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±10mV (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.