CHARACTERIZATION OF APICAL POTASSIUM CHANNELS INDUCED IN RAT DISTAL COLON DURING POTASSIUM ADAPTATION

1. Chronic dietary K+ loading stimulates an active K+ secretory proces s in rat distal colon, which involves an increase in the macroscopic a pical K+ conductance of surface epithelial cells. In the present study , the abundance and characteristics of K+ channels constituting this e nhanced apical K+ conductance were evaluated using patch clamp recordi ng techniques. 2. In isolated non-polarized surface cells, K+ channels were seen in 9 of 90 (10%) cell-attached patches in cells from contro l animals, and in 247 of 437 (57%) cell-attached patches in cells from K+-loaded animals, with a significant (P < 0.001) shift in distributi on density. Similarly, recordings from cell-attached patches of the ap ical membrane of surface cells surrounding the openings of distal colo nic crypts revealed identical K+ channels in 1 of 11 (9%) patches in c ontrol animals, and in 9 of 13 (69%) patches in K+-loaded animals. 3. In isolated surface cells and surface cells in situ, K+ channels had m ean slope conductances of 209 +/- 6 and 233 +/- 14 pS, respectively, w hen inside-out patches were bathed symmetrically in K2SO4 solution. Th e channels were sensitive to 'cytosolic' Ca2+ concentration, were volt age sensitive at 'cytosolic' Ca2+ concentrations encountered in coloni c epithelial cells, and were inhibited by 1 mM quinidine, 20 mM TEA or 5 mM Ba2+ ions. 4. The data show that dietary K+ loading increases th e abundance of Ca2+- and voltage-sensitive large-conductance K+ channe ls in the apical membrane of surface cells in rat distal colon. These channels constitute the enhanced macroscopic apical K+ conductance pre viously identified in these cells, and are likely to play a critical r ole in the active K+ secretory process that typifies this model of col onic K+ adaptation.