CA2-DEPENDENCE OF A MAXI-K+ CHANNEL FROM RABBIT DISTAL COLON EPITHELIUM( ACTIVATION AND PH)

To determine if their properties are consistent with a role in regulat ion of transepithelial transport, Ca2+-activated K+ channels from the basolateral plasma membrane of the surface cells in the distal colon h ave been characterized by single channel analysis after fusion of vesi cles with planar lipid bilayers. A Ca2+-activated K+ channel with a si ngle 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 potentia l of 0 mV to 300 nm at -40 mV. A decrease in pH at the cytoplasmic fac e of the K+ channel reduced the Ca2+ sensitivity dramatically. A loss of the high sensitivity to Ca2+ was also observed after incubation wit h MgCl2, possibly a result of dephosphorylation of the channels by end ogenous phosphatases. Modification of the channel protein may thus exp lain the variation in Ca2+ sensitivity between studies on K+ channels from the same tissue. High affinity inhibition (K0.5 = 10 nm) by chary bdotoxin of the Ca2+-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, c harybdotoxin should be a useful tool for examination of the role of ma xi K+ channels. The high sensitivity for Ca2+ and the properties of th e activator site are in agreement with an important regulatory role fo r the high conductance K+ channel in the epithelial cells.