Alterations in gastric acid secretion following hepatic portal injections of D-glucose and its anomers

Glucose-responsive neurons in the brainstem

AREA POSTREMA: The influence on feeding behavior caused by ablation of the area postrema (AP) in rodents indicates the participation of this structure in the control of ingestion. Two types of glucose responsive neurons were identified in the AP: one is characterized by increasing the discharge rate in response to glucose (glucoreceptor type) and the other by decreasing the discharge rates in response to glucose (glucose sensitive type). These glucose responsive neurons may participate in glycemic homeostasis. NUCLEUS OF SOLITARY TRACT: The glucose responsive neurons exist within the caudal portion of nucleus of the solitary tract (NTS), a relay station in visceral afferents. Two types similar to the AP were also recognized. It is confirmed that hepatic glucose sensitive afferents terminate on some of the glucose sensitive neurons. This convergence may serve as a fail-safe mechanism. In addition, the NTS involving complex neural networks of excitatory and inhibitory interneurons may be concerned with integration of glycemic information. DORSAL MOTOR NUCLEUS OF VAGUS: Some neurons within the dorsal motor nucleus of the vagus (DMV) were identified as the glucose responsive ones. Both types were also recognized. It is confirmed by antidromic activation that these glucose responsive DMV neurons send their axons toward the gastric or coeliac branch that innervates either the stomach, intestine or pancreas. Some of the DMV neurons may subserve an enteroceptor function by themselves. They may also play a role in the brainstem neural control of glycemic homeostasis as the fail-safe mechanism.

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.

Secretion of gastric acid inhibited by oxytocin injected into the hypothalamic paraventricular nucleus in the rat

Gastric acid output was examined following oxytocin injection into the hypothalamic paraventricular nucleus (PVN) or into the vagus nucleus (X) of the medulla in rats with insulin-hypoglycemia. Gastric acid output was reduced following the injection of nanomolar quantities of oxytocin into these nuclei, and the response was dose-dependent. It was also noted that there was a synergistic action on the response by the peptide between PVN and X. The acid response was blocked by section of the vagus nerve at the subdiaphragmatic level or by prior administration of atropine sulfate. These observations suggest that oxytocin in the PVN is active in suppressing gastric acid secretion, and the vagal response is characteristic of activation of the PVN and X.

Gastric vagal fibres distribution and acid secretion induced by portal infusion of D-glucose

The vagal glucose signal pathway relevant to hepatic portal control of gastric acid secretion was examined in bilaterally adrenalectomized rats. The decrease in acid output after portal glucose injection was blocked by section of the hepatic branch which originates in the ventral vagus trunk below the diaphragm. After cervical vagotomy, the reduction in acid output was the same whether the section had been done on the right or the left side. The acid response was strongly inhibited by prior section of the dorsal vagus trunk at the celiac level. These results suggest that the hepatic glucose signal evoking an inhibitory action on the secretion of gastric acid has a specific pathway from the liver to the stomach, and that there is functional laterality in this pathway.

Gastric vagal functional distribution in the secretion of gastric acid produced by sweet taste

The sweet signal pathway relevant to lingual control of gastric acid secretion was examined in bilaterally adrenalectomized rats. The increase in the acid output after lingual glucose application was completely blocked by prior section of both sides of the vagus nerve at the cervical level. However, the reduction in the acid output showed no laterality following vagotomy on either side. At the subdiaphragmatic level, the acid response was mainly suppressed by section of the dorsal vagus trunk. Both sides of trunk vagotomy abolished the acid response. These results suggest that the sweet signal evoking gastric acid secretion has a specific pathway from the tongue to the stomach, and that there is functional laterality in this pathway in the visceral cavity.

