Participation of nitric oxide in the nicotine-induced relaxation of the cat lower esophageal sphincter.

The participation of nitric oxide in the relaxation of the cat lower esophageal sphincter muscle strip in response to electrical field stimulation or administration of nicotine was studied. The nicotine-induced relaxation was mediated via a neuronal pathway, since it was inhibited by administration of hexamethonium or tetrodotoxin. Inhibition of nitric oxide biosynthesis by N-nitro-L-arginine decreased the relaxation induced by nicotine (50 microM) or field stimulation. With the maximal concentration of N-nitro-L-arginine (1 mM) electrical field stimulation-induced relaxation was abolished, while nicotine-induced relaxation decreased by 70%. L-Arginine (1 mM) partly restored this relaxation. Desensitization of P2x receptors by alpha, beta methylene-adenosine 5-triphosphate (alpha, beta-m-ATP) did not change the relaxation induced by either electrical field stimulation or administration of nicotine. It is therefore suggested that the field stimulation-induced relaxation is mediated by the release of nitric oxide, but in the nicotine-produced relaxation is only partly due to nitric oxide, other factor(s) might be also be involved.

[Gastrointestinal sphincters--pharmacologic viewpoints]. / Gastrointestinale Sphinkteren--pharmakologische Gesichtspunkte.

Bearing in mind the specificity of the functions of the gastrointestinal sphincters (GIS), we studied the responses of the lower esophageal sphincter (LES), the pyloric sphincter (PS), the ileocecal sphincter (ICS) and the internal anal sphincter (IAS) to noradrenaline (NA), acetylcholine (ACh), PGE1, PGF2 alpha and the peptide bombesin (B). The electrical and contractile activities as well as the response to field electrical stimulation (FES) were recorded. All sphincters except for PS responded to NA with depolarization and contractions. ACh at concentrations higher than 5 x 10(-6)M elicited a biphasic response from LES: depolarization followed by hyperpolarization and respectively contractions and relaxation. At concentrations higher than 10(-6)M ACh produced hyperpolarization and relaxation in IAS. Data were obtained about the modulating role of presynaptic N-cholinoreceptors in the release of a non-adrenergic, non-cholinergic (NANC) inhibitory neurotransmitter leading to hyperpolarization and relaxation in LES and IAS under the effect of exogenous ACh. PGE1 and PGF2 alpha increased the tone and decreased the FES-induced relaxation in LES. In PS PGE1 evoked relaxation and completely inhibited the response to FES. The pharmacological analysis showed that PGs modulated not only the adrenergic and cholinergic but also the NANC neurotransmission. Evidence is presented concerning the role of bombesin in the adrenergic, cholinergic, and NANC neurotransmission in LES.

Bombesin-induced changes in membrane potential-dependent phasic contractions of cat gastric muscle.

Electrical and contractile activities of smooth muscle strips isolated from the circular muscle layer of cat gastric antrum were studied using the sucrose gap technique. Bombesin (10(-8) mol/l) depolarized the gastric muscle; this was accompanied by an increase in the strip tone, in the plateau action potential frequency and in both the frequency and the amplitude of the spike potentials as well as by a shortening of the plateau action potential duration. Both the frequency and the amplitude of the phasic contractions increased thereafter. The changes in the frequency of the plateau action potentials and contractions were not influenced either by antagonists of cholinergic and adrenergic receptors or by TTX. In the presence of the Ca antagonists D600 (10(-6) mol/l) and nifedipine (10(-7) mol/l) or in Ca-free medium containing EGTA the effect of bombesin on the frequency of the plateau action potentials and phasic contractions remained unchanged; however, spike potentials were not observed and no increase in the amplitude of phasic contractions occurred. UV-light inactivation of nifedipine restored the typical bombesin effect on the electrical and contractile activities of the gastric smooth muscle. The present data suggest that the effect of bombesin on the frequency of both plateau action potentials and phasic contractions is not linked with Ca2+ influx.

Effects of bombesin on neurotransmission in cat duodenal smooth muscle.

The effects of tetradecapeptide bombesin on contractile activity and field electrical stimulation (FES) evoked contractions of duodenal smooth muscle were studied. Bombesin produced an increase in the tone as well as the amplitude and frequency of the spontaneous contractions. The atropine-sensitive contraction and nonadrenergic noncholinergic relaxation in responses to FES which were tetrodotoxin-sensitive significantly increased in amplitude after bombesin. This suggests that bombesin not only provokes release of acethylcholine but can also modulate the release of noncholinergic nonadrenergic neurotransmitter in the enteric nervous system.

Leucine-enkephalin as a modulator of neurotransmission in cat lower esophageal sphincter.

Interactions between cholinergic, adrenergic and noncholinergic nonadrenergic inervation in lower esophageal sphincter determine the character of smooth muscle responses to field electrical stimulation. The neurogenic responses of muscle strips from the proximal (LES1), middle (LES2) and lower (LES3) parts of lower esophageal sphincter (LES) are: primary relaxation and secondary contraction in LES1, relaxation in LES2 and primary contraction followed by primary relaxation in LES3. Leucin-enkephalin in concentration of 0.1 microM and 1 microM leads to: increasing of amplitude of the primary relaxation and inhibition of secondary contraction in LES1, enhancing of amplitude of relaxation in LES2 and converts the response in LES3 from two into three component one so that the primary relaxation and the primary contraction are followed by secondary poststimulation relaxation. These effects are most probably the results of inhibition of noradrenaline and acetylcholine releasing under the influence of leucine-enkephalin and of alteration of the correlation between the neurotransmitters. It is assumed that Leucine-enkephalin operates in LES as a neuromodulator inhibiting both the adrenergic and cholinergic neurotransmission.

