A comparison of the central and peripheral antimuscarinic effects of atropine and methylatropine injected systemically and into the cerebral ventricles

We compared the relative abilities of atropine sulfate and methylatropine, injected i.v. and into the cerebral ventricles (icv), to block pharmacological responses mediated through central and peripheral muscarinic receptors. The hypotensive response to i.v. injection of acetylcholine (peripheral muscarinic receptors) was inhibited 50% by i.v. injection of 14.3 nmol (5.5 micrograms)/kg methylatropine and 147.8n molar equivalents (50 micrograms)/kg atropine sulfate. A similar degree of inhibition followed icv injection of 49.4 nmol/kg methylatropine and 384.2 nmol equivalents/kg atropine sulfate, indicating significant leakage out of the ventricular space. The pressor response to icv injection of neostigmine (central muscarinic receptors) also was inhibited more effectively by icv methylatropine than by atropine sulfate. Methylatropine was not effective in blocking central muscarinic receptors when injected i.v.     

Dose-dependent inhibition of cold air-induced bronchoconstriction by atropine

We undertook a study to demonstrate whether inhalation of atropine could inhibit cold air-induced bronchoconstriction in a dose-dependent fashion. In seven subjects with asthma we assessed the effects of placebo and of various doses of inhaled atropine (0.13-2.08 mg) on a base-line specific airway resistance (sRaw) and on the increase in sRaw produced by 5 min of voluntary eucapnic hyperventilation with subfreezing air at -17 degrees C. We also assessed the effect of the lowest doses of atropine on the increase in sRaw produced by five breaths of 1.0% metacholine. Atropine in doses of 0.13 or 0.26 mg caused a maximal reduction in base-line sRaw and completely inhibited the effect of 1.0% methacholine on sRaw, but it did not inhibit the bronchomotor response to cold air. Higher doses of atropine did inhibit the effect of cold air on sRaw in a dose-dependent fashion. The dose of atropine required to inhibit this effect of cold air varied with the increase in sRaw produced by cold air after placebo. These results suggest that cold air causes bronchoconstriction through vagal pathways and that higher doses of antimuscarinic agents are required to inhibit vagally mediated bronchoconstriction than those required to reduce base-line airway tone or to inhibit the effects of a large dose of an inhaled muscarinic agonist.     

Muscarinic receptors involved in airway vascular leakage induced by experimental gastro-oesophageal reflux

Gastro-oesophageal acid reflux may cause airway responses such as cough, bronchoconstriction and inflammation in asthmatic patients. Studies in humans or in animals have suggested that these responses involve cholinergic nerves. The purpose of this study was to investigate the role of the efferent vagal component on airway microvascular leakage induced by instillation of hydrochloric acid (HCl) into the oesophagus of guinea-pigs and the subtype of muscarinic receptors involved. Airway microvascular leakage induced by intra-oesophageal HCl instillation was abolished by bilateral vagotomy or by the nicotinic receptor antagonist, hexamethonium. HCl-induced leakage was inhibited by pretreatment with atropine, a non-specific muscarinic receptor antagonist, and also by pretreatment with either pirenzepine, a muscarinic M(1) receptor antagonist, or 4-DAMP, a muscarinic M(3) receptor antagonist. Pirenzepine was more potent than atropine and 4-DAMP. These antagonists were also studied on airway microvascular leakage or bronchoconstriction induced by intravenous administration of acetylcholine (ACh). Atropine, pirenzepine and 4-DAMP inhibited ACh-induced airway microvascular leakage with similar potencies. In sharp contrast, 4-DAMP and atropine were more potent inhibitors of ACh-induced bronchoconstriction than pirenzepine. Methoctramine, a muscarinic M(2) receptor antagonist, was ineffective in all experimental conditions. These results suggest that airway microvascular leakage caused by HCl intra-oesophageal instillation involves ACh release from vagus nerve terminals and that M(1) and M(3) receptors play a major role in cholinergic-mediated microvascular leakage, whereas M(3) receptors are mainly involved in ACh-induced bronchoconstriction.     

