Effect of nicotine on the enterochromaffin like cells of the oxyntic mucosa of the rat

Nicotinic regulation of energy homeostasis

INTRODUCTION

The ability of nicotine, the primary psychoactive substance in tobacco smoke, to regulate appetite and body weight is one of the factors cited by smokers that prevents them from quitting and is the primary reason for smoking initiation in teenage girls. The regulation of feeding and metabolism by nicotine is complex, and recent studies have begun to identify nicotinic acetylcholine receptor (nAChR) subtypes and circuits or cell types involved in this regulation.

DISCUSSION

We will briefly describe the primary anatomical and functional features of the input, output, and central integration structures of the neuroendocrine systems that regulate energy homeostasis. Then, we will describe the nAChR subtypes expressed in these structures in mammals to identify the possible molecular targets for nicotine. Finally, we will review the effects of nicotine and its withdrawal on feeding and energy metabolism and attribute them to potential central and peripheral cellular targets.

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.

The contrasting influence of chronic nicotine intake on gastrin and gastric acid secretion

The effects of chronic nicotine treatment on gastric acid secretion stimulated by subcutaneous injection of pentagastrin, as well as on serum gastrin levels and the stomach parietal cell population, were examined. Rats drank a solution of nicotine 25 micrograms/mL tap water for periods of 10, 30 or 45 days. Pentagastrin increased the gastric secretory volume and acid output in pylorus-ligated control animals that drank tap water. Animals given nicotine in their drinking water for 10, 30 or 45 days showed increased basal gastric secretion and acid output. Pentagastrin produced maximum stimulatory effects at lower dose levels of 50 micrograms/kg in the 10-day treatment group and 25 micrograms/kg in the 30- or 45-day treatment groups; however, the maximum responses to pentagastrin in all nicotine-treated batches were comparable to those of their corresponding controls. Serum gastrin levels remained unchanged from the 10th day of nicotine treatment, whereas the levels in the control animals continued to rise with age. Nicotine 25 micrograms/mL drinking water given for 10, 30 or 45 days caused no significant changes in the parietal cell population, mucosal surface area or mucosal thickness. These findings are consistent with the idea that chronic nicotine administration, for at least 10 days, will lead to increased muscarinic receptor sensitivity; basal acid secretion is consequently elevated, and this in turn may depress gastrin secretion.

Physical exercise increases plasma concentrations of nicotine during treatment with a nicotine patch

The effect of physical exercise on plasma nicotine concentrations was studied in eight healthy subjects treated with a nicotine patch releasing 14 mg 24 h-1. After 11 h of patch application, plasma nicotine concentrations were measured before and after 20 min of moderate bicycle exercise, or 20 min of rest. Mean plasma nicotine concentration increased from 9.8 to 11.0 ng ml-1 (P = 0.015) during physical exercise, and fell non-significantly from 10.5 to 10.2 ng ml-1 during rest. The increase in plasma nicotine concentration during exercise is similar to that observed for transdermal nitroglycerin, and may be related to an exercise-induced increase in blood flow in the patch area.

Effects of smoking and nicotine on the gastric mucosa: a review of clinical and experimental evidence

Epidemiological and experimental evidence have shown that nicotine has harmful effects on the gastric mucosa. The mechanisms by which cigarette smoking or nicotine adversely affect the gastric mucosa have not been fully elucidated. In this report, clinical and experimental data are reviewed. The effects of nicotine from smoking on gastric aggressive or defensive factors are discussed. Nicotine potentiates gastric aggressive factors and attenuates defensive factors; it also increases acid and pepsin secretions, gastric motility, duodenogastric reflux of bile salts, the risk of Helicobacter pylori infection, levels of free radicals, and platelet-activating factor, endothelin generation, and vasopressin secretion. Additionally, nicotine impairs the therapeutic effect of H2-receptor antagonists and decreases prostaglandin synthesis, gastric mucosal blood flow, mucus secretion, and epidermal growth factor secretion. Although many of the studies provide conflicting results, the bulk of the evidence supports the hypothesis that nicotine is harmful to the gastric mucosa.