Effects of transmural field stimulation in isolated muscle strips from human esophagus

Distension contraction plots of pharyngeal/esophageal peristalsis: next frontier in the assessment of esophageal motor function

Esophageal peristalsis consists of initial inhibition (relaxation) followed by excitation (contraction), both of which move sequentially in the aboral direction. Initial inhibition results in receptive relaxation and bolus-induced luminal distension, which allows propulsion by the contraction with minimal resistance to flow. Similar to the contraction wave, luminal distension has unique waveform characteristics in normal subjects; both are modulated by bolus volume, bolus viscosity, and posture, suggesting a possible cause-and-effect relationship between the two. Distension contraction plots in patients with dysphagia with normal bolus clearance [high-amplitude esophageal contractions (HAECs), esophagogastric junction outflow obstruction (EGJOO), and functional dysphagia (FD)] reveal two major findings: 1) unlike normal subjects, there is luminal occlusion distal to bolus during peristalsis in certain patients, i.e., with type 3 achalasia and nonobstructive dysphagia; and 2) bolus travels through a narrow lumen esophagus during peristalsis in patients with HAECs, EGJOO, and FD. Aforementioned findings indicate a relative dynamic obstruction to the bolus flow during peristalsis and reduced distensibility of esophageal wall in the bolus segment of the esophagus. We speculate that a normal or supernormal contraction wave pushing bolus against resistance is the mechanism of dysphagia sensation in significant number of patients. Representations of distension and contraction, combined with objective measures of flow timing and distensibility are complementary to the current scheme of classifying esophageal motility disorders based solely on the characteristics of contraction phase of peristalsis. Better understanding of the distensibility of the bolus-containing segment of the esophagus during peristalsis will lead to the development of novel medical and surgical therapies in the treatment of dysphagia in significant number of patients.

Translational neuropharmacology: the use of human isolated gastrointestinal tissues

Translational sciences increasingly emphasize the measurement of functions in native human tissues. However, such studies must confront variations in patient age, gender, genetic background and disease. Here, these are discussed with reference to neuromuscular and neurosecretory functions of the human gastrointestinal (GI) tract. Tissues are obtained after informed consent, in collaboration with surgeons (surgical techniques help minimize variables) and pathologists. Given the difficulties of directly recording from human myenteric neurones (embedded between muscle layers), enteric motor nerve functions are studied by measuring muscle contractions/relaxations evoked by electrical stimulation of intrinsic nerves; responses are regionally dependent, often involving cholinergic and nitrergic phenotypes. Enteric sensory functions can be studied by evoking the peristaltic reflex, involving enteric sensory and motor nerves, but this has rarely been achieved. As submucosal neurones are more accessible (after removing the mucosa), direct neuronal recordings are possible. Neurosecretory functions are studied by measuring changes in short-circuit current across the mucosa. For all experiments, basic questions must be addressed. Because tissues are from patients, what are the controls and the influence of disease? How long does it take before function fully recovers? What is the impact of age- and gender-related differences? What is the optimal sample size? Addressing these and other questions minimizes variability and raises the scientific credibility of human tissue research. Such studies also reduce animal use. Further, the many differences between animal and human GI functions also means that human tissue research must question the ethical validity of using strains of animals with unproved translational significance.

Different responsiveness of excitatory and inhibitory enteric motor neurons in the human esophagus to electrical field stimulation and to nicotine

To compare electrical field stimulation (EFS) with nicotine in the stimulation of excitatory and inhibitory enteric motoneurons (EMN) in the human esophagus, circular lower esophageal sphincter (LES), and circular and longitudinal esophageal body (EB) strips from 20 humans were studied in organ baths. Responses to EFS or nicotine (100 microM) were compared in basal conditions, after N(G)-nitro-l-arginine (l-NNA; 100 microM), and after l-NNA and apamin (1 microM). LES strips developed myogenic tone enhanced by TTX (5 microM) or l-NNA. EFS-LES relaxation was abolished by TTX, unaffected by hexamethonium (100 microM), and enhanced by atropine (3 microM). Nicotine-LES relaxation was higher than EFS relaxation, reduced by TTX or atropine, and blocked by hexamethonium. After l-NNA, EFS elicited a strong cholinergic contraction in circular LES and EB, and nicotine elicited a small relaxation in LES and no contractile effect in EB. After l-NNA and apamin, EFS elicited a strong cholinergic contraction in LES and EB, and nicotine elicited a weak contraction amounting to 6.64 +/- 3.19 and 9.20 +/- 5.51% of that induced by EFS. EFS elicited a contraction in longitudinal strips; after l-NNA and apamin, nicotine did not induce any response. Inhibitory EMN tonically inhibit myogenic LES tone and are efficiently stimulated both by EFS and nicotinic acetylcholine receptors (nAChRs) located in somatodendritic regions and nerve terminals, releasing nitric oxide and an apamin-sensitive neurotransmitter. In contrast, although esophageal excitatory EMN are efficiently stimulated by EFS, their stimulation through nAChRs is difficult and causes weak responses, suggesting the participation of nonnicotinic mechanisms in neurotransmission to excitatory EMN in human esophagus.