Adrenergic innervation of the dorsal vagal motor nucleus: possible involvement in inhibitory control of gastric acid and pancreatic insulin secretion

Morphological and physiological approaches were used to investigate the possible role of an adrenergic innervation of the dorsal vagal complex in the control of basal gastric acid and pancreatic insulin secretion in the rat. The use of retrograde-tracing methods with injections of True Blue or of wheat-germ agglutinin into the stomach or pancreas first confirmed that most vagal preganglionic neurons innervating these two viscera are localized in the dorsal motor nucleus of the vagus, a number of them connected to both viscera. Light- and electron-microscopic investigation of the organization of adrenergic neuronal structures immunoreactive to phenylethanolamine-N-methyltransferase within this medullary nucleus further revealed: (i) that adrenergic axons establish profuse synaptic connections of the symmetrical type with perikarya and dendrites of this nucleus, and (ii) that several of these adrenergic fibers are connected with retrogradely labeled neurons innervating the stomach and/or pancreas. Lastly, measurements of basal gastric acid output and plasma insulin clearly indicated that both visceral secretions are rapidly and conspicuously decreased by local infusion of 2 nM adrenaline within the dorsal vagal complex. Taken together, these data strongly suggest that the adrenergic innervation of the dorsal medulla oblongata is involved in direct synaptic inhibition of the parasympathetic preganglionic neurons of the vagus that control secretion of gastric acid and pancreatic insulin.

Gastrin-17 injected into the hypothalamic paraventricular nucleus can induce gastric acid secretion in rats

Injections of picomolar quantities of gastrin-17 into the hypothalamic paraventricular nucleus increased gastric acid output in anesthetized rats. The response was dose-dependent, and it was blocked by atropin and by vagotomy. The same doses, injected intravenously, intraventriculary or into sites far from the nucleus, did not increase the output. Cholecystokinin-8 injected into the nucleus had no effect on the acid output.

Inhibition of gastric acid secretion in humans by glucagon during euglycemia, hyperglycemia, and hypoglycemia

The effect of intravenous infusion of glucagon in a dose of 85 pmol/kg/hr on submaximal pentagastrin-stimulated gastric acid secretion was studied in eight healthy volunteers. The study was repeated four times in each subject. By a glucose-insulin clamp technique blood glucose levels were kept constant during the studies at 5.0 mmol/liter (euglycemic clamp), 2.5 mmol/liter (hypoglycemic clamp), or 7.0 mmol/liter (hyperglycemic clamp) on three different days. Glucose and insulin were not infused during one control day study. During glucagon infusion, plasma glucagon levels increased but the level reached was lower during the hyperglycemic condition when compared to euglycemic and hypoglycemic conditions. Glucagon infusion inhibited gastric acid secretion during hyper- and euglycemic conditions but not during hypoglycemic conditions. Hyperglycemia caused a modest but significant inhibition of acid secretion. Serum gastric concentrations were unaltered during glucagon infusion regardless of the level of blood glucose. The present observations indicate that the inhibitory effect of glucagon is independent of the glucagon-induced hyperglycemia, but the effect is lost when blood glucose is below a certain limit, suggesting that blood glucose may have a modulating effect on gastric acid secretion.

D-glucose anomers in the nucleus of the tractus solitarius can reduce gastric acid secretion of rats

The infusion of alpha-, beta-, or equilibrated (alpha:36%, beta:64%) D-glucose solution in or in the vicinity of the nucleus of the tractus solitarius decreased gastric acid output caused by insulin in rats with bilateral adrenalectomy. This effect was not reproduced after vagotomy at the cervical level. Of the three forms of D-glucose solution, the effect of beta-D-glucose was greatest. The infusion of equitonic NaCl, however, produced no change in the acid output. These results suggest that blood beta-D-glucose may predominantly activate a brain mechanism which through the vagus nerve modulates gastric acid secretion at the medullary level.

