Kvβ1.3 reduces the degree of stereoselective bypivacaine block of Kv1.5 channels

Background: Kv beta 1.3 subunit modifies the gating and the pharmacology of Kv1.5 channels, decreasing their sensitivity to block induced by drugs, suggesting that Kv beta 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 + Kv beta 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 + Kv beta 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 + Kv beta 1.3 channels. Therefore, the degree of stereoselectivity (theta) decreased from 9 to 4 in the presence of Kv beta 1.3. The decrease in potency to block Kv1.5 + Kv beta 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 Kv beta 1.3, stereoselectivity was abolished for V514A mutant channels (involved in bupivacaine binding but not in Kv beta 1.3 binding) but not for L510A (part of Kv beta 1.3 binding site). Conclusions: The degree of stereoselective block of Kv1.5 decreases from 9 to 4 when Kv beta 1.3 is present. L510 is determinant for the modulation of bupivacaine block, because it is the only residue of the S6 segment that binds to both bupivacaine and Kv beta 1.3. These findings support an overlapping binding site for drugs and Kv beta 1.3.