Cholinergic component of histamine-induced bronchoconstriction in newborn guinea pigs

The magnitude of parasympathetic reflex-mediated bronchoconstriction during histamine infusion was compared in anesthetized paralyzed newborn and adult guinea pigs. The animals were ventilated using a constant-flow ventilator, and the conductance and compliance of the respiratory system were continuously monitored. We found that reactivity to histamine infusion was less in newborns than in adults, because newborns required a larger dose of histamine than adults (300 vs. 125 ng.kg-1.s-1) to produce an equivalent decrease in conductance (42 +/- 13 vs. 42 +/- 15%). Vagal interruption by bilateral cervical vagotomy or muscarinic blockade with atropine (3 mg/kg) significantly reduced the bronchoconstrictor response to histamine in adults. By contrast, neither vagotomy nor atropine significantly changed this response in the newborns. These results indicate the lack of a vagal component in the bronchoconstriction that histamine induced in the newborns. Their relative unresponsiveness to histamine might partly be related to the fact that, in the newborn, histamine mainly acts directly via its airway receptors.     

Role of vagal C-fiber afferents in the bronchomotor response to lactic acid in the newborn dog.

We addressed the hypothesis that vagal C-fiber afferents and cyclooxygenase products are the mechanisms responsible for lactic acid (LA)-induced bronchoconstriction in the newborn dog. Perineural capsaicin and indomethacin were used to block conduction of vagal C fibers and production of cyclooxygenase products, respectively. Perineural capsaicin eliminated (85%) the increase in lung resistance (RL; 45 +/- 5.6%) due to capsaicin (25 microg/kg), whereas the increase in RL (54 +/- 6.9%) due to LA (0.4 mmol/kg) was only inhibited by 37 +/- 4.7% (P < 0.05). Atropine reduced LA-induced bronchoconstriction (42 +/- 2.1%) by an amount similar to that obtained with perineural capsaicin. However, inhibition was significantly increased when atropine was combined with indomethacin (61 +/- 2.7%; P < 0.05), implicating cyclooxygenase products in the LA-induced bronchoconstrictor response. We conclude that the mechanisms responsible for LA-induced bronchoconstriction in the newborn are 1) activation of vagal C-fibers, which, through projections to medullary respiratory centers, leads to activation of vagal cholinergic efferents; 2) production of cyclooxygenase products, which cause bronchoconstriction independent of medullary involvement; and 3) an unknown bronchoconstrictor mechanism, putatively tachykinin mediated. On the basis of our data, pharmaceutical targeting of pulmonary afferents would prevent multiple downstream mechanisms that lead to airway narrowing due to inflammatory lung disease.     

Muscarinic receptor subtypes: M1 and M2 biochemical and functional characterization

The heterogeneity of muscarinic receptors was examined in sympathetic ganglia and atria by “in vitro” binding techniques and functional studies. As tools we have used the classical antagonist atropine, the selective antagonist pirenzepine and the unique muscarinic agonist McN-A-343. In binding studies atropine showed similar affinities to muscarinic sites in ganglionic and atrial membranes with dissociation constants of 1.1 and 3.2 nM, respectively. In contrast, pirenzepine displayed a distinctly different binding profile. In atria it bound to an homogenous population of low affinity sites (diss. const. 620 nM) while in ganglia it revealed the presence of two sites: a major population of high affinity sites (diss. const. 11 nM) and a minor one of lower affinity (diss. const. 280 nM). The functional correlate of the receptor properties in the two tissues was studied in the pithed rat by measuring A) the increase of arterial pressure evoked by McN-A-343 through selective activation of muscarinic receptors in ganglia and B) the bradycardia elicited by acetylcholine release in the heart through vagal stimulation. Mirroring the “in vitro” binding data atropine inhibited both muscarinic responses in the same narrow range of doses (2–30 μg/kg i.v.) whereas pirenzepine showed similar potency to atropine in inhibiting ganglionic stimulation (ED50 4.1 μg/kg i.v.) but was almost two orders of magnitude weaker in blocking vagal bradycardia (ED50 172 μg/kg i.v.). These data suggest that McN-A-343 and pirenzepine act selectively on a common muscarinic receptor subtype, a finding which agrees with the view that muscarinic receptors are heterogenous and that excitatory ganglionic receptors (Ml) are distinguishable from those (M2) present in effector organs like smooth muscle and heart.     

