Small and maxi K+ channels in the basolateral membrane of isolated crypts from rat distal colon: single-channel and slow whole-cell recordings.

The patch-clamp technique was used to characterize K+ channel activity in the basolateral membrane of isolated crypts from rat distal colon. In cell-attached patches with KCl in the pipette, channels with conductances ranging from 6 pS to 80 pS appeared. With NaCl in the pipette and KCl in the bath in excised inside-out membrane patches a small-conductance channel with a mean conductance of 12 +/- 6 pS (n = 18) was observed. The channel has been identified as K+ channel by its selectivity for K+ over Na+ and by its sensitivity to conventional K+ channel blockers, Ba2+ and tetraethylammonium (TEA+). Changes of cytosolic pH did not attenuate channel activity. Activity of the 12-pS channel was increased by membrane depolarization and elevated cytosolic Ca2+ concentration. In addition, a maxi K+ channel with a mean conductance of 187 +/- 15 pS (n = 4) in symmetrical KCl solutions was only occasionally recorded. The maxi K+ channel could be blocked by Ba2+ (5 mmol/l) on the cytosolic side. Using the slow-whole cell recording technique under control conditions, a cell membrane potential of -70 +/- 10 mV (n = 18) was measured. By application of various K+ channel blockers such as glibenclamide, charybdotoxin, apamin, risotilide, Ba2+ and TEA+ in the bath, only Ba2+ and TEA+ depolarized the cell membrane. The present data suggest that the small K+ channel (12 pS) is involved in the maintenance of the cell membrane resting potential.