Vagal Integrity Research Articles

Vagal cooling blocks circulating neuropeptide Y (NPY), peptide YY (PYY), and pancreatic polypeptide (PP) release

Neuropeptide Y (NPY), peptide YY (PYY), and pancreatic polypeptide (PP) are regulatory peptides that constitute a new family of gastrointestinal and neural peptides. The influence of vagal integrity on NPY, PYY, and PP basal and postprandial release was evaluated using a new technique of reversible cryogenic cervical vagal blockade in an awake canine model. Cooling coils were placed around bilateral cervical vagal trunks in five dogs along with omocervical arterial catheters. Vagal transmission was monitored by pulse and arterial pressure monitoring. Cryogenic blockade of vagal nerves was performed by circulating a mixture of 0°C ethanol and water through the cooling coils. NPY, PYY, and PP were measured using standard radioimmunoassays. Vagal cooling decreased basal NPY and PYY levels ( P < 0.05) but not PP. After a standard meal, vagal cooling blocked the postprandial rise seen in circulating NPY and PP ( P < 0.05). These data demonstrate a technique of reversible vagal blockade to evaluate the role of cervical vagal integrity in gastrointestinal endocrinology. Cryogenic vagal blockade inhibits the postprandial rise of circulating PP into the circulation. Vagal pathways appear to contribute to fasting activity of PYY and NPY releasing cells. Inhibited meal-stimulated release of NPY supports a role for vagal modulation of postprandial NPY release into the circulation.

Meal-stimulated absorption of water and electrolytes in canine jejunum

After a meal, the absorption of water and electrolytes from the jejunal lumen increases. This meal-induced jejunal absorption occurs in jejunal segments out of normal gastrointestinal continuity. The experimental model used 25-cm proximal jejunal Thiry-Vella loops in awake dogs (n = 72 observations) to evaluate the mechanisms involved in meal-induced jejunal absorption, seeking to define the source or sources of the proabsorptive signal. Specifically, we evaluated the jejunal absorptive response to a standard meal, a standard meal plus cholinergic blockage using atropine, a sham-fed meal, a gavage-fed meal, and gastric distension with balloon and gavage water. Both the standard meal and the gavage-fed meal induced a prompt, sustained, and significant (P less than 0.0001) increase in the absorption of H2O, Na+, and Cl-. Atropine significantly reduced the magnitude of the postmeal absorptive response (P less than 0.05) compared with the standard meal alone. The sham-fed meal, gastric balloon distension, and gavage water did not alter jejunal absorption. Vagal nerve integrity after cervical esophageal manipulation was verified by gastric acid output and gastrin response to stimuli. These data support a role for cholinergic modulation of meal-stimulated jejunal absorption via a cephalic-phase-independent and gastric-distension-independent mechanism.

Upper gastrointestinal involvement in diabetes mellitus: study of esophagogastric function.

The aim of our study was to evaluate in 18 diabetic patients (11 with and 7 without evidence of autonomic neuropathy as revealed by common cardiovascular tests) alterations indicative of autonomic nervous involvement of the gastrointestinal tract, independently of the presence of suggestive symptoms. All patients, without evidence of obstructive or mucosal pathology of the upper gastrointestinal tract, underwent the following: 1) study of gastric emptying time of nonabsorbable radiopaque markers (90, 120, 150 and 210 min); 2) study of gastric acid secretion: basal (BAO) and peak (PAO) acid output after sham-feeding (PAOSF) and peak acid output after pentagastrin (PAOPENT).PAOSF/PAOPENT ratio is an index of vagal integrity; 3) esophageal manometry. Our data confirm that a delayed gastric emptying of undigestible solids is a frequent finding in diabetic subjects. This was highly significant (p less than 0.01) at 150 min after a standard meal, in patients with signs of autonomic neuropathy and was often associated with asymptomatic esophageal motor abnormalities. No correlation was found with index of vagal integrity, hormonal pattern and degree of glycemic control. Autonomic neuropathy cannot be considered the only explanation for gastric and esophageal abnormalities in decompensated diabetes.

