The role of the vagal innervation of the gut in insulin release in lactating ewes

The vagal innervation of the pancreas and upper gastrointestinal tract has been implicated in the release of insulin in response to hyperglycaemia in several mammalian species. Therefore the role of the vagal innervation of the abomasum and upper small intestine in the release of insulin and its effect on glucose metabolism was studied in lactating ewes because of the importance of glucose for milk synthesis and secretion in ruminants. Vagal fibres innervating the abomasum, pylorus and duodenum were sectioned in 3 lactating ewes (VAG); 3 control lactating ewes underwent sham- operations (SHAM). Seven and 14 days after the surgery, all animals were given an intravenous injection of glucose (100 mg/kg BW) and jugular blood samples collected for glucose and insulin determinations. Plasma glucose concentration (mean ± standard error) rose in the VAG group from a pre-injection level of 3.9 ± 0.2 mmol/L to 6.2 ± 0.2 mmol/ L and in the SHAM group from 3.7 ± 0.2 mmol/L to 6.3 ± 0.2 mmol/L. Maximum glucose concentration occurred at 5 minutes postinjection in both groups but declined more slowly in the VAG group than in the SHAM group and were significantly (P < 0.05) different at both 45 and 60 minutes post-injection. This resulted in a larger total area under the curve for the VAG group (P < 0.01) compared to the SHAM group. In the SHAM group, plasma insulin concentration rose from 685 ± 193 pg/mL to 3462 ± 649 pg/mL in response to the injection of glucose, whereas in the VAG group, plasma insulin response was smaller and reached a peak of 1530 ± 649 pg/mL from a basal level of 615 ± 193 pg/mL. The mean plasma insulin concentration after glucose injection in the VAG group was significantly (P < 0.05) lower than that for the SHAM group at both 5 and 10 minutes post-injection. These results indicate that vagal innervation of the stomach (abomasum), and duodenum modifies the pancreatic release of insulin in response to hyperglycaemia induced by intravenous glucose administration.