changes in plasma glucose and serum insulin were followed before, during and after infusion. Basal serum insulin concentrations were significantly lower in lactating sheep but there was no significant difference in plasma glucose concentrations. During glucose infusion, changes in serum insulin concentrations were diminished by comparison with those in dry animals. GLP stimulated insulin secretion during hyperglycaemia in both dry and lactating sheep but, proportionately, the response was significantly greater in the lactating animals. S28 inhibited glucose-stimulated insulin secretion in both dry and lactating sheep and there was no significant difference in the extent of the inhibition between the two physiological states. S28 infusion also inhibited the secretion of GLP from the intestinal tract and this effect was significantly greater in the lactating animals. The results demonstrate a difference in response in the lactating animal to GLP compared with S28. A possible explanation of the difference is a decreased sensitivity (i.e. increased Km) of the pancreas to the insulinotropic effects of GLP but an increased sensitivity to the inhibitory effects of S28 at tissues other than the pancreas in lactation.