Effects of calcium channel antagonists on the vagally mediated cardiac chronotropic response in dogs

A brief electrical stimulation of the vagus nerve may elicit a triphasic response comprising (i) an initial prolongation of the same or the next cardiac cycle, (ii) a return of the subsequent cardiac cycle to about the level prior to vagal stimulation, and (iii) a secondary prolongation of cardiac cycle length that lasts several beats. We compared the effects of two calcium channel antagonists, verapamil and nifedipine, on this triphasic response to vagal stimulation in chloralose-anesthetized, open-chest dogs. In the absence of vagal stimulation, nifedipine (doses of 10, 40, and 50 micrograms/kg for a total dose of 100 micrograms/kg, i.v.) and verapamil (two doses of 100 micrograms/kg each, i.v.) increased the cardiac cycle length (A-A interval) by 16% (429 +/- 20 to 496 +/- 21 ms) and 29% (470 +/- 33 to 605 +/- 54 ms), respectively. Nifedipine (100 micrograms/kg total) attenuated the initial vagally mediated prolongation of the A-A interval, from 474 +/- 19 to 369 +/- 42 ms above the basal A-A interval. Following the initial prolongation of the vagal effect, other A-A intervals were not affected. In contrast, verapamil potentiated the vagally mediated initial prolongation in cardiac cycle length at the first dose administered (100 micrograms/kg) from 492 +/- 17 to 561 +/- 14 ms, but other increases in dosages had no further effect. Thus these two calcium channel antagonists have different effects on the sinoatrial chronotropic responses caused by brief vagal stimulation.     

Nonadrenergic bronchodilation in adult and young guinea pigs

The contribution of the nonadrenergic inhibitory system to airway responses to infusion of 5-hydroxytryptamine (5-HT) was evaluated in anesthetized, tracheotomized, and paralyzed young (13 days) and adult (82 days) guinea pigs. Animals were mechanically ventilated by a constant flow ventilator. Compliance (C) and conductance (G) of the respiratory system were continuously monitored. Three series of experiments were performed involving intravenous pretreatment with 1) atropine (3 mg/kg) and propranolol (1 mg/kg); 2) atropine (3 mg/kg), propranolol (1 mg/kg), and phentolamine (2 mg/kg); and 3) atropine (3 mg/kg) and hexamethonium (2 mg/kg). 5-HT was then intravenously infused for 5 min at a rate of 40 ng.kg-1.s-1 in adults and 60 ng.kg-1.s-1 in young guinea pigs to obtain the same degree of bronchoconstriction in both groups. At the 3rd min of the infusion, bilateral cervical vagotomy was performed and C and G were measured at the maximal response, 1-2 min thereafter. Vagotomy increased bronchoconstriction (P less than 0.01) in both young animals and adults. Phentolamine did not modify this increase, but hexamethonium completely inhibited it. These results indicate that, in adult and young guinea pigs, 5-HT infusion induces reflex activation of the nonadrenergic inhibitory system, which in turn modulates the bronchoconstrictor responses to 5-HT. This neural modulation is not mediated by an alpha-adrenergic pathway.     

Central nervous system control of airway tone in guinea pigs: the role of histamine

The central nervous system (CNS) plays an important role in the reflex control of bronchomotor tone, but the relevant neurotransmitters and neuromodulators have not been identified. In this study we have investigated the effect of histamine. Anesthetized male guinea pigs were prepared with a chronically implanted intracerebroventricular (icv) cannula and instrumented for the measurement of pulmonary resistance (RL), dynamic lung compliance (Cdyn), tidal volume (VT), respiratory rate (f), blood pressure (BP), and heart rate (HR). Administration of histamine (2-30 micrograms) icv caused a significant (P less than 0.05) reduction of Cdyn with no change in RL, VT, and f. At a dose of 100 micrograms icv, histamine caused an increase in RL (202 +/- 78%), a reduction of Cdyn (77 +/- 9%), an increase in f (181 +/- 64%), and a reduction of VT (53 +/- 18%). There were no changes in BP and HR after 100 micrograms of icv histamine. In contrast, intravenous administration of histamine (0.1-2 micrograms/kg) caused a dose-dependent decrease in Cdyn and increase in RL that was associated with tachypnea at each bronchoconstrictor dose. Intravenous histamine (2 micrograms/kg) produced a fall in BP and an increase in HR. The bronchoconstrictor responses to icv histamine were completely blocked by vagotomy and significantly reduced by atropine (0.1 mg/kg iv), whereas vagotomy and atropine did not block the bronchospasm due to intravenous histamine. Additional studies indicated that the pulmonary responses due to icv histamine (100 micrograms) were blocked by pretreatment with the H1-antagonist chlorpheniramine (1 and 10 micrograms, icv). These data indicate that histamine may serve a CNS neurotransmitter function in reflex bronchoconstriction in guinea pigs.     

