Characterization and modulation of [I-125]iberiotoxin-D19Y/Y36F binding inthe guinea-pig urinary bladder

The radioligand binding characteristics of the Ca2+-activated K+ channel li gand [I-125]iberiotoxin-D19Y/Y36F were examined in guinea-pig urinary bladd er membranes. Saturation analysis revealed a single class of high affinity binding sites in the bladder with a K-D value of 45.6 pM and a B-max value of 112 fmol/mg protein. Specific binding was displaced by unlabeled iberiot oxin and penitrem A, but not by blockers of other classes of K+ channels in cluding a-dendrotoxin, margatoxin and apamin. The indole alkaloids, paxilli ne and verruculogen, significantly increased binding by 4.5- and 4.3-fold, respectively. Tetraacetic acid derivatives such as ethylenediamine tetraace tic acid and ethyleneglycoltetraacetic acid enhanced specific [I-125]iberio toxin-D19Y/Y36F binding about 2.5-fold, which was not attributable to calci um chelation. This increase was due to a significant change in ligand bindi ng affinity (K-D = 6.3 pM), but not due to a change in the B-max, indicatin g that these compounds may enhance toxin binding via allosteric interaction s. Collectively, these results demonstrate that the binding sites for [I-12 5]iberiotoxin-D19Y/Y36F present in the urinary bladder shows a pharmacologi cal profile typical of maxi-K+ channels and can be modulated, not only by p reviously known indole alkaloids, but also by tetraacetic acid analogs. (C) 2000 Elsevier Science B.V. All rights reserved.