Hepatic glucose signals vagally modulate the cyclicity of gastric motility in rats

The gastroduodenal branch of the common hepatic vagus regulates voluntary lard intake, fat deposition, and plasma metabolites in streptozotocin-diabetic rats

The common hepatic branch of the vagus nerve negatively regulates lard intake in rats with streptozotocin (STZ)-induced, insulin-dependent diabetes. However, this branch consists of two subbranches: the hepatic branch proper, which serves the liver, and the gastroduodenal branch, which serves the distal stomach, pancreas, and duodenum. The aim of this study was to determine whether the gastroduodenal branch specifically regulates voluntary lard intake. We performed a gastroduodenal branch vagotomy (GV) on nondiabetic, STZ-diabetic, and STZ-diabetic insulin-treated groups of rats and compared them with sham-operated counterparts. All rats had high steady-state corticosterone levels to maximize lard intake. Five days after surgery, all rats were provided with the choice of chow or lard to eat for another 5 days. STZ-diabetes resulted in a reduction in lard intake that was partially rescued by either GV or insulin treatment. Patterns of white adipose tissue (WAT) deposition differed after GV- and insulin-induced lard intake, with subcutaneous WAT increasing exclusively after the former and mesenteric WAT increasing exclusively in the latter. GV also prevented the insulin-induced reduction in the STZ-elevated plasma glucagon, triglycerides, free fatty acids, and total ketone bodies but did not alter the effect of insulin-induced reduction of plasma glucose levels. These data suggest that the gastroduodenal branch of the vagus inhibits lard intake and regulates WAT deposition and plasma metabolite levels in STZ-diabetic rats.

Intraportal glucose infusion and pancreatic islet blood flow in anesthetized rats

The aim of the study was to evaluate whether a selective increase in portal vein blood glucose concentration can affect pancreatic islet blood flow. Anesthetized rats were infused (0.1 ml/min for 3 min) directly into the portal vein with saline, glucose, or 3-O-methylglucose. The infused dose of glucose (1 mg. kg body wt(-1). min(-1)) was chosen so that the systemic blood glucose concentration was unaffected. Intraportal infusion of D-glucose increased insulin release and islet blood flow; the osmotic control substance 3-O-methylglucose had no such effect. A bilateral vagotomy performed 20 min before the infusions potentiated the islet blood flow response and also induced an increase in whole pancreatic blood flow, whereas the insulin response was abolished. Administration of atropine to vagotomized animals did not change the blood flow responses to intraportal glucose infusions. When the vagotomy was combined with a denervation of the hepatic artery, there was no stimulation of islet blood flow or insulin release after intraportal glucose infusion. We conclude that a selective increase in portal vein blood glucose concentration may participate in the islet blood flow increase in response to hyperglycemia. This effect is probably mediated via periarterial nerves and not through the vagus nerve. Furthermore, this blood flow increase can be dissociated from changes in insulin release.

Innervation of the liver: morphology and function

Although it has been known for many years that the liver receives a nerve supply, it is only with the advent of immunohistochemistry that this innervation has been analysed in depth. It is now appreciated not only that many different nerve types are present, but also that there are significant differences between species, especially in the degree of parenchymal innervation. This has stimulated more detailed investigation of the innervation of the human liver in both health and disease. At the same time, functional studies have been underlining the important roles that these nerves play in processes as diverse as osmoreception and liver regeneration. This article briefly reviews current understanding of the morphology and functions of the hepatic nerve supply.

Control of motor and secretory functions of the stomach by a portal glucose signal

D-glucose solution injected into the portal vein influences efferent tonic activities of the vagal nerve innervating the stomach. This suggested the existence of a neural connection between hepatic vagal branch afferents and gastric vagal efferents in the brain. Considering this observation together with findings indicating that electrical stimulation of the proximal cut end of the hepatic vagal branch changes acidity in the gastric perfusing fluid or pressure within the stomach, it has been presumed that hepatic afferent signals related to glucose may regulate the motor or secretory function of the stomach through a change in central nervous activity. Recently active interaction between the portal and medullary glucose signals in gastric function was discovered, and analysis of the characteristic features of the system is in progress.

Glucose signal in the nucleus of the vagus nerve modulates the cyclicity of gastric motility in rats

The cyclicity and intensity of gastric motility were examined following glucose injection into the nucleus of the vagus nerve (X) or into the nucleus of the solitary tract (SOL) in anesthetized rats. Enhanced gastric motility caused by insulin administration was influenced by 4 mM glucose (500 nl) injected into the X; glucose provoked a shift in the cyclicity power spectrum without any change in intensity. The peak power spectrum shifted from 4.0-5.0 cpm to 2.0-3.0 cpm, but not significant change in the cyclicity power spectrum was seen when the same dose of glucose was injected into the SOL. It was also noted that the power spectrum response to 4 mM glucose injection into the X was not modified when 4 mM glucose was injected into the SOL simultaneously. The results suggest that the medullary glucose signal in the X differentially modulates the cyclicity of gastric motility independent of the SOL.