Influence of tone on responses to acetylcholine in the rabbit pulmonary vascular bed

Pulmonary vascular responses to acetylcholine were compared under resting and high tone conditions of the intact-chest rabbit. Under resting tone conditions, intralobar injections of acetylcholine increased lobar arterial pressure in a dose-related manner. The pressor responses to acetylcholine under resting conditions were blocked by meclofenamate, indomethacin, atropine, and pirenzepine. When lobar vascular resistance was raised to a high steady level, low doses of acetylcholine decreased lobar arterial pressure, whereas higher doses elicited a biphasic response with the pressor component predominating at the highest dose studied. Under high tone conditions, only the pressor component of the response was blocked by meclofenamate or indomethacin, whereas pressor and depressor responses were blocked by atropine or the 600-micrograms/kg iv dose of pirenzepine. Pressor responses to acetylcholine under resting and high tone conditions were blocked by pirenzepine (50 micrograms/kg iv), whereas gallamine had no effect on responses to acetylcholine. The 50-micrograms/kg iv dose of pirenzepine had no effect on depressor responses or the depressor component of the response to acetylcholine. The present data support the concept that acetylcholine has significant cyclooxygenase-dependent pressor activity in the rabbit pulmonary vascular bed and suggest that this response is mediated by a muscarinic M1-type receptor. These data also show that, under high tone conditions, a vasodilator response or a vasodilator component of a biphasic response is unmasked. This response is not dependent on the release of cyclooxygenase products and is mediated by a muscarinic receptor that is neither of the M1- nor the M2-type.     

Dose-response curves of central and peripheral airways to nicotine injections in dogs

The dose-response curves of the central and peripheral airways to intravenously injected nicotine were studied in 55 anesthetized dogs. With intact vagi, nicotine caused a dose-dependent increase in central airway resistance (Rc) similar to the increase in peripheral airway resistance (Rp) at concentrations ranging from 4 to 64 micrograms/kg. However, the responses of both Rc and Rp fell progressively when sequential doses of nicotine greater than 256 micrograms/kg were administered. With intact vagi and the administration of propranolol, there was a greater increase in Rp than in Rc at a nicotine dose of 64 micrograms/kg (P less than 0.05). With vagotomy, the responsiveness of both central and peripheral airways to nicotine decreased with doses of nicotine less than 64 micrograms/kg, but with doses of nicotine greater than 256 micrograms/kg the suppressive effect of nicotine on both Rc and Rp was less than that seen with intact vagi. Under conditions in which the vagi were cut and atropine administered, the responsiveness of nicotine was even further depressed. Combinations either of atropine and chlorpheniramine or atropine and phenoxybenzamine also completely blocked reactions to nicotine. Additionally reactions to nicotine were completely blocked by hexamethonium. These results suggest that nicotine increases both Rc and Rp mainly through a vagal reflex and stimulation of the parasympathetic ganglia.     

Effect of route of atropine delivery on bronchospasm from cold air and methacholine

We undertook a study to determine whether the apparent disparity between the dose of inhaled atropine required to inhibit the bronchoconstriction induced by inhaled methacholine and the dose required to inhibit the bronchoconstriction induced by eucapnic hyperpnea with cold air is a function of the route of administration of atropine. In six subjects with asthma, we constructed dose-response curves to inhaled methacholine and to eucapnic hyperpnea with cold air after treatment with inhaled atropine (0.5 mg delivered) and intravenous placebo, with inhaled placebo and intravenous atropine (0.5 mg injected), and with inhaled and intravenous placebos. Atropine by either route shifted the dose-response curves to both cold air and to methacholine to the right. In every subject, however, inhaled atropine caused a markedly greater rightward shift of the inhaled methacholine dose-response curve than did intravenous atropine, whereas inhaled and intravenous atropine had similar effects on the cold air dose-response curve. These findings suggest that the apparent disparity between the doses of atropine required to inhibit methacholine- and cold air-induced bronchoconstriction may be a function of the route of administration of atropine and thus does not imply a nonmuscarinic action of atropine. The findings support the view that cold air causes bronchoconstriction via muscarinic pathways.     

