Active K Transport Across Rabbit Distal Colon: Relation to Na Absorption and Cl Secretion

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We measured isotopic unidirectional fluxes of K to elucidate the mechanisms of active K transport across the distal colon of the rabbit. Separate pathways for active K absorption and active K secretion were detected using various transport inhibitors and stimulators. The rate and direction of net K transport depend on the activities of these two pathways. K absorption was reduced by orthovanadate (both solutions) or serosal Ba, consistent with ATPase-dependent uptake of K across the apical membrane and exit via a Ba-sensitive basolateral K conductance. K secretion was inhibited by serosal ouabain or mucosal Ba, indicating that K secretion involves basolateral uptake via the Na-K pump and apical exit via a Ba-sensitive K conductance. Active K secretion appears to be electrogenic, since inhibition by ouabain produced equivalent changes in the net K flux and short-circuit current. Addition of bumetanide to the serosal solution or the removal of either Na or Cl from the serosal solution inhibited K secretion; mucosal solution amiloride was without effect. These results indicate that this K secretory process is independent of electrogenic Na absorption but is mechanistically similar to Cl secretory processes. Both epinephrine and prostaglandin E2 (PGE2) stimulate K secretion, but only PGE2 also stimulates Cl secretion. The response to these secretogogues suggests that the mechanisms underlying K and Cl secretion are closely linked but can be regulated independently.