A Na⁺- and Cl⁻-activated K⁺ Channel in the Thick Ascending Limb of Mouse Kidney

This study investigates the presence and properties of Na⁺-activated K⁺ (K_Na) channels in epithelial renal cells. Using real-time PCR on mouse microdissected nephron segments, we show that Slo2.2 mRNA, which encodes for the K_Na channels of excitable cells, is expressed in the medullary and cortical thick ascending limbs of Henle's loop, but not in the other parts of the nephron. Patch-clamp analysis revealed the presence of a high conductance K⁺ channel in the basolateral membrane of both the medullary and cortical thick ascending limbs. This channel was highly K⁺ selective (P_K/P_Na ∼ 20), its conductance ranged from 140 to 180 pS with subconductance levels, and its current/voltage relationship displayed intermediate, Na⁺-dependent, inward rectification. Internal Na⁺ and Cl⁻ activated the channel with 50% effective concentrations (EC₅₀) and Hill coefficients (n_H) of 30 ± 1 mM and 3.9 ± 0.5 for internal Na⁺, and 35 ± 10 mM and 1.3 ± 0.25 for internal Cl⁻. Channel activity was unaltered by internal ATP (2 mM) and by internal pH, but clearly decreased when internal free Ca²⁺ concentration increased. This is the first demonstration of the presence in the epithelial cell membrane of a functional, Na⁺-activated, large-conductance K⁺ channel that closely resembles native K_Na channels of excitable cells. This Slo2.2 type, Na⁺- and Cl⁻-activated K⁺ channel is primarily located in the thick ascending limb, a major renal site of transcellular NaCl reabsorption.