LOCALIZATION OF CAMP-INDUCED AND ALDOSTERONE-INDUCED K+ SECRETION IN RAT DISTAL COLON BY CONDUCTANCE SCANNING

1. Aldosterone-and adrenaline-induced K+ secretion were investigated i n rat late distal colon using conductance scanning and Ussing chamber techniques. K+ secretion was unmasked by the K+ channel blocker tetrae thylammonium (TEA). Electrogenic Na+ absorption was inhibited by amilo ride. Rb+ net fluxes consistently measured about 80% of K+ secretion e stimated using change in short-circuit current (Delta I-SC) measuremen ts. 2. Partial block of K+ absorption by mucosal ouabain did not chang e TEA-sensitive K+ secretion. Thus, K+ absorption and K+ secretion are not coupled. 3. Additivity of Rb+ fluxes as well as Delta I-SC caused by 3 nM aldosterone (6 h in vitro incubation) and, subsequently, adre naline suggested additivity of aldosterone-induced and cAMP-mediated K + secretion in the presence of amiloride. 4. Conductance scanning unde r 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 (Delta I-SC=0.3 m u mol h(-1) cm(-2)), which increased during sham incubation. 5. Aldost erone (3 nM, 6 h in vitro incubation) resulted, after correction for t he basal K+ secretion, in a K+ secretion of Delta I-SC=0.9 mu mol 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. 6. Adrenaline (5 mu M) caused, in fresh tissue, a K+ secretion of Delta I-SC=1.2 mu mol 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)). 7. We conc lude that K+ secretion induced by aldosterone in physiological concent ration is restricted to surface epithelium, whereas cAMP-mediated K+ s ecretion is located equally in crypts and surface epithelium.