Effect of a thromboxane receptor antagonist on PGD2- and allergen-induced bronchoconstriction

In this study we investigated the effect of the selective and potent thromboxane A2 (TxA2) receptor antagonist GR32191 on smooth muscle contraction induced by the TxA2 analogue U46619, prostaglandin (PG) D2, PGF2 alpha, and methacholine (MCh) in guinea pig airways in vitro and the airways response provoked by inhaled PGD2 and MCh in asthmatic subjects in vivo. GR32191 antagonized competitively the contractile responses of all three prostanoids to a similar degree but had no effect on MCh-induced contractions. In asthmatic subjects GR32191, in a single oral dose of 80 mg, did not affect base-line airway caliber or MCh-induced broncho-constriction but caused significant inhibition of PGD2-induced bronchoconstriction, displacing the concentration-response curves to the right by greater than 10-fold. The effect of the same oral dose of GR32191 on allergen-induced immediate bronchoconstriction was subsequently investigated in allergic asthmatic subjects. In individual subjects, GR32191 inhibited to varying degrees the overall bronchoconstrictor response, with the maximum effect occurring between 10 and 30 min after allergen challenge. These studies suggest that prostanoids contribute to the immediate bronchoconstriction induced by inhaled allergen in allergic asthmatics, and that this effect is mediated by stimulation of a thromboxane receptor.     

Atropine pretreatment enhances airway hyperreactivity in antigen-challenged guinea pigs through an eosinophil-dependent mechanism

Airway hyperreactivity in antigen-challenged animals is mediated by eosinophil major basic protein (MBP) that blocks inhibitory M(2) muscarinic receptors on parasympathetic nerves, increasing acetylcholine release onto M(3) muscarinic receptors on airway smooth muscle. Acutely, anticholinergics block hyperreactivity in antigen-challenged animals and reverse asthma exacerbations in the human, but are less effective in chronic asthma. We tested whether atropine, given before antigen challenge, affected hyperreactivity, M(2) receptor function, eosinophil accumulation, and activation. Sensitized guinea pigs received atropine (1 mg/kg ip) 1 h before challenge and 6 h later. Twenty-four hours after challenge, animals were anesthetized, vagotomized, paralyzed, and ventilated. Airway reactivity to electrical stimulation of the vagi and to intravenous acetylcholine was not altered by atropine pretreatment in nonsensitized animals, indicating that atropine was no longer blocking postjunctional muscarinic receptors. Antigen challenge induced airway hyperreactivity to vagal stimulation that was significantly potentiated by atropine pretreatment. Bronchoconstriction induced by acetylcholine was not changed by antigen challenge or by atropine pretreatment. M(2) receptor function was lost in challenged animals but protected by atropine pretreatment. Eosinophils in bronchoalveolar lavage and within airway tissues were significantly increased by challenge but significantly reduced by atropine pretreatment. However, extracellular MBP in challenged airways was significantly increased by atropine pretreatment, which may account for reduced eosinophils. Depleting eosinophils with antibody to IL-5 before challenge prevented hyperreactivity and significantly reduced MBP in airways of atropine-pretreated animals. Thus atropine pretreatment potentiated airway hyperreactivity by increasing eosinophil activation and degranulation. These data suggest that anticholinergics enhance eosinophil interactions with airway nerves.     

Effects of morphine on hexamethonium-sensitive and -resistant excitatory responses of stomach to stimulation of vagal trunk in cats

Electrical stimulation of the vagal trunk with 10 Hz in frequency, 3 ms in duration and 15 volt in intensity for 10 s in cats produced an excitatory response of the stomach and the response was composed of two phases, an initial rapid excitation during stimulation period and the late multi-peak response after stimulation period. The initial response was inhibited by the administrations of hexamethonium (10 mg/kg, i.v.) and atropine (100 micrograms/kg, i.v.). The late response was not inhibited by hexamethonium but was inhibited by atropine (100 micrograms/kg, i.v.). The hexamethonium-sensitive initial excitation was not affected by the administration of morphine and gamma-aminobutyric acid (GABA). On the other hand, the hexamethonium-resistant late response was attenuated by the treatment with morphine (1 to 10 mg/kg, i.v.) and GABA (100 to 500 micrograms/kg, i.v.). Such inhibitory actions of morphine and GABA on the late response were antagonized by picrotoxin. From these results, it was concluded that morphine might inhibit specifically the hexamethonium-resistant late excitatory response of the stomach without affecting the hexamethonium-sensitive initial excitatory response and the inhibitory effect of morphine on the late response of stomach might be due to action of GABA released from the intramural neurons of gastric walls in cats.     

