Mechanosensitive ion channels in interstitial cells of Cajal and smooth muscle of the gastrointestinal tract.

Normal gastrointestinal (GI) motility is required to mix digestive enzymes and food and to move content along the GI tract. Underlying the complex motor patterns of the gut are electrical events that reflect ion flux across cell membranes. Smooth muscle electrical activity is directly influenced by GI interstitial cells of Cajal, whose rhythmic oscillations in membrane potential in part determine the excitability of GI smooth muscle and its response to neuronal input. Coordinated activity of the ion channels responsible for the conductances that underlie ion flux in both smooth muscle and interstitial cells is a requisite for normal motility. These conductances are regulated by many factors, including mechanical stress. Recent studies have revealed mechanosensitivity at the level of the ion channels, and the mechanosensor within the channel has been identified in many cases. This has led to better comprehension of the role of mechanosensitive conductances in normal physiology and will undoubtedly lead to understanding of the consequences of disturbances in these conductances.

Species dependent expression of intestinal smooth muscle mechanosensitive sodium channels.

A mechanosensitive Na(+) current carried by Na(v)1.5 is present in human intestinal circular smooth muscle and contributes to regulation of intestinal motor function. Expression of this channel in different species is unknown. Our aim was to determine if Na(+) currents and message for the alpha subunit of the Na(+) channel (SCN5A) are found in circular smooth muscle cells of human, dog, pig, mouse and guinea pig jejunum. Currents were recorded using patch clamp techniques. Message for SCN5A was investigated using laser capture microdissection and reverse transcription polymerase chain reaction (RT-PCR). Na(+) currents were identified consistently in human and dog smooth muscle cells; however, Na(+) current was not found in pig (0/20) or guinea pig smooth muscle cells (0/21) and found only one mouse cell (1/21). SCN5A mRNA was found in circular muscle of human, dog, and mouse, but not in pig or guinea pig, and not in mouse longitudinal or mucosal layers. In summary, SCN5A message is expressed in, and Na(+) current recorded from, circular muscle layer of human and dog but not from pig and guinea pig. These data show that there are species differences in expression of the SCN5A-encoded Na(v)1.5 channel, suggesting species-specific differences in the electrophysiological response to mechanical and depolarizing stimuli.

Opiate-induced oesophageal dysmotility.

BACKGROUND: Opiates have well characterized (troublesome) untoward effects on the gastrointestinal tract. Opioid bowel dysfunction has been a subject of research and even drug design, but surprisingly little is known with regard to clinical effects of opiates on the oesophagus. AIM: To characterize opiate effects on motor function of the oesophagus in patients presenting with dysphagia. METHODS: Retrospective review of 15 patients with dysphagia referred for oesophageal manometry while on chronic opiates. Manometry was completed during opiate use and in three cases, after opiates were discontinued. RESULTS: All patients demonstrated motility abnormalities. Incomplete lower oesophageal sphincter (LOS) relaxation (11.5 +/- 1.6 mmHg) was seen in most cases. Ten patients demonstrated nonperistaltic contractions in > or =3 of 10 swallows. Additional abnormalities included high amplitude contractions; triple peaked contractions; and increased velocity. The average resting lower oesophageal sphincter (LOSP) met criteria for hypertensive LOS in three patients. These features were suggestive of spasm or achalasia. Repeat manometry off opiates was performed in three cases. LOS relaxation was noted to be complete upon repeat manometry in these cases. There was also improved peristalsis and normal velocity. CONCLUSIONS: A range of manometric abnormalities were seen in patients with dysphagia in the setting of opiate use: impaired LOS relaxation, high amplitude/velocity and simultaneous oesophageal waves. These data suggest that the oesophagus is susceptible to the effects of opiates and care must be taken before ascribing dysphagia to a primary oesophageal motility disorder in patients taking opiates.

Achalasia: physiology and etiopathogenesis.

Achalasia is a disorder of esophageal motility that has been well documented for over 300 years. Despite this, the initiating factor or factors and the underlying mechanisms leading to the characteristic features of achalasia, the absence of distal esophageal peristalsis and abnormal lower esophageal sphincter relaxation, are still not well understood. Recent work has shed light on changes in neurotransmission and cell signaling in the lower esophagus and lower esophageal sphincter that lead to achalasia. A number of recent reviews have thoroughly discussed diagnostic and therapeutic modalities and the reader is referred to these for in-depth review of these topics. The focus of this review will be on our current understanding of the physiology of esophageal peristalsis and lower esophageal sphincter function as it relates to achalasia and on available evidence for etiology and proposed pathophysiologic mechanisms.

Differences in rates of benzene metabolism correlate with observed genotoxicity.

Benzene (BZ) requires oxidative metabolism via cytochrome P450 2E1 (CYP 2E1) to exert its hematotoxic and genotoxic effects. Male mice are two- to threefold more sensitive to the genotoxic effects of BZ as measured by micronuclei induction and sister chromatid exchanges. The purpose of our study was to investigate sex-related differences in the metabolism of BZ, phenol (PHE) and hydroquinone (HQ) in order to understand the metabolic basis for sex-dependent differences in BZ genotoxic susceptibility in mice. Rates of BZ oxidation were quantitated using closed chamber gas uptake studies with male and female B6C3F1 mice exposed to initial low (400-500 ppm), intermediate (1200-1300 ppm), and high (2600-2800 ppm) BZ concentrations. Acetone-pretreated and diethyldithiocarbamate-pretreated male mice were also studied to determine the extent to which induction and inhibition of CYP 2E1, respectively, would alter in vivo BZ oxidation rates. Elimination of PHE and HQ from blood was also compared in male and female mice to complement previously reported data on sex-related differences in urinary excretion of conjugated metabolites following iv administration of PHE. Based on PBPK model analysis, the optimized rate of metabolism (Vmax) of BZ was almost twofold higher in male mice (14.0 mumol/hr-kg) than in female mice (7.9 mumol/hr-kg); both male and female mice gas-uptake data were well fit with a KM of 3.0 microM. Pretreatment of male mice with 1% acetone in drinking water for 8 days to specifically induce CYP 2E1 enhanced the rate of BZ oxidation by approximately fivefold (Vmax = 75 mumol/hr-kg), while diethyldithiocarbamate pretreatment (320 mg/kg ip 30 min prior to uptake study) completely inhibited BZ oxidation (Vmax = 0 mumol/hr-kg). Thus, both pretreatment regimens are potentially useful investigative tools to study the metabolic basis for benzene toxicity. Elimination of PHE from blood was significantly faster in male mice, while elimination of HQ did not differ between male and female mice. Previous data indicated male mice produce more of the oxidized and conjugated metabolite, HQ glucuronide, after PHE administration, suggesting that HQ production from PHE is greater in male mice. Taken together, these data support the hypothesis that the greater sensitivity of male mice to the genotoxic effects of BZ compared to females is a function of greater oxidative metabolism toward both BZ and PHE in male mice. These data also suggest that differences in hepatic human CYP 2E1 activity may be an important factor to consider when evaluating human risk for benzene-induced hematotoxic and genotoxic effects.