Initiation of distension-induced descending peristaltic reflex in opossum esophagus: role of muscle contractility

The balloon distension (BD)-induced descending peristaltic reflex in the opossum smooth muscle esophagus is abolished in vitro when a Ca(2+)-free Krebs solution is placed at the site of distension, suggesting that either synaptic transmission occurs at the origin of the reflex or initiation of the reflex requires the development of muscle tension in response to BD. To test the latter possibility, an 8- to 10-cm length of smooth muscle esophagus was placed in a dual-chamber organ bath, isolating the stimulating (orad) from the recording site (aborad). Nifedipine addition to the orad chamber (i.e., site of distension) inhibited the BD-induced "off" contractions in both chambers in a concentration-dependent manner. However, the aborad response to electrical field stimulation (EFS) was unaffected. Atropine addition to the orad chamber had no effect on BD or EFS responses in either chamber. To examine the effects of these agents on tonic contractility, an isobaric barostat was employed. Pressure-volume curves were not altered by Ca(2+)-free Krebs solution, nifedipine, or TTX, suggesting that resting esophageal tone is not dependent on neural factors or muscle contractility. However, both Ca(2+)-free Krebs solution and nifedipine markedly decreased phasic contractions over the top of the distending bag. These observations suggest that local, stretch-induced phasic muscle contraction is required for initiation of the BD-induced descending peristaltic reflex.

Human esophageal smooth muscle cells express muscarinic receptor subtypes M(1) through M(5)

Receptor characterization in human esophageal smooth muscle is limited by tissue availability. We used human esophageal smooth muscle cells in culture to examine the expression and function of muscarinic receptors. Primary cultures were established using cells isolated by enzymatic digestion of longitudinal muscle (LM) and circular muscle (CM) obtained from patients undergoing esophagectomy for cancer. Cultured cells grew to confluence after 10-14 days in medium containing 10% fetal bovine serum and stained positively for anti-smooth muscle specific alpha-actin. mRNA encoding muscarinic receptor subtypes M(1)-M(5) was identified by RT-PCR. The expression of corresponding protein for all five subtypes was confirmed by immunoblotting and immunocytochemistry. Functional responses were assessed by measuring free intracellular Ca(2+) concentration ([Ca(2+)](i)) using fura 2 fluorescence. Basal [Ca(2+)](i), which was 135 +/- 22 nM, increased transiently to 543 +/- 29 nM in response to 10 microM ACh in CM cells (n = 8). This response was decreased <95% by 0.01 microM 4-diphenylacetoxy-N-methylpiperidine, a M(1)/M(3)-selective antagonist, whereas 0.1 microM methoctramine, a M(2)/M(4)-selective antagonist, and 0.1 microM pirenzepine, a M(1)-selective antagonist, had more modest effects. LM and CM cells showed similar results. We conclude that human smooth muscle cells in primary culture express five muscarinic receptor subtypes and respond to ACh with a rise in [Ca(2+)](i) mediated primarily by the M(3) receptor and involving release of Ca(2+) from intracellular stores. This culture model provides a useful tool for further study of esophageal physiology.

Effect of adrenoceptor agonists on striated muscle strips of the canine oesophagus

1. Acute psychological stress, which could be related to the release of a large amount of catecholamines, may cause oesophageal motility disorders. Therefore, the aim of our study was to elucidate the influence of adrenoceptor agonists on the striated muscle portion of the oesophagus by use of isolated strips from dogs. 2. Contractions were evoked in isolated striated muscle strips by electrical field stimulation (1 pulse min-1, 1 ms/pulse, submaximal voltage). The effects induced by administration of adrenoceptor agonists alone or in the presence of antagonists were tested to determine the nature of the adrenoceptors on this muscle preparation. 3. The administration of both the natural adrenoceptor agonists, adrenaline and noradrenaline, and the synthetic beta-adrenoceptor agonists, isoprenaline (beta 1 + beta 2), dobutamine (beta 1) or ritodrine (beta 2), enhanced the amplitude of the contractions induced by electrical stimulation in a concentration-dependent manner. The maximum responses were 82.6 (adrenaline), 66.2 (noradrenaline), 86.2 (isoprenaline), 34.6 (dobutamine) and 80.8% (ritodrine). The EC20 values obtained were respectively 2 nM, 0.2 microM, 0.91 nM, 3 microM and 80 nM. The administration of the alpha 1-adrenoceptor agonist, phenylephrine, also enhanced the contractile response in a concentration-dependent manner (EC20 value = 0.3 microM) and the maximum response was 64.6%, but the administration of the alpha 2-adrenoceptor agonist, clonidine, did not influence the contractile response. These data suggest the involvement of beta 2- and possibly alpha 1-adrenoceptors in the responses of these adrenoceptor agonists. 4 The selective P2-adrenoceptor antagonist ICI 118551 (3-100nM) shifted the concentration-effect curves for noradrenaline, phenylephrine and ritodrine to the right in a concentration-dependent manner.ICI 118551 (3 nM) also shifted the concentration-effect curves for adrenaline and isoprenaline to the right, but increasing the concentration of ICI 118551 did not cause any further antagonist activity until a concentration of 100 nM, when a further rightward shift was obtained.5. The selective alpha 1-adrenoceptor antagonist, prazosin (30-300 nM), did not affect the increased contractile responses induced by adrenaline, noradrenaline, phenylephrine, isoprenaline or ritodrine.6. In conclusion, it appears that beta2-adrenoceptors are present in the striated muscle portion of the canine oesophagus, where they mediate an enhancement of contractile responses evoked by electrical stimulation. The alpha l-agonist, phenylephrine, appears to interact with beta2-adrenoceptors on this preparation.beta 3-Adrenoceptors have already been demonstrated in smooth muscle from various parts of the gastrointestinal tract, and our study does not exclude the possibility that there is an additional population of beta 3-receptors in the canine striated muscle part of the oesophagus.