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Am J Physiol Gastrointest Liver Physiol 277: G22-G30, 1999;
0193-1857/99 $5.00
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Vol. 277, Issue 1, G22-G30, July 1999

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

Morten Grunnet1, Hans-Günther Knaus2, Christina Solander1, and Dan A. Klaerke1

1 Department of Medical Physiology, The Panum Institute, University of Copenhagen, DK-2200 Copenhagen N, Denmark, and 2 Institute of Biochemical Pharmacology, University of Innsbruck, A-6020 Innsbruck, Austria

The Ca2+-activated maxi K+ 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 K+ channels in colon epithelium by binding of iberiotoxin (IbTX), a selective 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 (K0.5 = 4.5 pM), indicating that the maxi K+ 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 K+ channels are predominantly expressed in the Na+-absorbing surface cells, as compared with the Cl--secreting crypt cells. However, aldosterone stimulation of this tissue induced by a low-Na+ diet does not change the total number of maxi K+ channels.

distal colon epithelium, iberiotoxin binding, aldosterone


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