Gastric nitrite processing in the surgically altered maximal and minimal bile reflux ferret model.

Ingested nitrate and nitrite have been shown to contribute to endogenous, N-nitroso compound formation in man and experimental animals. N-nitroso compounds have long been suspected of contributing to higher levels of gastric cancer in various populations. Reconstructive gastric surgery to treat ulcers is accompanied by a change in bile reflux, gastritis and an increased incidence of gastric cancer in humans. To evaluate possible connections between gastric nitrite processing, reconstructive surgery and gastric cancer, the surgically altered domestic ferret, Mustela putorius furo, was used as an experimental model. The aim of the study was to determine if surgery would alter the stomach in a way which would increase gastric nitrite concentration, and thereby enhance the likelihood of gastric N-nitroso compound formation. Three groups of ferrets, one control group (n = 6) and two groups of surgically altered ferrets, one to simulate maximal bile reflux (MABR, n = 6), and the other to model minimal bile reflux (MIBR, n = 7), were studied. Each group's response to an exogenously administered dose of sodium nitrite did not differ significantly with respect to rate of gastric nitrite absorption, with half-lives in the 13-min range. Permeability of gastric mucosa to nitrite did not differ between controls and MIBR ferrets. Mean doubling time of gastric nitrate appeared slowed in surgically altered ferrets. Mean rate of gastric emptying was the same in the three groups, but appeared delayed initially in MIBR ferrets. Thiocyanate concentrations, pH and HCl secretion, all parameters which have been shown to affect gastric nitrite processing, did not differ significantly between groups. Gastric mucosal endoscopic biopsies obtained at 6-month intervals showed no clear difference in degree of mucosal inflammation and/or dysplasia in the three groups. These findings indicate that gastric mucosal neoplasia has not occurred in this model and that changes in parameters favoring gastric N-nitrosation, even if relevant to the disease process, are not apparent at this time.

Theoretical model for predicting rates of nitrosamine and nitrosamide formation in the human stomach.

A mathematical model has been developed to estimate the rates of formation of nitrosamines and nitrosamides in the human stomach, under a variety of physiological and environmental conditions. The model combines a detailed description of the kinetics of N-nitrosation with mass balance equations which account for gastric emptying, dilution and absorption. The simulations were based on a typical schedule of dietary inputs, and included variations in gastric pH and in the volume of the stomach contents over a 24-h period. Consideration of these transient phenomena allowed a distinction to be made between amines or amides present in the diet and in gastric or salivary secretions. A comparison of the theoretical results with available data on the nitrosation of proline suggests that the model accurately predicts gastric rates of nitrosamine formation under control conditions, and correctly represents the strong catalytic effects of thiocyanate and the inhibitory effects of ascorbic acid or ascorbate ion. The results further suggest that nitrosoproline (NPro) excretion is not an accurate index of gastric nitrosation under physiological (low-dose) conditions, even when corrections are made for dietary intake of NPro. The predicted rates of formation of N-nitrosodimethylamine (NDMA), even for a diet high in dimethylamine, were found to be a factor of approximately 10(2) to 10(3) lower than published estimates of the dietary exposure to preformed NDMA. Thus, these findings do not support the hypothesis that gastric formation of NDMA from dietary dimethylamine poses a serious additional health risk. The results are presented in a graphical and tabular form which makes it possible to readily estimate the rates of formation of other nitrosamines or nitrosamides in the stomach, under various assumed conditions.

Use of ascorbic acid to inhibit nitrosation: kinetic and mass transfer considerations for an in vitro system.

