Potassium transport by rabbit descending colon

Unidirectional mucosal- to-serosal (J(m→s)) and serosal-to-mucosal (J(s→m)) fluxes of potassium and sodium were determined simultaneously on paired sections of descending colon from the same rabbit under short-circuit conditions. In 13-16 pairs of tissues, net potassium secretion and sodium absorption averaged 0.49 ± 0.08 and 4.0 ± 0.8 μeq.cm^-2.h^-1, respectively. Short-circuit current (J(sc)) averaged 3.7 ± 0.4 μeq.cm^-2.h^-1 and was approximately equal to the algebraic sum of net potassium and sodium fluxes. Treatment of both sides of the colon with 10^-4M ouabain reduced the J(sc) and transmural potential difference to near zero. Ouabain abolished net potassium secretion by reducing J(s→m)(K) and abolished net sodium absorption by inhibiting J(m→s)(Na). In the presence of ouabain, net potassium absorption averaging 0.15 ± 0.07 μeq.cm^-2.h^-1 (n = 11) was observed. In the presence of 10^-3 M 2,4-dinitrophenol, both net potassium and net sodium fluxes were abolished, primarily as a result of a reduction in J(s→m)(K) and J(m→s)(Na) without altering J(s→m)(K) and J(m→s)(Na). These results suggest that the rabbit descending colon has the capacity to secrete and possibly to absorb potassium by active mechanisms requiring metabolic energy. Comparison of potassium and sodium fluxes suggest that the paracellular pathway in the rabbit colon is not potassium selective.