Quantification and distribution of Ca2+-activated maxi K+ channels in rabbit distal colon

The Ca2+-activated maxi K+ channel is an abundant channel type in the dista l colon epithelium, but nothing is known regarding the actual number and pr ecise localization of these channels. The aim of this study has therefore b een to quantify the maxi K+ channels in colon epithelium by binding of iber iotoxin(IbTX), a elective peptidyl ligand for maxi K+ channels. In isotope flux measurements 75% of the total K+ channel activity in plasma membranes from distal colon epithelium is inhibited by IbTX (K-0.5 = 4.5 pM), indicat ing that the maxi K+ channel is the predominant channel type in this epithe lium, Consistent with the functional studies, the radiolabeled double mutan t I-125-IbTX-D19Y/Y36F binds to the colon epithelium membranes with an equi librium dissociation constant of similar to 10 pM. The maximum receptor con centration values (in fmol/mg protein) for I-125-IbTX-D19Y/Y36F binding to colon epithelium are 78 for surface membranes and 8 for crypt membranes, su ggesting that the maxi K+ channels are predominantly expressed in the Na+-a bsorbing surface cells, as compared with the Cl--secreting crypt cells. How ever, aldosterone stimulation of this tissue induced by a low-Na+ diet does not change the total number of maxi K+ channels.