title: Ca2+ activation and pH dependence of a maxi K+ channel from rabbit distal colon epithelium.
abstract: To determine if their properties are consistent with a role in regulation of transepithelial transport, Ca(2+)-activated K+ channels from the basolateral plasma membrane of the surface cells in the distal colon have been characterized by single channel analysis after fusion of vesicles with planar lipid bilayers. A Ca(2+)-activated K+ channel with a single channel conductance of 275 pS was predominant. The sensitivity to Ca2+ was strongly dependent on the membrane potential and on the pH. At a neutral pH, the K0.5 for Ca2+ was raised from 20 nM at a potential of 0 mV to 300 nM at -40 mV. A decrease in pH at the cytoplasmic face of the K+ channel reduced the Ca2+ sensitivity dramatically. A loss of the high sensitivity to Ca2+ was also observed after incubation with MgCl2, possibly a result of dephosphorylation of the channels by endogenous phosphatases. Modification of the channel protein may thus explain the variation in Ca2+ sensitivity between studies on K+ channels from the same tissue. High affinity inhibition (K0.5 = 10 nM) by charybdotoxin of the Ca(2+)-activated K+ channel from the extracellular face could be lifted by an outward flux of K+ through the channel. However, at the ion gradients and potentials found in the intact epithelium, charybdotoxin should be a useful tool for examination of the role of maxi K+ channels. The high sensitivity for Ca2+ and the properties of the activator site are in agreement with an important regulatory role for the high conductance K+ channel in the epithelial cells.
authors.affiliation: Biomembrane Research Center, August Krogh Institute, Copenhagen University, Denmark.
authors.forename: D A
authors.initials: DA
authors.lastname: Klaerke
authors.forename: H
authors.initials: H
authors.lastname: Wiener
authors.forename: T
authors.initials: T
authors.lastname: Zeuthen
authors.forename: P L
authors.initials: PL
authors.lastname: Jørgensen
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002462:Cell Membrane / Q000737:chemistry / Q000502:physiology / Q000648:ultrastructure; D018999:Charybdotoxin; D003106:Colon / Q000737:chemistry / Q000166:cytology / Q000502:physiology*; D004558:Electric Stimulation; D004847:Epithelial Cells; D004848:Epithelium / Q000737:chemistry / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D008051:Lipid Bilayers; D008564:Membrane Potentials / Q000502:physiology; D015221:Potassium Channels / Q000032:analysis / Q000187:drug effects / Q000502:physiology*; D011817:Rabbits; D012604:Scorpion Venoms / Q000494:pharmacology
chemical_list: D008051:Lipid Bilayers; D015221:Potassium Channels; D012604:Scorpion Venoms; D018999:Charybdotoxin; D002118:Calcium