ISSN:
1432-1424
Keywords:
choroid plexus
;
brush border membrane
;
Ca2+-activated K+ channels
;
calcium
;
barium
;
TEA
;
intracellular pH
;
cerebrospinal fluid secretion
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Chemistry and Pharmacology
Notes:
Summary The properties of Ca2+-activated K+ channels in the apical membrane of theNecturus choroid plexus were studied using single-channel recording techniques in the cell-attached and excised-patch configurations. Channels with large unitary conductances clustered around 150 and 220 pS were most commonly observed. These channels exhibited a high selectivity for K+ over Na+ and K+ over Cs+. They were blocked by high cytoplasmic Na+ concentrations (110mm). Channel activity increased with depolarizing membrane potentials, and with increasing cytoplasmic Ca2+ concentrations. Increasing Ca2+ from 5 to 500nm, increased open probability by an order of magnitude, without changing single-channel conductance. Open probability increased up to 10-fold with a 20-mV depolarization when Ca2+ was 500nm. Lowering intracellular pH one unit, decreased open probability by more than two orders of magnitude, but pH did not affect single-channel conductance. Cytoplasmic Ba2+ reduced both channel-open probability and conductance. The sites for the action of Ba2+ are located at a distance more than halfway through the applied electric field from the inside of the membrane. Values of 0.013 and 117mm were calculated as the apparent Ba2+ dissociation constants (K d (0 mV) for the effects on probability and conductance, respectively. TEA+ (tetraethylammonium) reduced single-channel current. Applied to the cytoplasmic side, it acted on a site 20% of the distance through the membrane, with aK d (0 mV)=5.6mm. A second site, with a higher affinity,K d (0 mV)=0.23mm, may account for the near total block of chanel conductance by 2mm TEA+ applied to the outside of the membrane. It is concluded that the channels inNecturus choroid plexus exhibit many of the properties of “maxi” Ca2+-activated K+ channels found in other tissues.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01870998
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