Effects of PGE1 and PGF2 alpha on the responses of gastrointestinal sphincters to field stimulation.

The study concerned the effects of PGE1 and PGF2 alpha on the spontaneous tone and the responses to field stimulation of isolated strips from: lower esophageal sphincter (LES), pyloric sphincter (PS) and ileocaecal sphincter (ICS) of cats. PGE1 and PGF2 alpha (0.3 microM) increased the LES tone and abolished the PS contractions. PGF2 alpha increased the tone of ICS while PGE1 had no effect at all. The effects of the prostaglandins (PG) were reduced when the preparations were pretreated with atropine, propranolol and phenoxybenzamine (1 microM). PGE1 and PGF2 alpha decreased the LES relaxation in response to field stimulation (0.5 msec, 2 Hz for 10 sec at supramaximal current). No responses of PS to field stimulation were observed after PG treatment. PGF2 alpha increased the amplitude of the relaxation response in the ICS to field stimulation. PGE1 did not change the effect of field stimulation in ICS. The effects of PGE1 and PGF2 alpha on the LES and ICS responses to field stimulation were not influenced by adrenergic and cholinergic blocking agents. The data suggest a PG effect on non-adrenergic, non-cholinergic (purinergic) transmission in the gastrointestinal sphincter smooth muscle on field stimulation. Due to the high concentration of PG used this effect was probably a pharmacological one.

Character of the gastric and duodenal electrical activity after blocking of the adrenoreactive structures.

Experiments are carried out on six dogs with chronically implanted electrodes in the gastric wall and in the duodenum. The electrical activity of the stomach (EGMG) and of the duodenum (EEG) is recorded. The myoelectrical complex of the stomach consists of four phases. The first phase records only slow potentials and lasts 40--60 min. The appearance of spike activity in the duodenum is always preceded by EGMG activation. Intravenous injection of the specific blocker of the alpha-adrenergic structures (phentolamine -- 1 mg/kg) leads to the appearance of spike activity in EGMG and EEG. 50--60 per cent of the slow waves being accompanied by spike potentials. At the same time the frequency of the slow potentials from EGMG is also decreased. Application of beta-adrenoblocker (propranolol -- 2 mg/kg) does not change subatantially EGMG and EEG. Simultaneous administration of alpha- and beta-adrenoblockers prolongs spiking time in the stomach and duodenum (spike potentials are recorded for 120--180 min, duration in the background 40--50 min). The influence of the interactions of cholinergic and adrenergic structures on the character of EGMG and EEG is discussed.

Inhibitory action of acetylcholine on the smooth muscle from the lower esophageal sphincter.

The effect of acetylcholine (Ach) on smooth-muscle strips isolated along the transversal axis of cat lower esophageal sphincter (LES) is studied. Ach in low concentrations (10(-11)--10(-9) g/ml) causes contraction of the muscle strips. Increase of the concentration to 10(-8) g/ml leads to biphasic effect: contraction with relaxation. Inhibitory response predominates at Ach 10(-6) and 10(-5) g/ml. Atropine (10(-6) M) eliminates the excitatory phase but it has no effect on the second relaxation phase. Propranolol (10(-6), 2 X 10(-6) M) as well as phentolamine turn the inhibitory response to Ach into contraction. Noradrenaline leads to LES contraction while isoprenaline induces relaxation. In smooth-muscle LES strips from cats pretreated with reserpine (1 mg/kg for 3 days), Ach in the concentrations used (10(-5), 10(-6) g/ml) leads to contraction. The changes observed are membrane-dependent -- the contraction is accompanied by depolarization, relaxation by hyperpolarization. The inhibitory effect of Ach on LES smooth muscle is discussed in the light of the hypothesis of Burn and Rand (1960) about the release of noradrenaline under the effect of Ach.

Increase in the intestinal slow-wave frequency below the transection through electrical stimulation.

Experiments are carried out on dogs with chronically implanted leading-off electrodes on both sides of the transection at the level of the proximal duodenum. A pair of stimulating electrodes are also implanted below the transection. Electrical stimulation with impulses synchronized with the slow waves generated by the segment proximal to the section (amplitude 3-7 V, delay 0.2 s and duration 0.5 s) as well as with periodic impulses (18-21 cpm, duration 0.2-0.5 s and amplitude 3-10 V) results in shortening of the period T of several slow waves from the distal segment. Subsequently the frequency of the slow waves from this segment approaches that of the proximal segment. The restoration of the slow-wave plateau at the beginning of the small intestine depends both on the amplitude and on the frequency of the stimulating impulses. On the background of antagonists of the cholinergic (atr. sulf. 200 microgram/kg) and of the adrenergic systems (propranolol 2 mg/kg and phentolamine 1 mg/kg), increasing the frequency of the slow waves of the segment which is distal to the section requires an increase in the amplitude of the stimulating impulses. The role of the intrinsic nervous system for the assimilation of the slow-wave rhythm from the distal segment by the proximal one is discussed.