Release of gastrin by vagal stimulation in the acid secretory response to food.

Gastric pouches were constructed in 8 dogs; in 4 they were of denervated type and in 4 the innervation was intact. An oesophageal fistula was then prepared in each dog. The acid secretory response to oral, tube and sham feeding was determined before and after denervation of the pyloric antrum. The results support the view that vagal release of gastrin makes a relatively small contribution to the total acid secretory response to food.     

Stomach-brain communication by vagal afferents in response to luminal acid backdiffusion, gastrin, and gastric acid secretion

Vagal afferents play a role in gut-brain signaling of physiological and pathological stimuli. Here, we investigated how backdiffusion of luminal HCl or NH(4)OH and pentagastrin-stimulated acid secretion interact in the communication between rat stomach and brain stem. Rats were pretreated intraperitoneally with vehicle or appropriate doses of cimetidine, omeprazole, pentagastrin, dexloxiglumide (CCK(1) receptor antagonist), and itriglumide (CCK(2) receptor antagonist) before intragastric administration of saline or backdiffusing concentrations of HCl or NH(4)OH. Two hours later, neuronal activation in the nucleus of the solitary tract (NTS) and area postrema was visualized by c-Fos immunohistochemistry. Exposure of the rat gastric mucosa to HCl (0.15-0.5 M) or NH(4)OH (0.1-0.3 M) led to a concentration-dependent expression of c-Fos in the NTS, which was not related to gender, gastric mucosal injury, or gastropyloric motor alterations. The c-Fos response to HCl was diminished by cimetidine and omeprazole, enhanced by pentagastrin, and left unchanged by dexloxiglumide and itriglumide. Pentagastrin alone caused an omeprazole-resistant expression of c-fos, which in the NTS was attenuated by itriglumide and prevented by dexloxiglumide but in the area postrema was reduced by dexloxiglumide and abolished by itriglumide. We conclude that vagal afferents transmit physiological stimuli (gastrin) and pathological events (backdiffusion of luminal HCl or NH(4)OH) from the stomach to the brain stem. These communication modalities interact because, firstly, acid secretion enhances afferent signaling of gastric acid backdiffusion and, secondly, gastrin activates NTS neurons through stimulation of CCK(1) receptors on vagal afferents and of CCK(2) receptors on area postrema neurons projecting to the NTS.     

AV blocking due to asynchronous vagal stimulation in rats

The parasympathetic nervous system innervates the heart through two cervical vagal branches. The right vagal branch mainly influences the heart rate by the modulation of the rhythmogenesis of the sinoatrial node. The left branch predominantly influences the conduction properties of the atrioventricular (AV) node. We investigated the effect of asynchronous stimulation by the vagal nerves on the occurrence of irregularities in heart rate. In rats, the vagal nerves were isolated and cut. Different vagal stimulation patterns (continuous, pulsed) were applied. The heart was beating spontaneously under continuous vagal stimulation. In case of pulsed vagal stimulation, the atria were paced at different rates. Asynchronicity was induced by delaying the right stimulus with respect to the left stimulus (early right) or the left stimulus with respect to the right stimulus (early left). The value of the fraction of deviated R-R or P-Q intervals in the distribution in the histogram was used to characterize irregularities during a stimulation protocol (duration in case of continuous stimulation: 20 s; pulsed stimulation: 120 s). Under both stimulation patterns (continuous or pulsed), we found that early left vagal stimulation introduced a much larger fraction of deviated intervals in the R-R or P-Q histogram (in R-R: 29.1 +/- 4.9%; in P-Q: 12.90 +/- 1.95%) than early right vagal stimulation (in R-R: 7.4 +/- 2.0%; in P-Q: 1. 05 +/- 0.50%) or synchronous stimulation (in R-R: 8.2 +/- 3.6%; in P-Q: 2.15 +/- 0.75%). We conclude that early stimulation by the left vagal nerve can introduce irregularities in heart rate, mainly due to different degrees of AV nodal blockade.     

