Effect of ouabain on Na,K-ATPase and electrolyte transport in the dog ileum in vivo

SUMMARY It is currently believed that the rate and direction of sodium transport in the small intestine may be regulated by the activity of Na,K-ATPase in the basolateral cell membrane. We tested this hypothesis by selectively infusing ouabain, a known inhibitor of Na,K-ATPase, into the mesenteric artery supplying a perfused loop of ileum in 18 dogs. Before ouabain infusion there were significant correlations between the activity of Na,K-ATPase and net and lumen to plasma fluxes of sodium and chloride. After ouabain, there was no significant change in sodium and chloride trans-port, unidirectional fluxes or transmucosal potential difference, despite a 50 % reduction in Na,K-ATPase activity. Furthermore, there was no significant correlation between Na,K-ATPase activity and sodium or chloride transport after ouabain. The only statistically significant effect of ouabain infusion was a reduction in the rate of bicarbonate secretion. Thus, the results of our experiments suggest that mucosal Na,K-ATPase is not a rate-limiting step in the absorption of sodium and chloride in the dog ileum, though it may be an important facilitative factor. The normal small intestine in vivo usually absorbs physiological electrolyte solutions, although the rate of absorption is highly variable from subject to subject, and in the same subject on different days (Turnberg et al., 1970). In some apparently normal people and animals, the small intestine even secretes fluid, rather than absorbs it (Florey et al., 1941; Scott, 1965; Turnberg etal., 1970). In some animals, secretion is more common than absorption (Powell et al., 1968). One of the plausible explanations of variable rates of absorption-secretion is the activity of the baso-lateral membrane concentration of sodium and potassium-dependent ATPase (Na,K-ATPase) (Schultz et al., 1974). This enzyme is believed to be the biochemical counterpart of a sodium pump, which transports sodium in exchange for potassium across the basolateral membrane (Hokin, 1976). It is postulated that the Na,K-ATPase pumps more sodium out of the cell than potassium into the cel