Ascorbic acid and ascorbate ion (denoted together as ASC) inhibit nitrosation by competing for the nitrosating agents formed from nitrite (e.g. N2O3, NO+ and NOSCN). ASC is oxidized irreversibly by this reaction and the nitrite equivalents are reduced to nitric oxide (NO). In open, aerobic systems the effective stoichiometry of the reaction between ASC and nitrite is not fixed, but is determined by a competition between the physical removal of NO (and NO2) from the system and the oxidation of NO by dissolved O2. The oxidation of NO reconverts it to a nitrosating agent which may react again with the remaining ASC. To determine the conditions under which ASC is most effective as a nitrosation inhibitor, we examined the kinetics of the reactions between nitrite and ASC and between nitrite and proline. These reactions were studied in open shaker flasks as functions of pH, anion composition (SCN- and Cl-), temperature, and gas-liquid mass transfer rate. At lower mass transfer rates, at lower pH and/or in the presence of SCN- or Cl-, relatively more ASC was consumed by a given amount of nitrite. Increased temperature caused more or less ASC to be consumed by a given amount of nitrite, depending on the conditions. A mathematical model of the reactions and mass transfer steps was developed which describes each of these features. The model predicts the variable stoichiometry of the reaction between nitrite and ASC in open, aerobic systems, and clarifies the mechanisms by which ASC inhibits nitrosation.

Effects of ascorbic acid and thiocyanate on nitrosation of proline in the dog stomach.

To elucidate the factors governing the formation of N-nitrosamines in the stomach, the formation of N-nitrosoproline (NPro) was studied under controlled conditions, using a dog equipped with a Thomas cannula. Solutions containing nitrite, proline and in some cases ascorbic acid and/or SCN-, were infused into the stomach and samples taken to determine gastric [nitrite], [NPro], [ASC], [SCN-] and pH as functions of time. (Brackets indicate molar concentrations; ascorbic acid and ascorbate ion are denoted together by ASC.) Previous work showed that the rapid decline of [nitrite] in the stomach was due primarily to absorption. Additional experiments here in which ASC, proline or NPro were infused together with a non-absorbable marker, in the absence of nitrite, demonstrated that there was negligible absorption or secretion of these substances in the stomach. Thus, changes in [ASC] and [NPro] with time could be interpreted quantitatively in terms of rates of chemical reaction and dilution of the stomach contents. A mathematical model, based on mass balance equations for the various chemical species and chemical kinetic data obtained previously from in vitro studies, was developed for this purpose. The ability of ASC to inhibit nitrosation (by reaction with nitrite) was shown to be highly dependent on initial [ASC] and on the rate of entry of O2 into the stomach from blood. The rate of NPro formation in the absence of ASC and SCN-, the inhibitory effects on nitrosation of ASC and the catalytic effects of SCN-, were all accurately predicted by the mathematical model. This suggests that similar models may prove useful in estimating rates of intragastric N-nitrosation reactions in humans, under various assumed conditions.

Mechanisms for nitrite loss from the stomach.

Nitrite loss from the stomach was studied using dogs equipped with Thomas cannulas for direct access to the stomach lumen. Solutions containing sodium nitrite and non-absorbable volume marker (polyethylene glycol, PEG) were infused into the stomach, and samples were taken over 60 min to determine the concentration of 'total nitrite' (including NO2-, HNO2 and other species in equilibrium with NO2-) and rate of dilution of the stomach contents as a function of time. Changes in stomach volume were also measured. Nitrite loss was found to be very rapid, with total nitrite concentrations declining to less than half the initial levels in 10 min. The decay in total nitrite concentrations was due predominantly to gastric absorption, with small additional contributions from dilution of the stomach contents (inferred from PEG concentrations) and chemical reactions (from in vitro kinetic data). Results for initial nitrite concentrations varying over a range of 0.15-4.5 mM showed absorption to be first order in total nitrite. The permeability-area product for nitrite absorption (PA) was about 0.6 l/h, and was unaffected by the addition of 1 mM SCN- or Cl-. All of these results are consistent with nitrite absorption in the form of NO2- or HNO2. Buffering the infusate with HCO3- to increase luminal pH from approximately 2 to 7 caused a three-fold reduction in the apparent value of PA. When pentagastrin was used to stimulate acid secretion, nitrite absorption was only half as fast as when acid secretion was inhibited with cimetidine, or when no drug was given. This effect could not be explained by variations in luminal pH, and suggests that acid secretion either decreases PA or is accompanied by active secretion of nitrite. Based on these data, a mathematical model was developed to stimulate the physical and chemical factors governing nitrite concentrations in the stomach.