Abstract

Background: Aldosterone has a rapid, non-genomic, inhibitory effect on macroscopic basolateral K⁺ conductance in the human colon, reducing its capacity for Cl⁻ secretion. The molecular identity of the K⁺ channels constituting this aldosterone inhibitable K⁺ conductance is unclear.

Aim: To characterise the K⁺ channel inhibited by aldosterone present in the basolateral membrane of human colonic crypt cells.

Methods: Crypts were isolated from biopsies of healthy sigmoid colon obtained during colonoscopy. The effect of aldosterone on basolateral K⁺ channels, and the possible involvement of Na⁺:H⁺ exchange, were studied by patch clamp techniques. Total RNA from isolated crypts was subjected to reverse transcriptase-polymerase chain reaction (RT-PCR) using primers specific to intermediate conductance K⁺ channels (KCNN4) previously identified in other human tissues.

Results: In cell attached patches, 1 nmol/l aldosterone significantly decreased the activity of intermediate conductance (27 pS) K⁺ channels by 31%, 53%, and 54% after 1, 5 and 10, minutes, respectively. Increasing aldosterone concentration to 10 nmol/l produced a further 56% decrease in channel activity after five minutes. Aldosterone 1–10 nmol/l had no effect on channel activity in the presence of 20 μmol/l ethylisopropylamiloride, an inhibitor of Na⁺:H⁺ exchange. RT-PCR identified KCNN4 mRNA, which is likely to encode the 27 pS K⁺ channel inhibited by aldosterone.

Conclusion: Intermediate conductance K⁺ channels (KCNN4) present in the basolateral membranes of human colonic crypt cells are a target for the non-genomic inhibitory effect of aldosterone, which involves stimulation of Na⁺:H⁺ exchange, thereby reducing the capacity of the colon for Cl⁻ secretion.