Back-Diffusion – Fact or Fiction?

Abstract

Alterations in the concentration of acid in gastric juice secreted at different flow rates and disappearance of acid from the gastric lumen, when the gastric mucosa is exposed to acid luminal contents, have been interpreted as indicating ‘back-diffusion’ of acid into the gastric mucosa from the luminal contents. The loss of acid from the gastric contents increases when the mucosa is exposed to certain drugs or is diseased, giving rise to the suggestion that the increased degree of ‘back-diffusion’ of acid indicates mucosal damage, reflecting a breakdown of the gastric mucosal ‘barrier’ to back-diffusion of acid from the gastric lumen. The change in the ‘barrier’ properties of the gastric mucosa has been found to be associated with change in the electrical properties of the mucosa, so that alterations of the transmucosal potential difference has been considered to denote gastric mucosal damage. The case for every one of these hypotheses and for their underlying assumptions is discussed and found wanting for lack of direct evidence.

Trimoprostil decreases acid disappearance from the human stomach

“…Hydrogen ions 'disappear' from the gastric contents either by reacting with bicarbonate to form carbon dioxide and water or, it has been suggested, by diffusing into the gastric mucosa down the very large activity gradient of hydrogen ions. 13 The finding of this study that the disappearance of hydrogen ions decreased after administration of trimoprostil indicates one of four possibilities: (i) that trimoprostil had stimulated the secretion of small amounts of acid; (ii) had interfered with the inhibitory effects of ranitidine on gastric secretion; (iii) had inhibited the normal secretion of bicarbonate elicited by an acid load in the stomach' or (iv) had lessened the 'normal' back diffusion of hydrogen ions into the gastric mucosa,13 perhaps by 'strengthening the gastric mucosal barrier'.…”

RELATIONSHIP BETWEEN LESION FORMATION AND PERMEABILITY OF RAT GASTRIC MUCOSA TO H+ AND OTHER CATIONS

“…However, it should be pointed out that these latter parameters, particularly p.d. and net K+ ion flux, should be interpreted with some caution (Thjodleifsson & Wormsley, 1977;Fromm, 1979). For example, the transepithelial p.d.…”

Physiological insights into electrodiffusive maintenance of gastric mucus through sensitivity analysis and simulations

“…It has been known for some time that the the gastric epithelium also secretes bicarbonate, which can neutralize dissolved hydrogen ions and contributes to the protective nature of the mucus layer [ 10 , 11 ]. However, there has been some debate if hydrogen, once transported to the stomach lumen, diffuses normally back down its own concentration gradient towards the epithelium, and if not, what mechanisms retard its diffusive flux [ 12 ]. Finally, there is no clear understanding of how the secretory processes within the epithelium are coordinated with the physical processes governing molecular diffusion through the mucus layer in order to maintain healthy gastric function.…”