The Ca2+-activated maxi K+ channel was found in the apical membrane of everted rabbit connecting tubule (CNT) with a patch-clamp technique. The mean number of open channels (NP o ) was markedly increased from 0.007 ± 0.004 to 0.189 ± 0.039 (n= 7) by stretching the patch membrane in a cell-attached configuration. This activation was suggested to be coupled with the stretch-activation of Ca2+-permeable cation channels, because the maxi K+ channel was not stretch-activated in both the cell-attached configuration using Ca2+-free pipette and in the inside-out one in the presence of 10 mm EGTA in the cytoplasmic side. The maxi K+ channel was completely blocked by extracellular 1 μm charybdotoxin (CTX), but was not by cytoplasmic 33 μm arachidonic acid (AA). On the other hand, the low-conductance K+ channel, which was also found in the same membrane, was completely inhibited by 11 μm AA, but not by 1 μm CTX. The apical K+ conductance in the CNT was estimated by the deflection of transepithelial voltage (ΔV t ) when luminal K+ concentration was increased from 5 to 15 mEq. When the tubule was perfused with hydraulic pressure of 0.5 KPa, the ΔV t was only −0.7 ± 0.4 mV. However, an increase in luminal fluid flow by increasing perfusion pressure to 1.5 KPa markedly enhanced ΔV t to −9.4 ± 0.9 mV. Luminal application of 1 μm CTX reduced the ΔV t to −1.3 ± 0.6 mV significantly in 6 tubules, whereas no significant change of ΔV t was recorded by applying 33 μm AA into the lumen of 5 tubules (ΔV t =−7.2 ± 0.5 mV in control vs.ΔV t =−6.7 ± 0.6 mV in AA). These results suggest that the Ca2+-activated maxi K+ channel is responsible for flow-dependent K+ secretion by coupling with the stretch-activated Ca2+-permeable cation channel in the rabbit CNT.
The Journal of Membrane Biology – Springer Journals
Published: Jul 1, 1998
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