Deleterious action of vasopressin in gastroduodenal ulceration: experimental and clinical observations

The pharmacological actions of nicotine on the gastrointestinal tract

Increasing use of tobacco and its related health problems are a great concern in the world. Recent epidemiological findings have demonstrated the positive association between cigarette smoking and several gastrointestinal (GI) diseases, including peptic ulcer and cancers. Interestingly, smoking also modifies the disease course of ulcerative colitis (UC). Nicotine, a major component of cigarette smoke, seems to mediate some of the actions of cigarette smoking on the pathogenesis of GI disorders. Nicotine worsens the detrimental effects of aggressive factors and attenuates the protective actions of defensive factors in the processes of development and repair of gastric ulceration. Nicotine also takes part in the initiation and promotion of carcinogenesis in the GI tract. In this regard, nicotine and its metabolites are found to be mutagenic and have the ability to modulate cell proliferation, apoptosis, and angiogenesis during tumoriogenesis through specific receptors and signalling pathways. However, to elucidate this complex pathogenic mechanism, further study at the molecular level is warranted. In contrast, findings of clinical trials give promising results on the use of nicotine as an adjuvant therapy for UC. The beneficial effect of nicotine on UC seems to be mediated through multiple mechanisms. More clinical studies are needed to establish the therapeutic value of nicotine in this disease.

Smoking and the pathogenesis of gastroduodenal ulcer--recent mechanistic update

Peptic ulcer is a common disorder of gastrointestinal system and its pathogenesis is multifactorial, where smoking and nicotine have significant adverse effects. Smoking and chronic nicotine treatment stimulate basal acid output which is more pronounced in the smokers having duodenal ulcer. This increased gastric acid secretion is mediated through the stimulation of H2-receptor by histamine released after mast cell degranulation and due to the increase of the functional parietal cell volume or secretory capacity in smokers. Smoking and nicotine stimulate pepsinogen secretion also by increasing chief cell number or with an enhancement of their secretory capacity. Long-term nicotine treatment in rats also significantly decreases total mucus neck cell population and neck-cell mucus volume. Smoking also increases bile salt reflux rate and gastric bile salt concentration thereby increasing duodenogastric reflux that raises the risk of gastric ulcer in smokers. Smoking and nicotine not only induce ulceration, but they also potentiate ulceration caused by H. pylori, alcohol, nonsteroidal anti-inflammatory drugs or cold restrain stress. Polymorphonuclear neutrophils (PMN) play an important role in ulcerogenesis through oxidative damage of the mucosa by increasing the generation of reactive oxygen intermediates (ROI), which is potentiated by nicotine and smoking. Nicotine by a cAMP-protein kinase A signaling system elevates the endogenous vasopressin level, which plays an aggressive role in the development of gastroduodenal lesions. Smoking increases production of platelet activating factor (PAF) and endothelin, which are potent gastric ulcerogens. Cigarette smoking and nicotine reduce the level of circulating epidermal growth factor (EGF) and decrease the secretion of EGF from the salivary gland, which are necessary for gastric mucosal cell renewal. Nicotine also decreases prostaglandin generation in the gastric mucosa of smokers, thereby making the mucosa susceptible to ulceration. ROI generation and ROI-mediated gastric mucosal cell apoptosis are also considered to be important mechanism for aggravation of ulcer by cigarette smoke or nicotine. Both smoking and nicotine reduce angiogenesis in the gastric mucosa through inhibition of nitric oxide synthesis thereby arresting cell renewal process. Smoking or smoke extract impairs both spontaneous and drug-induced healing of ulcer. Smoke extract also inhibits gastric mucosal cell proliferation by reducing ornithine decarboxylase activity, which synthesises growth-promoting polyamines. It is concluded that gastric mucosal integrity is maintained by an interplay of some aggressive and defensive factors controlling apoptotic cell death and cell proliferation and smoking potentiates ulcer by disturbing this balance.

Vasopressin pressor receptor-mediated activation of HPA axis by acute ethanol stress in rats

The plasma arginine vasopressin (AVP), ACTH, and corticosterone levels and the hypothalamic corticotropin-releasing hormone (CRH) content were measured after oral administration of 1 ml of 75% ethanol to rats, a model known to induce acute gastric erosions and stress. Elevated plasma AVP, ACTH, and corticosterone levels were detected 1 h after ethanol administration. Treatment with the vasopressin pressor (V(1)) receptor antagonist [d(CH(2))(5)Tyr(Me)-AVP] before ethanol administration significantly reduced the ACTH and corticosterone level increases. A higher hypothalamic CRH content was measured at 30 or 60 min after ethanol administration. V(1) receptor antagonist injection, 5 min before ethanol administration, inhibited the rise in hypothalamic CRH content. The protein synthesis blocker cycloheximide prevented the hypothalamic CRH content elevation after stress. The AVP-, CRH-, and AVP + CRH-induced in vitro ACTH release in normal anterior pituitary tissue cultures was also prevented by pretreatment with the V(1) receptor antagonist. The results support the hypothesis that stress-induced AVP may not only act directly on the ACTH producing anterior pituitary cells but also indirectly at the hypothalamic level via the synthesis and release of CRH.

Gastric mucosal endogenous prostacyclin levels are different in Brattleboro rats compared with Wistar strain

Homozygous Brattleboro rats were investigated and compared to normal (physiological) Wistar strain rats regarding their gastric mucosal endogenous prostacyclin (PG-I(2)) level. It seems that the Brattleboro animals have a significantly lower level of this important protective material. Wistar rats having an artificial pituitary stalk lesion (which is the artificial equivalent of homozygous Brattleboro animals) showed no differences in endogenous mucosal prostacyclin level compared to normal Wistar rats. Therefore, we concluded that this hitherto unknown property of the homozygous Brattleboro rats is genetically determined.