Effect of Galanin and Vagal Integrity on Insulin Release in Dogs

In four conscious dogs infusions of glucose (1 g/kg/h) alone or glucose and galanin (2 micrograms/kg/h) were undertaken during cryogenic vagal blockade at -2 degrees C or following atropine (100 micrograms/kg i.v.). When compared to parenteral glucose alone, the addition of galanin substantially elevated plasma glucose and blunted plasma insulin responses. Vagal blockade or atropine failed to alter these effects of galanin on plasma insulin or glucose responses. Moreover, plasma levels of somatostatin, pancreatic glucagon, pancreatic polypeptide, growth hormone, and cortisol were unaffected by galanin infusions. Thus, the inhibition of plasma insulin responses to glucose by galanin is mediated by a nonvagal, noncholinergic mechanism and is independent of changes in either plasma pancreatic glucagon or somatostatin levels. Galanin at the dose employed in the study may have direct and selective actions on the B cell and thus play an important role in the neuromodulation of insulin release.

Vagal influences on parasternal intercostal muscle inspiratory shortening during hypercapnia and airway occlusion

To determine whether increases in electromyographic activity (EMG) are necessary for respiratory muscle shortening to occur during airway occlusion, respiratory changes in parasternal intercostal musle length were measured using sonomicrometry in 11 anesthetized dogs during unoccluded (UB) and occluded (OB) breaths both before and after vagotomy. During UB the extent of parasternal intercostal inspiratory shortening was greater after than before vagotomy both during oxygen breathing and during progressive hyperoxic hypercapnia. The relation between parasternal shortening, parasternal EMG, and tidal volume was not substantially affected by vagotomy. During OB parasternal intercostal EMG increased significantly compared to UB when the vagi were intact, but airway occulusion did not significantly change EMG activity post-vagotomy. However, both before and after vagotomy the parasternal intercostal shortened during OB in all animals. Parasternal intercostal inspiratory shortening during OB as a % of that during UB was significantly greater before compared to after vagotomy during oxygen breathing and moderate hypercapnia, but vagal integrity made no significant difference at high CO 2. These result suggest that (1) pulmonary stretch receptor afferents inhibit parasternal intercostal inspiratory shortening but in proportion to their inhibitory effects on parasternal intercostal EMG and tidal volume, and (2) even when the EMG stays constant the parasternal intercostal muscle does not contract isometrically during occluded breaths.

Influence of vagal integrity on gastrin and somatostatin release in dogs

Plasma gastrin and somatostatin responses to ingestion of a solid meal, to insulin hypoglycemia, and to intravenous infusion of gastrin-releasing peptide were measured in 4 conscious dogs with and without bilateral cryogenic blockade of the cervical vagus nerves. Vagal cooling to −2 °C abolished mealstimulated rises in plasma gastrin and somatostatin. Atropine did not modify the gastrin response to cooling but bethanechol reduced the magnitude of inhibition to 37% ± 9% without influencing plasma somatostatin. Gastrin-releasing peptide elevated postprandial plasma gastrin during vagal blockade to levels comparable to those with the vagus intact but did not alter the nadir plasma somatostatin response. The plasma gastrin and somatostatin rises associated with insulin hypoglycemia were similarly inhibited by cooling to −2 °C. Cooling to 12 °C, which selectively blocks vagaJ inhibitory pathways, had no effect on meal-stimulated gastrin release and partially decreased the plasma gastrin response to insulin hypoglycemia. Thus, gastrin release by food and by insulin hypoglycemia is mediated by a vagal nonmuscarinic excitatory pathway that is independent of changes in circulating plasma somatostatin but may include participation by the candidate neurotransmitter gastrin-releasing peptide.

Small intestinal motility in fasted and postprandial states: effect of transient vagosympathetic blockade.