Exercise training augments the dynamic heart rate response to vagal but not sympathetic stimulation in rats

We examined the transfer function of autonomic heart rate (HR) control in anesthetized sedentary and exercise-trained (16 wk, treadmill for 1 h, 5 times/wk at 15 m/min and 15-degree grade) rats for comparison to HR variability assessed in the conscious resting state. The transfer function from sympathetic stimulation to HR response was similar between groups (gain, 4.2 ± 1.5 vs. 4.5 ± 1.5 beats·min(-1)·Hz(-1); natural frequency, 0.07 ± 0.01 vs. 0.08 ± 0.01 Hz; damping coefficient, 1.96 ± 0.55 vs. 1.69 ± 0.15; and lag time, 0.7 ± 0.1 vs. 0.6 ± 0.1 s; sedentary vs. exercise trained, respectively, means ± SD). The transfer gain from vagal stimulation to HR response was 6.1 ± 3.0 in the sedentary and 9.7 ± 5.1 beats·min(-1)·Hz(-1) in the exercise-trained group (P = 0.06). The corner frequency (0.11 ± 0.05 vs. 0.17 ± 0.09 Hz) and lag time (0.1 ± 0.1 vs. 0.2 ± 0.1 s) did not differ between groups. When the sympathetic transfer gain was averaged for very-low-frequency and low-frequency bands, no significant group effect was observed. In contrast, when the vagal transfer gain was averaged for very-low-frequency, low-frequency, and high-frequency bands, exercise training produced a significant group effect (P < 0.05 by two-way, repeated-measures ANOVA). These findings suggest that, in the frequency domain, exercise training augments the dynamic HR response to vagal stimulation but not sympathetic stimulation, regardless of the frequency bands.     

A monoclonal antibody to free N-acetylneuraminic acid

A monoclonal antibody (70-A) to free N-acetylneuraminic acid was obtained by immunizing mice with its synthetic beta-glycoside, sodium O-[(5-acetamido-3,5-dideoxy-D-glycero-beta-D-galacto-2- nonulopyranosyl)onate]-(2----3)-1,2-di-O-tetradecyl-sn-glyce rol, followed by fusing the isolated spleen cells with mouse myeloma cells and cloning positive fusions. 70-A reacted with various synthetic beta-glycosides of N-acetylneuraminic acid and also with cytidine-5'-monophosphate-N- acetylneuraminic acid, known as its sole naturally occurring beta-glycoside. The inhibition assay showed that N-glycolylneuraminic acid had slightly lower reactivity than N-acetylneuraminic acid, but other monosaccharides tested, such as N-acetylglucosamine, N-acetylgalactosamine or N-acetylmannosamine, had no reactivity toward 70-A. Reactivity of 70-A with free N-acetylneuraminic acid was confirmed by measuring the specific binding of N-[14C]acetylneuraminic acid to the antibody. The association constant of 70-A with N-acetylneuraminic acid was determined to be 5.96.10(4) M-1 by equilibrium dialysis.     

Elevation of plasma gastrin and pancreatic polypeptide by electrical vagal stimulation in sheep: effects of sequential stimulation

Gastrin and pancreatic polypeptide (PP) are released into the circulation by vagal stimulation. The individual effects of the anterior and posterior vagal trunks on the release of these peptides are unknown. Four sheep were anaesthetized and studied acutely: both vagi were dissected at the hiatus and the trunks divided. In two sheep, the distal ends of the anterior trunks were stimulated for 5 min with an 8 V, 1 ms impulse at 10 Hz. After 1 h the posterior trunks were stimulated similarly. In the other two sheep, the posterior trunk was stimulated and after 1 h the anterior vagal trunk was stimulated. The anterior and posterior trunk equally stimulated the release of both gastrin and PP in four animals. The second stimulation in these four animals resulted in an 18-fold greater integrated response of gastrin and 20-fold greater response of PP. This potentiation to the second stimulus was observed in further experiments even when the same trunk, posterior or anterior, was stimulated twice. The similarity of influence of the anterior and posterior trunks for the release of PP suggests the existence of mechanisms for vagally stimulated PP release other than branches direct from the vagal trunks.     

