An inward rectifier K+ channel at the basolateral membrane of the mouse distal convoluted tubule:: similarities with Kir4-Kir5.1 heteromeric channels

In this study, K+ channels present in the basolateral membrane of the distal convoluted tubule (DCT) were investigated using patch-clamp methods. In addition, Kir4.1. Kir4.2 and Kir5.1 inward rectifier channels were investigated using RT-PCR and immunohistochemistry (Kir4.1). DCTs were microdissected from collagenase-treated mouse kidneys. One type of K+ channel was detected in about 50 % of cell-attached patches from the DCT basolateral membrane; this channel was inwardly rectifying and had an inward conductance (delta(in)) of similar to40 pS at an external [K+] of 145 mm. The current-voltage relationship was linear when inside-out patches were exposed to a Mg2+-free medium. Mg2+ at a concentration of 1.2 mm considerably reduced the outward conductance (delta(out)), yielding a delta(in)/delta(out) ratio of similar to4.7. The polycation spermine (5 X 10(-7) m) reduced the open probability (P-o) by 50 %. Channel activity was dependent upon the intracellular pH, with acid pH decreasing, and basic pH increasing, P-o. Internal ATP (2 mm) and Ca2+ (up to 10(-3) m) had no effect. Channel activity declined irreversibly when the inner side of the patch was exposed to Mg2+. Kir4.1, Kir4.2 and Kir5.1 mRNAs were all detected in the DCT. The Kir4.1 protein co-localised with the Na+-Cl-cotransporter, which is specific to the DCT, and was located on basolateral membranes. The DCT K+ channel differs from other functionally identified renal K+ channels with regard to its inhibition by spermine and insensitivity to internal ATP and Ca2+. At the current state of knowledge, the channel is similar to Kir4.1-Kir5.l and Kir4.2-Kir5.1 heteromeric channels, but not to Kir4.1 or Kir4.2 homomeric channels.