Verapamil is known as a potent blocker of currents through Kv1.3 channels. Earlier studies have shown that verapamil acts from the intracellular side of the channel close to the selectivity filter (Dreker et al., Mol Pharm, 2005, 68:966). We were therefore wondering whether the cation species that go through the Kv1.3 channel can influence the action of the phenylalkylamine verapamil. In order to answer this question we performed electrophysiological experiments in the whole-cell recording mode of the patch clamp technique and tested the effect of verapamil with either K⁺ or Rb⁺ as major cation in the intracellular pipette solution. The effect of verapamil on currents through Kv1.3 channels was different for K⁺_i compared to Rb⁺_i as judged by the cumulative reduction of peak currents elicited with 200 ms depolarizing pulses to +40 mV from a holding potential of -80 mV every 30 s in the presence of different verapamil concentrations. From these experiments we constructed dose-response curves and obtained a K_d value of 4.7 mM with K⁺_i and a K_d of 15.7 mM with Rb ⁺_i, respectively. Therefore the affinity of verapamil to block currents through Kv1.3 channels is reduced by a factor of ~3 for Rb⁺_i compared to K⁺_i indicating that the intracellular cations might interact with the verapamil binding site.

Supported by grants from the 4SC AG (Martinsried) and the Wilhelm-Sander Stiftung (2004.046.1)