In vitro potassium transport in the mouse small intestine.

Ingested K+ is believed to be absorbed mainly in the small intestine by passive diffusion through the paracellular pathway. To further clarify K+ absorption in the small intestine, we determined the unidirectional flux values of Rb+ in vitro by atomic absorption spectroscopy in the mouse ileum mounted in Ussing chambers under short-circuit conditions. The mucosal-to-serosal Rb+ flux (J(ms)) was larger than the serosal-to-mucosal Rb+ flux (J(sm)), resulting in positive net Rb+ absorption (J(net)). The effect of changing the transmucosal potential (V(t)) showed that J(ms) was composed of both a V(t)-dependent diffusion component and a V(t)-independent non-diffusion component, while J(sm) was composed mainly of a V(t)-dependent component. A forskolin treatment eliminated J(net) mainly due to the increase in J(sm). When animals were fed a low-Na diet, J(net) was mainly eliminated as a result of the increase in J(sm). These findings suggest that K+ is absorbed not only by passive diffusion through the paracellular pathway, but also by an active transport mechanism operating through the cellular pathway. In addition, cAMP and aldosterone may be involved in regulating intestinal K+ transport.