Pathways for K+ efflux in isolated surface and crypt colonic cells. Activation by calcium.

K+ -conductive pathways were evaluated in isolated surface and crypt colonic cells, by measuring (86)Rb efflux. In crypt cells, basal K+ efflux (rate constant: 0.24 +/- 0.044 min(-1), span: 24 +/- 1.3%) was inhibited by 30 mM TEA and 5 mM Ba2+ in an additive way, suggesting the existence of two different conductive pathways. Basal efflux was insensitive to apamin, iberiotoxin, charybdotoxin and clotrimazole. Ionomycin (5 microM) stimulated K+ efflux, increasing the rate constant to 0.65 +/- 0.007 min(-1) and the span to 83 +/- 3.2%. Ionomycin-induced K+ efflux was inhibited by clotrimazole (IC(50) of 25 +/- 0.4 microM) and charybdotoxin (IC(50) of 65 +/- 5.0 nM) and was insensitive to TEA, Ba2+, apamin and iberiotoxin, suggesting that this conductive pathway is related to the Ca2+-activated intermediate-conductance K+ channels (IK(ca)). Absence of extracellular Ca2+ did neither affect basal nor ionomycin-induced K+ efflux. However, intracellular Ca2+ depletion totally inhibited the ionomycin-induced K+ efflux, indicating that the activation of these K+ channels mainly depends on intracellular calcium liberation. K+ efflux was stimulated by intracellular Ca(2+) with an EC(50) of 1.1 +/- 0.04 microM. In surface cells, K+ efflux (rate constant: 0.17 +/- 0.027 min(-1); span: 25 +/- 3.4%) was insensitive to TEA and Ba2+. However, ionomycin induced K+ efflux with characteristics identical to that observed in crypt cells. In conclusion, both surface and crypt cells present IK(Ca) channels but only crypt cells have TEA- and Ba2+-sensitive conductive pathways, which would determine their participation in colonic K+ secretion.