We investigated vagal control of the migrating myoelectric complex (MMC) and postprandial pattern of the canine small intestine. Gastric and small intestinal motility were monitored in six conscious dogs. The vagosympathetic nerves, previously isolated in bilateral skin loops, were blocked by cooling. To feed, a meat-based liquid food was infused by tube into the gastric fundus. MMC phases I, II, III, and IV were observed in the fasted state. On feeding, the fed pattern appeared quickly in the proximal small bowel but was delayed distally. Vagal blockade abolished all gastric contractions and spiking activity as well as the small bowel fed pattern. During vagal blockade, the small bowel exhibited MMC-like migrating bursts of spikes in both the fasted and fed states. The migration and cycling of these bursts were not significantly different from the MMC, but the duodenal and jejunal phase II was absent or shortened. On termination of vagal blockade, normal fasting or fed activity reappeared but with a delay in the fed pattern distally. We conclude: the ileum is the least sensitive to vagal blockade; the fasting vagal influence is exerted primarily on phases I and II of the duodenal and jejunal MMC; the fed pattern throughout the entire small bowel is normally dependent upon vagal integrity; the phase III-like bursts of activity seen during vagal blockade likely represents the intrinsic small bowel MMC, which is vagally independent.

Vagal control of canine postprandial upper gastrointestinal motility

The role of the vagus nerves in the control of postprandial motility in the upper gastrointestinal tract was investigated in four dogs by use of a bilateral cervical cooling blockade technique. On administration of food, the fasting migrating motor complex (MMC) was replaced by the postprandial (feeding) pattern. Feeding pattern duration varied in a dose-dependent manner with either total volume or calories of food. During the feeding pattern, oscillations in lower esophageal sphincter (LES) pressure occurred at time intervals equivalent to the MMC cycle period. Twenty-one control feeding experiments and 17 postprandial vagal blockade experiments were performed, with a minimum of three of each type in each dog. Vagal blockade, initiated at times ranging from 15 min to 4 h after feeding and maintained for up to 5 h, abolished the postprandial activity in the upper gastrointestinal tract. During postprandial vagal blockade, LES pressure was abolished and bursts of contractions were observed only in the upper small bowel, a pattern resembling that observed during vagal blockade in the fasted state. These bursts occurred at the expected times relative to, and their cycle period was not significantly different from, that of the MMCs recorded prior to feeding. Vagal blockade started prior to feeding prevented initiation of the fed pattern, which appeared immediately on termination of the blockade. We conclude that initiation and maintenance of the postprandial pattern in the upper gastrointestinal tract with concurrent inhibition of the fasting MMC normally require vagal integrity. The "clock" controlling the MMC cycle period is not reset by feeding, but its effect on motility is suppressed.

Tachypnoeic response to amyl nitrite inhalation in the rabbit

Intratracheal inhalation of amyl nitrite, a non-specific smooth muscle relaxant, in the pentobarbitone/urethane anaesthetized rabbit caused reductions in tidal volume and both inspiratory and expiratory times, without a preceding apnoea, that were independent of the associated hypotension and of reflex influences from the carotid sinus region but dependent on supra-abdominal vagal integrity. In artificially ventilated, paralyzed rabbits amyl nitrite caused a pronounced sensitization of pulmonary stretch receptors (PSRs during the inflation phase, typically with a reduction in the level of activity during the deflation phase. The time course of the change in the pattern of PSR activity paralleled that of the tachypnoeic response. The sensitization of a small sample of rapidly adapting ‘irritant’ receptors was of a significantly shorter duration. A unitary analysis of non-myelinated vagal afferents was not attempted. The sensitization of these vagal afferents cannot be attributed to the smooth muscle relaxant properties of amyl nitrite since other relaxants (sodium nitrite, sodium nitroprusside, phentlamine) did not posses this capacity, and such properties would be expected to diminish their activities.

The role of vagal integrity in gastrin releasing peptide stimulated gastroenteropancreatic hormone release and gastric acid secretion.