Alterations in gastric acid secretion following hepatic vagotomy at a stage of development in rats

The relation of gastric acid secretion to the hepatic vagal section was examined in consideration of developmental stages of a rat. Acid outputs in rats deprived of food for 22 hr before the experiment were estimated with or without insulin. The animals were classified into five groups according to their body weights at the experiments (50, 100, 200, 300 and 400 g). Hepatic vagotomy was effective in decreasing acid output in all of the groups treated with food deprivation and insulin, and it was found that there was a close relationship between the output and glucose concentration in the portal blood. In rats treated only with food deprivation, hepatic vagotomy produced different effects for the five groups; the vagotomy failed to cause acid response when the rats weighed about 300 and 400 g, while in the animals weighing about 50, 100 and 200 g acid outputs were reduced following the vagotomy. It was noted that the sensitivity to hypoglycemia in the acid output was greater in the young rats than the older ones. Results suggest that function of the hepatic vagal nerve may be prominent in modulating acid secretion in an earlier stage of development when the animals are most sensitive to hypoglycemia.

Changes in water intake following hepatic vagotomy in young rats

Changes in water intake after hepatic vagotomy were examined in the body weights of 3 different rats (about 100, 200 and 280 g). The vagotomy reduced water intake only in the 100-g rats.

Changes in food intake after hepatic vagotomy at a stage of development in rats

Changes in food intake after hepatic vagotomy were examined in the body weights of three different rats (about 100, 200 and 280 g). The vagotomy reduced food intake only in the 100-g rats.

Vagal afferent stimulation-evoked gastric secretion suppressed by paraventricular nucleus lesion

Studies were performed to evaluate the possibility that the paraventricular nucleus (PVN) of the hypothalamus modulates gastric acid secretion by changing the sensitivity of the gastric secretory control mechanism to vagal afferent input. Under pentobarbital anesthesia, 17 rats were prepared with esophageal and pyloric catheters such that the stomach could be perfused continuously on a flow-through basis. Thus, acid secretion could be monitored throughout the experiment. Stimulating electrodes were attached to the central cut end of the cervical vagus nerve. Unilateral stimulation of cervical vagal afferents resulted in a substantial increase in gastric acid secretion. This vagal afferent-mediated increase in acid outflow was suppressed following a single PVN lesion ipsilateral to the side of afferent stimulated output. Given the nature of PVN connections with brainstem regions responsible for the elaboration of vago-vagal reflexes, our results suggest that the PVN may control gastric acid outflow by changing the gain of gastric vago-vagal reflexes.

Different effects of D-glucose anomers on enhanced secretion of gastric acid in rat

The injection of alpha-, beta-, or equilibrated (alpha: 36%, beta: 64%) D-glucose solution into the cranial side of the carotid artery decreased gastric acid output caused by insulin in rats with bilateral adrenalectomy. This effect was not reproduced after vagotomy at the cervical level. Of the 3 forms of D-glucose solution the effect of beta-D-glucose was greatest. The injection of isotonic NaCl solution, however, produced no change in acid output. These results suggest that blood beta-D-glucose may play a predominant role in activating a brain mechanism which controls gastric acid secretion via the vagus nerve.

Reflex control of the autonomic nervous system activity from the glucose sensors in the liver in normal and midpontine-transected animals

Intraportal venous infusion of glucose resulted in a significant post-infusion decrease in the mean discharge rate in the adrenal nerve and the hepatic branch of the splanchnic nerve. Intraportal venous infusion of the same amount of glucose caused a significant increase in discharge rate of the pancreatic branch of the vagus nerve. After section of the hepatic branch of the vagus nerve, the same procedure induced no change in discharge rates in any of these nerves. Repetitive electrical stimulation of the central stump of the hepatic branch of the vagus nerve also caused reflex changes in efferent discharge rates in these three nerves, an increase in discharge rate in the hepatic branch of the splanchnic nerve and adrenal nerve, and a decrease in the pancreatic branch of the vagus nerve in the normal animals and also in midpontine-transected animals. The reflex network may consist of glucose-sensitive hepatic afferents, a reflex center in the hindbrain, and vagal pancreatic efferents, adrenal and splanchnic-hepatic efferents. This network contributes a neural regulatory mechanism to the control of blood glucose levels.

