Kvβ1.3 Reduces the Degree of Stereoselective Bupivacaine Block of Kv1.5 Channels

Background: Kvbeta]1.3 subunit modifies the gating and the pharmacology of Kv1.5 channels, decreasing their sensitivity to block induced by drugs, suggesting that Kvbeta]1.3 competes with them for a binding site at Kv1.5 channels.

Methods: Currents generated by the activation of Kv1.5 and Kv1.5 + Kvbeta]1.3 channels expressed in HEK293 cells and Xenopus oocytes were recorded by using whole cell patch clamp and voltage clamp techniques.

Results: Block of Kv1.5, but not that produced on Kv1.5 + Kvbeta]1.3 channels, was voltage dependent. In both channels, bupivacaine block was time dependent. R(+)- and S(-)-bupivacaine blocked Kv1.5 with IC50 4.4 +/- 0.5 mu]m (n = 15) and 39.8 +/- 8.2 mu]m (n = 16; P < 0.05), respectively. These values increased fourfold for R(+)-bupivacaine (17.2 +/- 2.2 mu]m) and twofold for S(-)-bupivacaine (71.9 +/- 11.5 mu]m) in Kv1.5 + Kvbeta]1.3 channels. Therefore, the degree of stereoselectivity (theta]) decreased from 9 to 4 in the presence of Kvbeta]1.3. The decrease in potency to block Kv1.5 + Kvbeta]1.3 channels was the result of a less stable interaction between bupivacaine enantiomers and channels. Differences in stereoselectivity in each situation were due to a more favorable interaction between the channel and R(+)-bupivacaine. In the presence of Kvbeta]1.3, stereoselectivity was abolished for V514A mutant channels (involved in bupivacaine binding but not in Kvbeta]1.3 binding) but not for L510A (part of Kvbeta]1.3 binding site).