Angiotensin II attenuates myocardial interstitial acetylcholine release in response to vagal stimulation

Although ANG II exerts a variety of effects on the cardiovascular system, its effects on the peripheral parasympathetic neurotransmission have only been evaluated by changes in heart rate (an effect on the sinus node). To elucidate the effect of ANG II on the parasympathetic neurotransmission in the left ventricle, we measured myocardial interstitial ACh release in response to vagal stimulation (1 ms, 10 V, 20 Hz) using cardiac microdialysis in anesthetized cats. In a control group (n = 6), vagal stimulation increased the ACh level from 0.85 +/- 0.03 to 10.7 +/- 1.0 (SE) nM. Intravenous administration of ANG II at 10 microg x kg(-1) x h(-1) suppressed the stimulation-induced ACh release to 7.5 +/- 0.6 nM (P < 0.01). In a group with pretreatment of intravenous ANG II receptor subtype 1 (AT(1) receptor) blocker losartan (10 mg/kg, n = 6), ANG II was unable to inhibit the stimulation-induced ACh release (8.6 +/- 1.5 vs. 8.4 +/- 1.7 nM). In contrast, in a group with local administration of losartan (10 mM, n = 6) through the dialysis probe, ANG II inhibited the stimulation-induced ACh release (8.0 +/- 0.8 vs. 5.8 +/- 1.0 nM, P < 0.05). In conclusion, intravenous ANG II significantly inhibited the parasympathetic neurotransmission through AT(1) receptors. The failure of local losartan administration to nullify the inhibitory effect of ANG II on the stimulation-induced ACh release indicates that the site of this inhibitory action is likely at parasympathetic ganglia rather than at postganglionic vagal nerve terminals.     

Electrical stimulation of hybridoma cells producing monoclonal antibody to cAMP

Electrical stimulation was applied to hybridoma cells in order to activate metabolic activities and increase the monoclonal antibody production. Hybridoma cells that produce monoclonal antibody to adenosine 3':5'-cyclic monophosphate were placed on a transparent glass electrode immersed in medium and subjected to electric pulses (pulse shape, alternating rectangular; field strength, 4 X 10(3) V X m-1; frequency, 5 kHz; pulse mode, 0.5 min application and 4.5 min pause). After 48 h of incubation, the concentration of lactic acid in the medium reached 8.4 mM, approx. 30% higher than that obtained without electric stimulation. Similarly, cell growth rate was promoted by the electric stimulation, reaching a maximum stimulation after 40 h. When the hybridoma was cultured for 48 h with electrical stimulation, the antibody concentration in the medium reached 22.3 microgram X ml-1, approx. 10% higher than the control, with a concomitant 16% increase in cell concentration. Longer periods of electric pulse application, however, caused an inhibitory effect on the hybridoma growth. The most probable cause of the inhibition are reactive oxygen species such as superoxide and hydrogen peroxide, which are inevitably generated by electrolysis. The presence of superoxide dismutase (EC 1.15.1.1) reduced the inhibitory effects. In conclusion, metabolic activities including monoclonal antibody production were activated by the electrical stimulation.     

The effect of substance P on the regional lymph node antibody response to antigenic stimulation in capsaicin-pretreated rats

