Effects of cholera toxin on cellular and paracellular sodium fluxes in rabbit ileum

The diarrhea observed in patients which cholera is known to be related to secretion of water and electrolytes into the intestinal lumen. However, the exact mechanisms involved in these secretory processes have remained unclear. Although it is clear that purified toxin acts on epithelial cell metabolism, its activity on Na⁺ transport across intestinal mucosa is equivocal: reported either to prevent net Na⁺ absorption or to cause net secretion of Na⁺ from serosa to mucosa. Since total transmural Na⁺ fluxes across "leaky" epithelia involve very significant movement via a paracellular shunt pathway, we studied the effects of cholera toxin on the cellular and paracellular pathways of Na⁺ movement. Unidirectional Na⁺ fluxes were examined as functions of applied potential in control tissues and in tissues from the same animal treated with purified cholera toxin. Treatment of rabbit ileum in vitro with toxin stimulated the cellular component of serosa-to-mucosa Na⁺ flux (from 2.41 ± 0.49 μequiv./h per cm² under control conditions to 4.71 ± 0.43 μequiv./h per cm² after treatment with toxin, P < 0.01). The effect of cholera toxin on Na⁺ movement through the cells from mucosa to serosa appeared to be insignificant. Finally, a marked decrease in the Na⁺ permeability (P < 0.01) and no detectable significant changes in transference number for Na⁺ of the paracellular shunt pathway were observed following treatment with cholera toxin. These results provide direct evidence for the hypothesis that purified cholera toxin stimulates active sodium secretion but has minimal effect on sodium absorption.