Properties of an inwardly rectifying K+ channel in the basolateral membrane of mouse TAL

We investigated the properties of K+ channels in the basolateral membrane of the cortical thick ascending limb (CTAL) using the patch-clamp technique. Approximately 34% of cell-attached patches contained an inwardly rectifying K+ channel (K+-to-Na+permeability ratio ∼22), having an inward conductance (G in) of 44 pS and an outward conductance (G out) of ∼10 pS (G in/G out ∼ 4). Channel activity (NP o) increased with depolarization. When the cytosolic sides of inside-out patches were exposed to an Mg2+-free medium, the channel had aG in of 50 pS and was weakly inwardly rectifying (G in/G out ∼ 1). Cytosolic Mg2+ reduced G out, yielding a G in/G out of 3.8 at 1.3 mM Mg2+. Internal Na+ also yielded aG in/G out of 1.6 at 20 mM Na+. Spermine reduced NP o on inside-out membrane patches. Sensitivity to spermine at depolarizing voltages [half-maximal inhibitory concentration (K i) = 0.2 μM] was much greater than at hyperpolarizing voltages (K i = 26 μM). Half-inactivation by 0.5 μM spermine occurred at a clamp po...