The role of the vagus nerve in the control of gastrin releasing peptide (GRP) stimulated gastroenteropancreatic hormone release and gastric acid secretion was investigated in four conscious gastric fistula dogs using a technique of bilateral cryogenic vagal blockade. A 90-min infusion of GRP at a dose of 400 pmol X kg-1. h-1 produced significant elevations in plasma levels of gastrin, motilin, GIP, enteroglucagon, insulin, pancreatic glucagon, pancreatic polypeptide and VIP. Vagal blockade reversibly inhibited the rise of plasma PP and significantly blunted the elevation of plasma VIP. However, the GRP stimulated response of the other hormones investigated was not modified by vagal blockade. Similarly, the substantial secretion of gastric acid observed with GRP was not influenced by vagal blockade. Thus GRP acts predominantly via mechanisms which are independent of vagal integrity, findings that are in support of a major role for the local neuromodulation of hormone release and gastric acid secretion.

Relationship between porcine motilin-induced migrating motor complex-like activity, vagal integrity, and endogenous motilin release in dogs

Both the vagus nerve and motilin have been implicated in the initiation of phase III of the fasting migrating motor complex. To investigate the contribution of each, the effects of intravenous porcine motilin, vagosympathetic nerve blockade, and atropine were assessed. Intralumenal pressures of the lower esophageal sphincter, stomach, and upper small intestine, and plasma motilin levels were monitored. Porcine and canine motilin in plasma were distinguished by radioimmunoassay using two different antibodies. Injection of porcine motilin (75–370 pmol), initiated dose-dependent phase III-like motor activity in the lower esophageal sphincter, stomach, and small bowel if the vagi were intact; if the vagi were blocked, activity was initiated in the small bowel only. Moreover, a significant (p < 0.001), dose-dependent peak in canine plasma motilin was observed after the onset of the induced motor activity when the vagi were intact or blocked, with plasma motilin peaks comparable to those occurring spontaneously. In both intact and vagally blocked dogs, atropine abolished both the spontaneous motor activity and associated rise in motilin level, and also abolished porcine motilin-induced activity. However, a diminished, but significant (p < 0.01) peak in porcine motilin-induced canine motilin persisted in the presence of atropine. These results in dogs indicate that while spontaneous phase II motility in the upper gastrointestinal tract and phase III activity in the lower esophageal sphincter and stomach are dependent on vagally mediated excitatory pathways, spontaneous and induced phase III motor activity in the small intestine are dependent on nonvagal cholinergic innervation. Canine motilin release induced by porcine motilin is mediated primarily by a nonvagal cholinergic (muscarinic) pathway, with minor contributions from vagal noncholinergic, and nonvagal noncholinergic mechanisms. Because spontaneous or induced motilin release peaks well after the onset of phase III motor activity, it is unlikely that motilin is the primary factor responsible for initiation of the migrating motor complex in the dog. Motilin may, however, modulate motility produced by preexisting neural excitation.

Vagal control of colonic motility in the anaesthetized ferret: evidence for a non-cholinergic excitatory innervation.

Spontaneous colonic motility in the urethane-anaesthetized ferret consists of two distinct types of contraction which correspond to the patterns recorded myoelectrically in conscious animals. This motility was abolished or greatly reduced when nervous conduction was prevented in the cervical vagi by cooling to below 4 degrees C. On rewarming the nerves the colonic motility returned, after a short latency, to the pre-cool level. Atropine transiently abolished colonic motility. On its return the motility was significantly reduced but still sensitive to vagal integrity. Thus the atropine-resistant colonic motility was also abolished or markedly reduced by cooling the cervical vagi to below 4 degrees C. On rewarming there was a longer latency for the return of motility compared to that before atropinization. Electrical vagal stimulation produced, after a short latency, large-amplitude colonic contractions. Following atropine, the short-latency response to electrical vagal stimulation was replaced in the majority of animals by a long-latency response whose characteristics were quite different from those of the cholinergic response. These results are consistent with the vagus containing two functional motor pathways to the colon, one to cholinergic post-ganglionic neurones and the other operating via a non-cholinergic mechanism.

Behavioral effects of vagotomy in humans.

