HCO₃ ^– potentiates the cAMP-dependent secretory response of the human distal colon through a DIDS-sensitive pathway

Abstract.

We used the Ussing short-circuit technique to investigate the role of HCO₃ ^– in the adenosine 3′,5′-cyclic monophosphate (cAMP)-dependent secretory response of the human distal colon. In HCO₃ ^–-free 4-(2-hydroxyethyl)-1-piperazineethanesulphonic acid (HEPES)-Ringer's, forskolin (10 µmol l^–1 mucosal and serosal) evoked a sustained increase in short-circuit current (I _sc) (ΔI _sc=24±3 µA cm^–2, n=57). However, this was only 25% of the forskolin-stimulated I _sc response in HCO₃ ^–-Ringer's (ΔI _sc=84±8 µA cm^–2, n=57). The reduced response to forskolin in HCO₃ ^–-free HEPES-Ringer's was not due to inhibition of the secretory mechanism by HEPES, as replacing HCO₃ ^– with a different buffer, N-tris[hydroxymethyl)methyl-2-aminoethanesulphonic acid (TES), had a similar effect and inclusion of HEPES in the HCO₃ ^–-Ringer's did not reduce the secretory response. Similarly, it was not due to an indirect modulation of electrogenic Cl^– secretion, as the forskolin-stimulated bumetanide-sensitive I _sc was comparable in the two Ringer's. Rather it was due to the activation of a HCO₃ ^–-dependent I _sc which was inhibited by serosal 4,4′-diisothiocyano-stilbene-2,2′-disulphonate (DIDS). This DIDS-sensitive I _sc was not inhibited by acetazolamide, but it was inhibited by the replacement of bathing solution Cl^– with gluconate, suggesting a role for a Na⁺-dependent Cl^–/HCO₃ ^– exchanger in the cAMP-dependent secretory response of the human distal colon.