Refractoriness for ultrasonically nebulized distilled water and histamine after histamine challenge

Refractoriness for bronchial provocation frequently occurs after different challenge tests used to assess bronchial hyperresponsiveness in asthmatic patients. We investigated whether histamine inhalation could cause refractoriness for bronchoconstriction induced by ultrasonically nebulized distilled water (UNDW) and whether histamine causes tachyphylaxis for a subsequent histamine challenge in nine stable asthmatic patients. Preinhalation of histamine induced a significant diminished bronchoconstrictor response to UNDW cumulative dose of inhaled UNDW causing a 20% fall in forced expired volume in 1 s. The mean increased from 3.5 +/- 0.8 to 11.8 +/- 2.6 (SE) ml after histamine challenge (P less than 0.01). However, repeated inhalation of histamine did not change the bronchoconstrictor response to histamine within 1 h after rechallenge (P greater than 0.5). The magnitude of refractoriness for UNDW inhalation after preinhalation of histamine was correlated to the bronchoconstrictor response to histamine (r = 0.73, P less than 0.05). We conclude that inhaled histamine can induce refractoriness for UNDW, which seems to be related to the degree of bronchial hyperresponsiveness.     

Inhibitory effects of beta-alanine on the vagal efferent and afferent excitatory responses of the stomach to stimulation of vagal trunk in cats.

The effects of beta-alanine on the electrically evoked vagal efferent (hexamethonium-sensitive initial excitatory response) and afferent (hexamethonium-resistant delayed excitatory response) responses of the cat stomach were studied. beta-alanine (30 to 300 micrograms/kg, i.v.) dose-dependently inhibited both the efferent and afferent response. The IC50 values of beta-alanine on the efferent and afferent response were 296 +/- 65 micrograms/kg and 128 +/- 35 microgram/kg, respectively. Maximal inhibitory effects of beta-alanine (300 micrograms/kg, i.v.) appeared about 1 hr after the injection. Glycine and taurine (100 to 10,000 micrograms/kg) did not affect these responses. Treatment with hexamethonium (10 mg/kg, i.v.) prevented the efferent response, but augmented the afferent response. The treatment with hexamethonium abolished the inhibitory effect of beta-alanine on the afferent response. Both picrotoxin (100 and 500 micrograms/kg, i.v.) and bicuculline (2000 micrograms/kg, i.v.) antagonized the inhibitory effects of beta-alanine on the vagal efferent and afferent responses of the stomach. The present experiments clearly demonstrated that beta-alanine inhibited both the vagal efferent and afferent excitatory responses of stomach to electrical stimulation of vagal trunk in cats.     

Pharmacological investigation on the neurohumoral transmission of the vasomotor regulation.

Renal efferent sympathetic activity and its changes due to stimulation of the central stump of the vagal, sciatic and ulnar nerves were investigated. In addition, the effect on basal activity and sympathetic reflexes of drugs with well defined site of action was studied (diazepam, tofizopam, phentolamine, dihydroergotamine, chlorpromazine, reserpine, clonidine, atropine, methysergide and phenindamine). The sympathetic efferent activity and the changes in sympathetic reflexes allowed conclusions to be drawn as to the functional state of the vasomotor centre. Neither methysergide nor phenindamine inhibited efferent sympathetic activity or influenced sympathetic reflexes. These findings exclude the possibility of serotonin or histamine being the transmitter substance in the vasomotor neurone. Experiments with atropine revealed that the muscarinic action of acetylcholine does not figure in the sympathetic inhibitory or excitatory reflex processes. Of the drugs investigated only diazepam and clonidine inhibited efferent sympathetic activity. Clonidine was more selective and acted in much lower doses (20 micrograms/kg) than diazepam (0.5--1 mg/kg). The alpha blocking agents inhibited the viscero-sympathetic inhibitory reflex arch more intensely than the somato-sympathetic inhibitory one. The transmitter is presumably noradrenaline. The sympathetic excitatory reflexes were decreased by diazepam and tofizopam and increased by clonidine and phentolamine. The other substances were ineffective. As to the transmitter substance figuring in the sympathetic excitatory reflexes no unequivocal answer could be obtained in the present experiments.     