Animal experiments and clinical observations have demonstrated significant effects of vagotomy on body weight. Weight loss or inability to regain are partly due to impaired motility and secretomotor activity of the vagus nerve causing disturbances in digestion which, however, are not sufficient to explain most of the weight deficit after vagotomy in animals or morbidly obese patients. The body weight deficit is also due to reduced caloric intake with changes in the quantity and quality of food and liquid intake, the latter accounting for more than one-third of the total reduction in caloric intake. Obese patients have consistent decreases in hunger ratings after vagotomy and also reveal changed hedonic ratings and estimations of taste intensity. Validation of vagotomy studies requires tests of vagal integrity to confirm the completeness of the surgery and rule out regeneration of nerve tissue or recruitment of function. Tests of completeness of vagotomy are difficult to perform and evaluate in morbidly obese patients due to insulin resistance. The finding of an inadequate gastric acid response to insulin hypoglycemia implies a defect hypothalamic response to hypoglycemic stress in these patients. A new postoperative test of completeness of vagotomy based on disrupted drinking after intravenous hypertonic saline challenge is introduced as an attractive alternative to the potentially hazardous insulin test.

Reduced sinus arrhythmia in diabetic autonomic neuropathy: diagnostic value of an age-related normal range.

A study was carried out to establish a normal range for use in a deep breathing test of cardiac vagal integrity in diabetes mellitus. The change in heart rate resulting from taking a deep breath was recorded in 174 healthy subjects aged 16-89 years. Results were expressed as the ratio of the longest R-R interval during expiration to the shortest R-R interval during inspiration. This ratio declined appreciably with age (p less than 0.001) but was not significantly related to resting heart rate. An age-related normal range was constructed and its diagnostic value investigated in 134 diabetics (aged 15-70 years) with various degrees of neuropathy. The sensitivity for indicating autonomic dysfunction was high: six false-negative and one false-positive result were obtained. Measurement of this age-dependent ratio, which may be made with any electrocardiographic apparatus, provides a simple, accurate diagnostic screen for autonomic neuropathy in the clinic.

Truncal vagotomy abolishes the somatostatin response to insulin-induced hypoglycemia in man.

The mechanism whereby insulin-induced hypoglycemia stimulates release of immunoreactive somatostatin (SRIF-LI) into the peripheral circulation in man is unknown. We have measured the plasma SRIF-LI response to insulin-induced hypoglycemia in 16 healthy subjects and five subjects with prior truncal vagotomy. Mean nadir of plasma glucose was similar in the two groups (37 +/- 5 and 34 +/- 2 mg/dl in the control group). Hypoglycemia induced a brisk rise in plasma pancreatic polypeptide (hPP) concentrations in healthy subjects (maximum concentration 1563 +/- 245 pg/ml) whereas in none of the postvagotomy subjects was there a significant change in hPP concentrations, indicative of completeness of the truncal vagotomy. In healthy subjects SRIF-LI concentrations rose for a basal of 168 +/- 19 pg/ml to a maximum of 254 +/- 30 at 39 +/- 4 minutes (p less than 0.005) with an incremental area of 2.8 +/- 1 ng . min/ml above basel. In vagotomized subjects, the mean basal SRIF-LI concentration of 166 +/- 22 pg/ml was not significantly different from that in healthy subjects. After insulin injection, SRIF-LI concentration fell with a net decrement of -3.2 +/- 1 ng . min /ml below the basal. It is concluded that the SRIF-LI response to insulin-induced hypoglycemia is dependent upon vagal integrity. Section of the vagus unmasks a suppressive effect of insulin action or its metabolic or hormonal consequences on the concentration of SRIF-LI in plasma.

The role of the antrum in the modulation of plasma pancreatic polypeptide levels

The role of pancreatic polypeptide (PP) in gastrointestinal physiology is not well understood. In order to elucidate its function, we have undertaken a detailed investigation into the modulation of its release into plasma. This study was designed to examine the role of the antrum in the mediation of plasma pancreatic polypeptide release by comparing the effect of a histologically proven total antrectomy on the release of pancreatic polypeptide by food ingestion or bombesin infusion. Extragastric vagal integrity was maintained by meticulous preservation of the omenta of the greater and lesser gastric curvatures. Basal PP plasma levels were significantly decreased throughout the 6-month postantrectomy time period in which the animals were studied. The meal-stimulated release of PP was abolished by antrectomy and sequential meal tests over a 6-month period indicated no restoration of meal-stimulated PP release. Bombesin-stimulated PP release was unimpaired except in the immediate postoperative period. Within 2 months of surgery the PP response to bombesin infusion had been restored to preoperative levels. The data are thus consistent with the hypothesis that the gastric antrum plays a role in maintenance of plasma pancreatic polypeptide levels. The evidence provided by this study supports the suggestion that an antral factor responsible for modulation of PP release might well be a bombesin-like peptide.

