Lack of effects of an acute hepatic vagotomy on insulin and catecholamine responses in rats following exercise.

The purpose of the present investigation was to evaluate the effects of an acute hepatic vagotomy on hormonal responses to hyperglycemic and hypoglycemic challenges in rats previously submitted to an exercise protocol. Two experiments were conducted. In a first experiment, 8-week trained (TR) and untrained (UNTR) rats, subdivided into acutely hepatic vagotomized (HV) and sham-operated (SHM) groups, were submitted to an intraperitoneal glucose tolerance test (0.5 g/kg) under anesthesia. Training was associated with a tendency (P = 0.07) for blood glucose levels to be less elevated (at time point 10 min), and with a significant (P < 0.01) lower glucose/insulin ratio following the glucose injection. The HV did not have any effects on these responses. In a second experiment, non-exercised rats and a group of rats submitted to an acute bout of exercise (treadmill, 60 min, 26 m/min, 5% slope) 24 h before the experiment, each one of these two groups being subdivided into acutely HV and SHM groups, were submitted to an insulin-induced hypoglycemia protocol, under anesthesia. Blood glucose concentrations were decreased significantly (P < 0.01) to approximately 40 mg/dl in all groups 60 and 80 min after the insulin injection. Plasma adrenaline and noradrenaline levels were increased significantly (P < 0.01) in all groups. The catecholamine increase was not influenced by the HV or the acute exercise bout. The present results do not indicate an implication of the hepatic vagus nerve on hormonal responses to hyper and hypoglycemia following exercise.

Effects of dietary manipulations and glucose infusion on glucagon response during exercise in rats.

The purpose of the present investigation was to test the hypothesis that blood glucose concentration is not always related to glucagon response during exercise. Three groups of rats were submitted to a prolonged (3-h) swimming exercise. Two groups of rats had their normal food intake restricted by 50% the night before the experiment. One of these two groups of rats was intravenously infused with glucose throughout exercise to maintain euglycemia. The third group of rats swam while under normal dietary conditions. Plasma glucose, sampled in arterial blood, was reduced (P < 0.05) at 75, 105, 150, and 170 min of exercise (from approximately 130 to 110 mg/dl) in the food-restricted animals without glucose infusion, whereas a significant (P < 0.05) increase was measured in the two other groups during exercise. A significant (P < 0.01) difference in the mean integrated areas under the glucose-concentration curve was found only between the fed and the two food-restricted groups. Plasma insulin concentrations decreased (P < 0.05) similarly in all groups during exercise, whereas plasma epinephrine and norepinephrine concentrations increased significantly (P < 0.01) in all groups. Despite differences between groups in plasma glucose response during exercise, and despite the absence of any decrease in exercising blood glucose levels in at least two of the three groups, plasma glucagon responses were increased (P < 0.05) similarly in all groups (from approximately 250 to 550 pg/ml) at the end of the exercise period. The increase in glucagon was significant after 90 min of exercise in the food-restricted groups, with or without glucose infusion, but only after 140 min in the fed group. These results indicate that the glucagon response during exercise is not always linked to the decrease in plasma glucose.

Nutritional status influences the insulin response produced by acute hepatic vagotomy.

It has been established that the liver, through the afferent pathway of the vagus nerve, can influence insulin secretion. The purpose of the present study was to determine if this influence can be altered by different nutritional status aimed at inducing metabolic changes in the liver. This was carried out by comparing the insulin response 30 min after sectioning of the hepatic vagus branch in five experimental conditions: a normal (NCD) and a medium-fat (MFD) for 3 weeks, both with and without an overnight fast, and after an overloading liver glycogen protocol (normal diet). All experiments were conducted using anesthetized, adrenodemedullated rats. Blood was collected before and after (30 min) the hepatic vagotomy (HV) or a sham operation (SHM). As expected, liver glycogen levels were significantly (p < 0.01) lower in the fasted than in the fed condition, and were approximately 50% higher (p < 0.01) in the overloaded than in the normally fed condition. Basal insulin concentrations were also lower (p < 0.01) in the fasted compared to the fed groups, but were significantly (p < 0.01) increased by the medium-fat diet. Plasma glucose levels were significantly (p < 0.01) decreased by the overnight fast, but were not affected by the hepatic vagotomy. Plasma catecholamine concentrations were similar in all experimental conditions. Insulin concentrations were significantly (p < 0.05) increased by the HV, compared to SHM rats, in all experimental conditions (from 50% to 75%). The extent of this response was altered by the diet manipulations as the HV-induced insulin increase was greater (p < 0.01) in the MFD than in the NCD groups, whether fed or fasted. Furthermore, and contrary to our expectations, high hepatic glycogen contents did not reduce the insulin response to an acute hepatic vagotomy. These results indicate that the insulin increase induced by an acute HV is influenced by the prevailing metabolic conditions, and suggest that the hepatic vagus nerve exerts a constant inhibition on insulin secretion, independently of the hepatic glycogen content.

[Effect of removal of the adrenal medulla on the hormonal response induced by acute hepatic vagotomy]. / Effet d'une médullosurrénalectomie sur la réponse hormonale induite par une vagotomie hépatique aiguë.

The purpose of the present study was to determine the effects of an acute hepatic vagotomy on plasma insulin and catecholamines levels in fasted and fed rats with and without adrenodemedullation. Rats were first divided into two groups: adrenodemedullated (ADM) and non-adrenodemedullated rats, each group being subdivided into a normally fed and a fasted (14 hrs) group. Anesthetized rats were first ADM or sham operated, and a jugular catheter was inserted. The first blood sample was taken 30 min later (time 0). Rats were then hepatic vagotomized (HV) or sham operated, and the second blood sample was taken 30 min after the HV. Results (0 vs 30 min) indicate an increase in plasma glucose, epinephrine, and norepinephrine concentrations in rats non-adrenodemedullated. This effect was independent of the nutritional state and the integrity of the vagus nerve. A weakly significant (P < 0.07) effect of the HV on insulin response was found in ADM rats, independently of the fed or fast state (different levels of hepatic glycogen content). These results indicate the necessity of using ADM animals to study the effects of an acute HV. They also suggest that the hepato-pancreatic axis is not influenced by the level of hepatic glycogen.