Characteristics of Sodium Flux from Serosa to Mucosa in Rabbit Ileum

Abstract

Sodium flux from serosa to mucosa, JN, in rabbit ileum in vitro has been studied as a function of applied electrical potential at equal sodium concentrations in the bathing solutions. The results indicate that J involves two pathways, a diffusional flux through a paracellular shunt pathway and a flux that is independent of applied potential and presumably involves a transcellular pathway. The latter pathway comprises approximately 25 % of J in Ringer's solution containing 10 mM glucose and 25 mM bicarbonate. It is stimulated significantly by theophylline unaffected by removal of glucose or addition of ouabain but is reduced to negligible values by anoxia, dinitrophenol, and replacement of all chloride and bicarbonate by isethionate. Thus this component of J:L has a number of characteristics consistent with involvement in a specific secretory process mediating an electrically neutral secretory transport of sodium plus anion from serosa to mucosa. In addition to stimulating this process, theophylline significantly reduced the permeability of the paracellular shunt pathway to sodium.Recent studies on ion transport across rabbit ileum have raised certain questions about mechanisms involved in movement of ions from the serosal to the mucosal side of the tissue. For example, Frizzell and Schultz (1) have suggested that nearly all of the Na flux from serosa to mucosa across in vitro preparations of rabbit ileum occurs via the intercellular shunt pathway they have described (see also Rose and Schultz [2]). On the other hand, Powell et al. have proposed that there may be an active secretory process for Na in guinea pig ileum (3) and rabbit ileum (4, 5) that accounts for a part of the observed serosal-to-mucosal flux and that this process may be stimulated by cholera toxin (6) and theophylline (5). Such a process might be expected to involve transcellular movement of Na and would not be consistent with purely passive Na movement from serosa to mucosa via a shunt pathway. The situation is further complicated by the suggestion of Field et al. (7,8) that the THE JOURNAL OF GENERAL PHYSIOLOGY

Na and Cl transport and short-circuit current in rabbit ileum

“…This equality in diffusional Na fluxes is consistent with the concept that there should be no net diffusion of any ion across the tissue under short-circuit conditions in the steady state. It is also in agreement with the PD dependent diffusional Na fluxes via the paracellular shunt pathway described by Desjeux et al [7]. The linear relationship between JsY2 and [Na] might suggest that the J~ is entirely attributable to simple ionic diffusion [24].…”

“…The results strongly suggest that jya represents Isc and that the difference between the j N~ and I~ at 140 mM is due to an onset of an "electrically neutral" NaC1 flux from serosa to mucosa. In addition, the observation by Desjeux et al [7] that the transcellular serosa-to-mucosa Na flux obtained under various conditions has no clear connection with I~c also lends support to this suggestion.…”

“…The technique for separating the unidirectional transmural Na flux into a diffusional (presumably via a paracellular shunt pathway) and a nondiffusional (presumably via a transcellular pathway) component was similar to that used by Desjeux et al [7], Frizzell and Schultz [14], and Schultz and Zalusky [23]. In brief, two adjacent pieces of tissue from the same animal were bathed in identical solution.…”

“…The linear relationship between JsY2 and [Na] might suggest that the J~ is entirely attributable to simple ionic diffusion [24]. However, the compelling evidence of a serosa-to-mucosa PD independent transcellular Na flux obtained by Desjeux et al [7] indicates that the J~ at 140 mM Na contains two components. The question of whether the Jff~ at 140 mM is completely diffusional or contains two components may be clarified by the relationship between the tissue conductance and [Na], and by the relationship between the diffusional Na flux and [Na] determined in the next experiments.…”

Antigen Absorption by the Jejunal Epithelium of Children with Cow's Milk Allergy

“…This conclusion is based on the following data: 1) The electrical tissue conductance which is a good index of the electrolyte paracellular pathway in the small intestine (24) was not increased at any stages of the disease.…”