AGI July 40/1

Grunnet, Morten, Hans-Günther Knaus, Christina Solander, and DanA. Klaerke. Quantification and distribution of Ca21-activated maxi K1 channels in rabbit distal colon. Am. J. Physiol. 277 (Gastrointest. Liver Physiol. 40): G22– G30, 1999.—The Ca21-activated maxi K1 channel is an abundant channel type in the distal colon epithelium, but nothing is known regarding the actual number and precise localization of these channels. The aim of this study has therefore been to quantify the maxi K1 channels in colon epithelium by binding of iberiotoxin (IbTX), a selective peptidyl ligand for maxi K1 channels. In isotope flux measurements 75% of the total K1 channel activity in plasma membranes from distal colon epithelium is inhibited by IbTX (K0.5 5 4.5 pM), indicating that the maxi K1 channel is the predominant channel type in this epithelium. Consistent with the functional studies, the radiolabeled double mutant 125I-IbTX-D19Y/Y36F binds to the colon epithelium membranes with an equilibrium dissociation constant of ,10 pM. The maximum receptor concentration values (in fmol/mg protein) for 125I-IbTX-D19Y/Y36F binding to colon epithelium are 78 for surface membranes and 8 for crypt membranes, suggesting that the maxi K1 channels are predominantly expressed in the Na1-absorbing surface cells, as compared with the Cl2-secreting crypt cells. However, aldosterone stimulation of this tissue induced by a low-Na1 diet does not change the total number of maxi K1 channels.