Influence of extracellular pH on the modulatory effect of zinc ions on Kv1.3 potassium channels
In the present study we applied the whole-cell patch-clamp technique to study the influence of extracellular pH (pHo) on the modulatory effect exerted by zinc ions (Zn2+) on voltage-gated potassium channels Kv1.3 expressed in human lymphocytes. Obtained data provide evidence that lowering of pHo from the 7.35 to 6.4 slowed significantly the current activation rate, shifted the activation midpoint by about 16 mV towards positive membrane potentials and reduced the current amplitude to about 0.55 of the control value. In contrast, raising the pHo from 7.35 to 8.4 did not affect significantly the activation midpoint and current amplitude. Application of Zn2+ in the concentration range from 100 µM to 1 mM at pHo=6.4 slowed additionally the activation rate, shifted the activation midpoint by about 20 mV towards positive membrane potentials and reduced the current amplitude in a concentration-dependent manner. The total effect exerted by Zn2+ and protons at pHo = 6.4 was more significant than the effect exerted by Zn alone. Both the magnitude of the shift and the degree of current inhibition by Zn2+ were independent on pHo in the range from 6.4 to 8.4. The data might suggest that the effects exerted by protons and zinc ions occur independently on each other and probably involve different mechanisms. Changing the holding potential from -90 mV to -60 mV at pHo=7.35 abolished the Zn2+-induced inhibition of the current amplitudes at concentrations below 300 µM. At pHo=6.4 the total inhibition caused by Zn2+ and protons was also diminished, however, a significant reduction was observed at 100 µM concentration. In contrast, changing the holding potential did not change the Zn2+- and proton-induced shift of the activation midpoint. Altogether, obtained data suggest that extracellular protons exert the modulatory effects that are additive to the effects exerted by Zn2+ on the channels. Possible physiological significance of these additive effects is discussed.