Inhibition of gastric acid secretion elicited by D-glucose anomers in man

Acid outputs from the stomach were measured after venous administration of D-glucose and its optical anomers in men with insulin hypoglycemia. A significant decrease in gastric acid output was noted after the administration of 277 mM alpha-D-glucose, 277 mM optically equilibrated D-glucose consisting of 36% alpha-anomer and 64% beta-anomer, or 277 mM beta-D-glucose. The effect of beta-D-glucose was most potent in the three forms of D-glucose. NaCl solution, however, produced no appreciable change in the acid outputs. Our findings suggested that, in humans, beta-D-glucose in the blood may play an important role in the activation of glucose-sensitive mechanisms controlling vagally mediated secretion of gastric acid.

Glucose-sensitive afferent nerve fibers in the liver and their role in food intake and blood glucose regulation

The discharge rates of glucose-sensitive hepatic vagal afferents and glucose concentrations in the portal vein showed an inverse relationship in experiments performed using isolated and perfused liver preparations and in in vivo experiments on the guinea pig. The rate of discharge of these afferents in the rat was facilitated following administration of insulin and inhibited after application of glucagon and CCK. Hepato-pancreatic, -adrenal and -hepatic reflexes initiated through vagal hepatic afferents were demonstrated in the rabbit, guinea pig or rat. The results obtained suggest that glucose-sensitive vagal afferents from the liver play an important role in the control of food intake as well as in the control of blood glucose levels.

The afferent and preganglionic parasympathetic innervation of the rat liver, demonstrated by the retrograde transport of horseradish peroxidase

The afferent and parasympathetic preganglionic innervations of the rat liver were investigated by the use of retrograde transport of horseradish peroxidase (HRP). Vagal nerve fibers reach the rat liver by way of the left and right hepatic nerves, which originate from the homonimous abdominal vagal trunks. Three different experimental protocols were used: (i) intraparenchymal HRP injections; (ii) retrograde HRP injection through the common bile duct; (iii) HRP application to the central end of the severed hepatic nerves. The technical problems inherent in these 3 methods were experimentally investigated, with regard to the possible leakage of HRP from the liver after retrograde injection. It is concluded that leakage of HRP occurs, but it is not sufficient to cause non-specific labeling. Neurons of the lower thoracic dorsal root ganglia (DRG) are bilaterally labeled following intraparenchymal and retrograde HRP injections. Bilateral labeling of the nodose ganglia (NGs) following retrograde injection is still observed after subdiaphragmatic section of the left abdominal vagus, whereas cervical transection prevents labeling of the ipsilateral NG. Labeling produced by exposure of the left hepatic nerve to HRP is prevented by subdiaphragmatic transection of the left abdominal vagus. Efferent neurons located bilaterally in the dorsal motor nucleus (DMN) and in the left nucleus ambiguus (NA) are labeled following retrograde HRP injection. Only ipsilateral labeling is observed after HRP application to the cut left hepatic nerve. HRP exposure of the left abdominal vagus yields bilateral labeling in both DMN and NA. It is concluded that: (i) the afferent innervation of the rat liver is provided by the lower thoracic DRG and by the NGs of both vagi, mostly by the left; partial crossing of vagal afferent fibers takes place at thoracic level; and (ii) the liver receives efferent fibers bilaterally from the DMN and from the left NA; the DMN neurons project to the liver via the homolateral hepatic nerves, and those of the NA via the left hepatic nerve.

Central connections of the hepatic branch of the vagus nerve: a horseradish peroxidase histochemical study

The hepatic branch of the vagus nerve has been implicated as an important source of afferent input controlling both physiological and behavioral homeostasis. In addition, it is clear that parasympathetic efferents to the liver can significantly alter hepatic functions. In order to begin physiological studies on the nature of hepatic afferent and efferent relations, it will be necessary to understand the central anatomical organization of the components of this small visceral nerve. By carefully exposing and dissecting the hepatic branch of the vagus and applying crystalline horseradish peroxidase (HRP) to it, we were able to elucidate a predominant pattern of afferent terminations within the left subnucleus gelatinosus, the medial division of the left solitary nucleus and the left lateral edge of the area postrema. Efferent nuclei were concentrated in the left dorsal motor nucleus of the vagus (DMN) with a few scattered neurons located in the right DMN as well as the left anterior nucleus ambiguous.