External K+ relieves the block but not the gating shift caused by Zn2+ in human Kv1.5 potassium channels

1. We used the whole-cell recording technique to examine the effect of extracellular Zn2+ on macroscopic currents due to Kv1.5 channels expressed in the human embryonic kidney cell line HEK293. 2. Pits of a Boltzmann function to tail current amplitudes showed that 1 mM Zn2+ shifted the half-activation voltage from -10.2 +/- 0.4 to 21.1 +/- 0.7 mV and the slope factor increased from 6.8 +/- 0.4 to 9.4 +/- 0.7 mV. The maximum conductance in 1 mM Zn2+ and with 3.5 mM K-0(+) was 33 +/- 7 % of the control value. 3. In physiological saline the apparent K-D for the Zn2+ block was 650 +/- 24 muM and was voltage independent. A Hill coefficient of 1.0 +/- 0.03 implied that block is mediated by the occupation of a single binding site. 4. Increasing the external concentration of K+ ([K+](0)) inhibited the block by Zn2+. Estimates of the apparent K-D of the Zn2+ block in 0, 5 and 135 mM K+ were 69, 650 and 2100 muM, respectively. External Cs+ relieved the Zn2+ block but was less effective than K+. Changing [K+](0) did not affect the Zn2+-induced gating shift. 5. A model of allosteric inhibition fitted to the relationship between the block by Zn2+ and the block relief by external K+ gave K-D estimates of similar to 70 muM for Zn2+ and similar to 500 muM for K+. B. We propose that the gating shift and the block caused by Zn2+ are mediated by two distinct sites and that the blocking site is located in the external mouth of the pore.