The undecapeptide substance P (SP) contained in primary afferent nerves is thought to mediate that part of the neurogenic inflammatory response consisting of vasodilation and plasma extravasation. This response is diminished in rats pretreated as neonates with the neurotoxin capsaicin. It is not known whether primary afferent nerves influence cellular responses of the immune response to antigenic stimulation. Using 6- to 12-wk-old Sprague-Dawley rats pretreated as neonates with capsaicin, we examined the regional lymph node response to a s.c. antigenic stimulus of sheep red blood cells. The number of cells secreting antigen-specific antibody in these animals was reduced by more than 80% using direct and indirect plaque assay methods. The reduced antibody response in capsaicin-pretreated animals was reversed by a s.c. infusion of SP given over a 4-hr period at the injection site immediately after antigen stimulation. This response had a threshold at approximately 1.0 X 10(-5) M SP. SP1-7 (1.0 X 10(-5) M) was without effect but an infusion of SP5-11 (1.0 X 10(-5) M) reversed the effects of capsaicin treatment indicating a carboxyl-terminal effect of SP. The results suggest that the reduced response of capsaicin-treated animals to an antigenic stimulus is due to an effect of capsaicin on the SP-containing primary afferent nerves rather than a toxic effect of capsaicin on the immune system.     

High plasma norepinephrine attenuates the dynamic heart rate response to vagal stimulation

To better understand the pathophysiological significance of high plasma norepinephrine (NE) concentration in regulating heart rate (HR), we examined the interactions between high plasma NE and dynamic vagal control of HR. In anesthetized rabbits with sinoaortic denervation and vagotomy, using a binary white noise sequence (0-10 Hz) for 10 min, we stimulated the right vagus and estimated the transfer function from vagal stimulation to HR response. The transfer function approximated a first-order low-pass filter with pure delay. Infusion of NE (100 microg. kg(-1) x h(-1) iv) attenuated the dynamic gain from 6.2 +/- 0.8 to 3.9 +/- 1.2 beats x min(-1) x Hz(-1) (n = 7, P < 0.05) without affecting the corner frequency or pure delay. Simultaneous intravenous administration of phentolamine (1 mg x kg(-1) x h(-1)) and NE (100 microg x kg(-1) x h(-1)) abolished the inhibitory effect of NE on the dynamic gain (6.3 +/- 0.8 vs. 6.4 +/- 1.3 beats x min(-1) x Hz(-1), not significant, n = 7). The inhibitory effect of NE at infusion rates of 10, 50, and 100 microg x kg(-1) x h(-1) on dynamic vagal control of HR was dose-dependent (n = 5). In conclusion, high plasma NE attenuated the dynamic HR response to vagal stimulation, probably via activation of alpha-adrenergic receptors on the preganglionic and/or postganglionic cardiac vagal nerve terminals.     

GRP mediates an inhibitory response of gut-related vagal motor neurons to PVN stimulation

We previously characterized neurons in the dorsal motor nucleus of the vagus (DMNV) that were modulated by electrical stimulation of the PVN and by gastrointestinal distention. Bombesin has been identified in a subset of PVN neurons projecting to the DMNV. It is currently unknown whether this neurotransmitter is involved in descending communication from PVN to DMNV neurons. In this study we determined whether the specific bombesin antagonist, N-acetyl-GRP(20-26), influenced (1) the basal firing rate of DMNV neurons and (2) the response to electrical current stimulation of the PVN. Our results indicate that N-acetyl-GRP(20-26), significantly attenuated the inhibitory response of DMNV neurons to PVN stimulation. These results provide a possible mechanism by which bombesin regulates gastrointestinal function, body temperature homeostasis, and feeding behaviors.     

Lack of Gastric Inhibitory Polypepetide (GIP) response to vagal stimulation in the rat

The effect of sham feeding on the plasma concentration of gastric inhibitory polypeptide (GIP) was studied in unrestrained rats bearing chronic gastric fistulas and jugular catheters. While no increase of plasma GIP concentration could be detected during sham feeding (fistula open), during normal feeding (fistula closed), plasma GIP concentrations rose rapidly. In contrast to GIP, plasma insulin concentrations showed a rapid and phasic response during sham feeding in the absence of changes of glycemia. In anesthetized rats electrical stimulation of the vagus nerve was without any effect on plasma GIP concentration, while plasma insulin increased rapidly by as much as 150 percent. It is concluded that under the conditions used, the full gastric and/or intestinal phases of food ingestion are necessary to trigger GIP release, and that vagal activation alone is unable to stimulate GIP release in the rat.