Aldosterone Stimulates K Secretion across Mammalian Colon Independent of Na Absorption

K transport across guinea pig (Cavia porcellus) distal colon was measured in vitro using isotopically determined unidirectional fluxes. Aldosterone stimulated electrogenic Na absorption, as measured by amiloride-sensitive short-circuit current (I _sc ), and reduced net K absorption from +2.5 ± 0.2 µEq/cm ² per hr to +0.8 ± 0.3 µEq/cm ² per hr (mean ± SEM). Amiloride addition to the mucosal solution did not enhance net K absorption, as expected if inhibiting active Na absorption would reduce active K secretion as in the distal nephron. The amiloride-insensitive I _sc was -1.0 ± 0.2 µEq/cm ² per hr (mean ± SEM) and was inhibited by mucosal addition of Ba, a K channel blocker. Addition of bumetanide to the serosal solution also inhibited this negative I _sc , and K transport returned to the control level of net absorption. Thus, the amiloride-insensitive, negative I _sc is consistent with active K secretion stimulated by aldosterone. This stimulation of an active K secretory pathway by aldosterone occurred without altering the active K absorption pathway that also is present. These results indicate that the aldosterone-stimulated K secretory pathway operates independently of the amiloride-sensitive Na absorption pathway, which also is stimulated by aldosterone.