Competition between Mg2+ and spermine for a cloned IRK2 channel expressed in a human cell line.

A cloned inwardly rectifying K+ channel, IRK2, was expressed in a human cell line, human embryonic kidney (HEK) 293T. Its electrophysiological properties were examined using the patch clamp technique in the whole‐cell, cell‐attached and inside‐out patch configurations. 2. The cells transfected with IRK2 cDNA exhibited a K+ current which showed classical properties of inwardly rectifying K+ channels at both whole‐cell and single‐channel levels. 3. In the inside‐out patch configuration, intracellular Mg2+ (Mg2+i blocked the outward currents in a voltage‐dependent and virtually time‐independent manner. Mg2+i (1‐100 microM) caused a decrease in the unitary current amplitude of the IRK2 channel by inducing subconducting levels. 4. In the absence of Mg2+i, intracellular spermine blocked the outwardly flowing IRK2 currents in a voltage‐ and time‐dependent manner. Spermine (1‐100 nM) did not affect the unitary channel current amplitude but reduced the channel open probability. The spermine block showed a slower time and steeper voltage dependence than the Mg2+i++ block. 5. When both these blockers were present, Mg2+i apparently attenuated the inhibitory effect of spermine on the outwardly flowing IRK2 currents. This interaction was voltage and time dependent, and could be well explained by a model in which Mg2+i and spermine competitively bind to the channel with their individual first‐order kinetics. This competition would induce time‐dependent transits of the channel between the Mg2+i ‐and spermine‐blocked states via a single open state, thereby preserving a certain size of persistent outward currents at depolarized potentials.