Variability in the muscle composition of rat esophagus and neural pathway of lower esophageal sphincter relaxation

Generation of Spontaneous Tone by Gastrointestinal Sphincters

An important feature of the gastrointestinal (GI) muscularis externa is its ability to generate phasic contractile activity. However, in some GI regions, a more sustained contraction, referred to as "tone," also occurs. Sphincters are muscles oriented in an annular manner that raise intraluminal pressure, thereby reducing or blocking the movement of luminal contents from one compartment to another. Spontaneous tone generation is often a feature of these muscles. Four distinct smooth muscle sphincters are present in the GI tract: the lower esophageal sphincter (LES), the pyloric sphincter (PS), the ileocecal sphincter (ICS), and the internal anal sphincter (IAS). This chapter examines how tone generation contributes to the functional behavior of these sphincters. Historically, tone was attributed to contractile activity arising directly from the properties of the smooth muscle cells. However, there is increasing evidence that interstitial cells of Cajal (ICC) play a significant role in tone generation in GI muscles. Indeed, ICC are present in each of the sphincters listed above. In this chapter, we explore various mechanisms that may contribute to tone generation in sphincters including: (1) summation of asynchronous phasic activity, (2) partial tetanus, (3) window current, and (4) myofilament sensitization. Importantly, the first two mechanisms involve tone generation through summation of phasic events. Thus, the historical distinction between "phasic" versus "tonic" smooth muscles in the GI tract requires revision. As described in this chapter, it is clear that the unique functional role of each sphincter in the GI tract is accompanied by a unique combination of contractile mechanisms.

Bodyweight, not age, determines oesophageal length and breaking strength in rats

BACKGROUND/PURPOSE

Delayed primary repair is still the method of choice in the management of long-gap oesophageal atresia in many centres, but the timing of anastomoses varies. Some assume the infant's bodyweight to be an important factor, whereas others prefer age. We therefore aimed to clarify whether age or bodyweight determined oesophageal length in a rodent model.

METHODS

We explanted the oesophagi of 20 Sprague-Dawley rats, aged 15 to 444 days (n = two per time point), measured bodyweight, oesophageal length, weight, and linear breaking strength to measure tissue resilience. Univariate and multivariate regression analyses were conducted to determine the influence of age and bodyweight on oesophageal length and linear breaking strength.

RESULTS

All parameters were highly correlated (R > 0.8), except for age and linear breaking strength (R = 0.65). Both age and bodyweight were univariate significant predictors of oesophageal length, weight, and linear breaking strength (p < 0.0001). Multivariate analyses showed bodyweight to be a significant predictor of oesophageal length (p < 0.0001), whereas age was not (p = 0.18) [adjusted R² = 0.9031]. This was also true for linear breaking strength (p = 0.0007 and p = 0.97, respectively) [adjusted R² = 0.71]. Moreover, the influence of age was negligible, as the adjusted R² and the regression coefficient of bodyweight and its 95% confidence interval were almost identical between univariate und multinomial regressions.

CONCLUSIONS

Only weight determines oesophageal length and tissue resilience in rodents, whereas age is irrelevant. If a similar relationship exists in humans, it may facilitate choosing the optimum time point for delayed primary anastomosis.

LEVEL OF EVIDENCE

IV - Experimental Paper.

The Physiology of Eructation

Eructation is composed of three independent phases: gas escape, upper barrier elimination, and gas transport phases. The gas escape phase is the gastro-LES inhibitory reflex that causes transient relaxation of the lower esophageal sphincter, which is activated by distension of stretch receptors of the proximal stomach. The upper barrier elimination phase is the transient relaxation of the upper esophageal sphincter along with airway protection. This phase is activated by stimulation of rapidly adapting mechanoreceptors of the esophageal mucosa. The gas transport phase is esophageal reverse peristalsis mediated by elementary reflexes, and it is theorized that this phase is activated by serosal rapidly adapting tension receptors. Alteration of the receptors which activate the upper barrier elimination phase of eructation by gastro-esophageal reflux of acid may in part contribute to the development of supra-esophageal reflux disease.

Morphological, immunocytochemical, and functional characterization of esophageal enteric neurons in primary culture

The enteric nervous system of the esophagus plays an important role in its sensory and motor functions. Although the esophagus contains enteric neurons, they have never been isolated and characterized in primary culture. We isolated and cultured enteric neurons of the rat esophagus and determined their morphological appearance, chemical coding for neurotransmitters, and functional characteristics. After primary culture for 2 wk, dendrites and axons appeared in the enteric neurons, which usually have one axon and several dendrites. Although the size of neuronal bodies varied from Dogiel type I to type II, their average size was 39 ± 1.8 μm in length and 23 ± 1.4 μm in width. Immmunocytochemical studies revealed that over 95% of these cells were positively stained for two general neuronal markers, PGP 9.5 or Milli-Mark Fluoro. Chemical coding showed that the neurons were positively stained for choline acetyltransferease (53 ± 6%) or nNOS (66 ± 13%). In functional studies, membrane depolarization and stimulation of several G protein-coupled receptors (GPCRs) induced Ca²⁺ signaling in the esophageal enteric neurons. The GPCR stimulation was found to induce both intracellular Ca²⁺ release and extracellular Ca²⁺ entry. The functional expressions of Ca²⁺ channels (voltage-gated Ca²⁺ channels and store-operated channels) and Ca²⁺ pump (sarcoplasmic reticulum Ca²⁺-ATPase) were also demonstrated on these neurons. We have grown, for the first time, esophageal enteric neurons in primary culture, and these contain excitatory and inhibitory neurotransmitters. The functional integrity of GPCRs, Ca²⁺ channels, and Ca²⁺ pump in these neurons makes them a useful cell model for further studies.