Bombesin: A vagally independent stimulator of gastrin release

In this study we demonstrate that bombesin is a potent stimulant of canine gastrin and gastric acid secretion. Bombesin was shown to release almost fourfold more gastrin than a meal by a mechanism which is independent of vagal integrity. An exogenously administered cholinomimetic agent released only minimal amounts of gastrin after vagotomy and virtually none in the intact canine model. The combination of bombesin and bethanechol caused a significant increase in acid secretion compared with bombesin alone and was associated with a marked inhibition of gastrin release both before and after vagotomy. Gastric acid secretion in response to bombesin was not significantly altered by vagotomy. We interpret these data to reflect the sensitivity of bombesin-stimulated gastrin release to gastric luminal pH, although they also suggest the existence of a cholinergic inhibitory mechanism for gastrin release. The data provided by this study are consistent with the hypothesis that bombesin might function as a paracrine neuromodulator of gastrin release. In view of its presence in the nerves and mucosa of human stomach and its potent actions on gastrin release and gastric acid secretion, bombesin may play a role in gastric physiology.

Pancreatic polypeptide. Release following surgery for duodenal ulcer disease.

The pancreatic polypeptide (PP) response to food has been measured by radioimmunoassay in patients with duodenal ulcer and 3 months following proximal gastric vagotomy (PGV), 18 and 49 months following truncal vagotomy and pyloroplasty (TV), 35 months following Billroth II gastrectomy (BII), and 35 months following truncal vagotomy and antrectomy (TV&A). Basal PP levels and those in response to food were similar in DU and PGV, but these values were significantly higher than those 18 months or 49 months after TV, or after BII and TV&A. The responses in the latter four groups were similar and in particular, the levels 18 and 49 months after TV were the same. These results indicate that the release of PP by food in unoperated patients consists of two phases, a primary phase which requires both intact vagi and an intact stomach and a secondary phase which also depends on vagal innervation and normal gastric anatomy. Disturbances in vagal innervation or gastric integrity lead to profound changes in PP release which may be due to interruption of neural arcs or loss of gastric hormones. Unlike others, we have been unable to document a return of PP secretion towards normality with time after TV.

Vagal influences on the pancreatic response to intraluminal trypsin.

The significance of vagal influences on the pancreatic response to intraluminal trypsin was investigated in conscious rats surgically prepared with bile-pancreatic fistulae. Vagotomy as well as cholinergic blockage depressed the hypersecretion of protein in fistula rats. Contrary to in control rats intraduodenal trypsin infusion did not change the protein output after vagotomy or cholinergic blockage. Indirect vagal stimulation induced by insulin hypoglycemia did not further increase the protein output in rats with their bile-pancreatic juice deviated from the intestine. In these latter rats, however, intraduodenal infusion of trypsin markedly inhibited the protein secretion. The results suggest that vagal integrity is essential for the response of the pancreas to intraduodenal trypsin.

Effect of Atropine on Food-Stimulated Gastrin Release After Truncal Vagotomy in Man

Studies have been performed on man after truncal vagotomy to ascertain the effect of 1.2 mg of atropine sulfate on basal and postprandial immunoreactive gastrin. Atropine had no effect on basal gastrin, but it caused a significant increase in both the peak postprandial gastrin (135 to 240 pg per ml without the 139 to 308 pg per ml with atropine) and the integrated gastrin response (10.2 and 15.5 ng-min per ml over 2 hr respectively). This indicates that vagal integrity is not essential for the enhancement of the gastrin response by atropine and implies a direct effect on the antral gastrin cell.