Effect of subdiaphragmatic vagotomy and cholinergic agents in the hypothalamic-pituitary-adrenal axis activity.

In the present study, we examined whether the vagus nerve is involved in mediating the stimulation of hypothalamic-pituitary-adrenal (HPA) axis by cholinergic muscarinic and nicotinic agonists, carbachol and nicotine. The site of HPA axis muscarinic stimulation was determined using peripheral (i.p.) and intracerebroventricular (i.c.v.) administration of carbachol, atropine sulphate (AtrS) and atropine hydrobromide (AtrBr). The i.p. carbachol-(0.5 mg/kg)-induced corticosterone response was significantly reduced by i.p. pretreatment with AtrBr (0.1 mg/kg), but was not diminished by i.c.v. AtrS (0.1 mug). The increase in corticosterone secretion induced by i.c.v. carbachol (2 microg) was totally abolished by i.c.v. pretreatment with AtrS (0.1 microg) but was not altered by i.p. AtrBr. Subdiaphragmatic vagotomy performed 2 weeks earlier substantially decreased the i.p. carbachol (0.2 mg/kg)-induced ACTH response and markedly augmented ACTH and corticosterone response to a higher dose of carbachol (0.5 mg/kg) in comparison with the responses in sham operated rats. Vagotomy abolished the stimulatory effect of i.p. nicotine in a low dose (1 mg/kg) on ACTH and corticosterone secretion; the ACTH response to higher dose (2.5 mg/kg) was considerably reduced, while corticosterone response remained unaffected. These results suggest that carbachol given i.c.v. evokes considerable corticosterone response by stimulation of central cholinergic muscarinic receptors. A major part of the i.p. carbachol-induced corticosterone secretion results from peripheral cholinergic muscarinic receptor stimulation. Subdiaphragmatic vagotomy moderately intensified the carbachol-induced ACTH and corticosterone secretion. Vagotomy significantly reduced the nicotine-induced ACTH secretion, possibly by the involvement of vagal afferents. The nicotine-induced corticosterone secretion is not exclusively regulated by circulating ACTH but by various intra-adrenal regulatory components.     

Effect of atropine on plasma gastrin and somatostatin concentrations during sham feeding in man

The purpose of these studies was to measure circulating gastrin and somatostatin concentrations during sham feeding in humans and to evaluate the effect of two doses of intravenous atropine on circulating concentrations of these peptides. Gastric acid and bicarbonate secretion and pulse rate were also measured. Sham feeding increased plasma gastrin concentrations by approximately 15 pg/ml but had no effect on plasma somatostatin-like immunoreactivity (SLI). A small dose of atropine (5 micrograms/kg) augmented plasma gastrin concentrations during sham feeding significantly (P less than 0.01), but did not affect plasma SLI. Atropine also significantly inhibited gastric acid secretion and gastric bicarbonate secretion (by 62% and 52%, respectively), but pulse rate was not affected. A larger dose of atropine (15 micrograms/kg intravenously) suppressed plasma gastrin concentrations significantly compared to the smaller 5 micrograms/kg atropine dose (P less than 0.02), so that plasma gastrin concentrations when 15 micrograms/kg atropine was given were not significantly different from those during the control study. 15 micrograms/kg atropine reduced gastric acid and bicarbonate secretion by 81% and 66%, respectively, and also increased pulse rate by 15 min-1. These studies indicate that small doses of atropine enhance vagally mediated gastrin release in humans, probably by blocking a cholinergic inhibitory pathway for gastrin release. Although the nature of this cholinergic inhibitory mechanism is unclear, we found no evidence to incriminate somatostatin. Our finding that the larger dose of atropine reduced serum gastrin concentrations compared with the smaller dose suggests that certain vagal-cholinergic pathways may facilitate gastrin release.