RESUMEN
The Ussing chamber technique was used to measure unidirectional Rb(+) fluxes under short-circuit conditions across tissue sheets from proximal, central, and distal jejunum of rats. Whereas the proximal and central parts of the jejunum did not show any net transport of Rb(+), there was a net secretion of around 0.2 micromol hr(-1) cm(-2) in the distal segment. This secretion could not be influenced significantly by mucosal application of K(+) channel blockers such as Ba(2+) (5 mm), tetraethylammonium (20 mm) or quinine (1 mm). Serosal ouabain (1 mm) blocked net secretion by increasing mucoserosal flux. Blockers of H(+)/K(+) ATPases could not alter net fluxes of Rb(+). Stimulation of Cl(-) secretion by forskolin (10 microm) or of Na(+) absorption by serine (10 mm) failed to influence the observed secretion of Rb(+). Adrenaline (10 microm) also had no effect on Rb(+) fluxes. Blocking Na(+)/H(+) exchange by 5-(N-Ethyl-N-isopropyl)-amilorid (100 microm) blocked net secretion by increasing mucoserosal flux, as did the addition of Na(+) acetate (30 mm) to the mucosal solution. We conclude that the distal jejunum of the rat secretes K(+) under short-circuit conditions. This secretion does not seem to occur via K(+) channels, but through a pH dependent mechanism.
