Localization of cAMP- and aldosterone-induced K+ secretion in rat distal colon by conductance scanning

摘要

Aldosterone- and adrenaline-induced K+ secretion were investigated in rat late distal colon using conductance scanning and Ussing chamber techniques. K+ secretion was unmasked by the K+ channel blocker tetraethylammonium (TEA). Electrogenic Na+ absorption was inhibited by amiloride. Rb+ net fluxes consistently measured about 80 % of K+ secretion estimated using change in short-circuit current (ΔISC) measurements.Partial block of K+ absorption by mucosal ouabain did not change TEA-sensitive K+ secretion. Thus, K+ absorption and K+ secretion are not coupled.Additivity of Rb+ fluxes as well as ΔISC caused by 3 nM aldosterone (6 h in vitro incubation) and, subsequently, adrenaline suggested additivity of aldosterone-induced and cAMP-mediated K+ secretion in the presence of amiloride.Conductance scanning under control conditions revealed a small TEA-sensitive K+ conductivity in surface epithelium (0.3 ± 0.2 mS cm−2) but not in crypts, as well as a small basal K+ secretion in surface epithelium (ΔISC = 0.3 μmol h−1 cm−2), which increased during sham incubation.Aldosterone (3 nM, 6 h in vitro incubation) resulted, after correction for the basal K+ secretion, in a K+ secretion of ΔISC = 0.9 μmol h−1 cm−2. Aldosterone induced a TEA-sensitive conductivity of 1.1 ± 0.3 mS cm−2 in surface epithelium, but not in crypts.Adrenaline (5 μm) caused, in fresh tissue, a K+ secretion of ΔISC = 1.2 μmol h−1 cm−2 and equal conductivity changes in crypts (0.7 ± 0.2 mS cm−2) and surface epithelium (0.7 ± 0.1 mS cm−2).We conclude that K+ secretion induced by aldosterone in physiological concentration is restricted to surface epithelium, whereas cAMP-mediated K+ secretion is located equally in crypts and surface epithelium.