Block of hKv1.5 channels by bupivacaine is stereoselective, with (R)-(+)-bupivacaine being 7-fold more potent than (S)-(-)-bupivacaine. The study of the effects of chemically related enantiomers on these channels may help to elucidate the structural determinants of stereoselective hKv1.5 channels block by local anesthetics. In this study, we analyzed the effects of (R)-(+)-ropivacaine, (R)-(+)-mepivacaine, and (S)-(-)-mepivacaine on hKv1.5 channels stably expressed in Ltk- cells. (R)-(+)-Ropivacaine inhibited hKv1.5 current and induced a fast initial decline superimposed to the slow inactivation during the application of depolarizing pulses, which reached steady state at the end of 250-msec depolarizing pulses. The concentration-dependence block induced by (R)-(+)-ropivacaine yielded a KD value of 32 +/- 1 microM [i.e., 2.5-fold more potent than (S)-(-)-ropivacaine]. (R)-(+)-Ropivacaine block also was voltage dependent, with a fractional electrical distance (delta) of 0.156 +/- 0.003 (n = 14) referred to the inner surface. Both (S)-(-)- and (R)-(+)-mepivacaine blocked hKv1.5 channels, with KD values of 286.8 +/- 34.1 and 379.0 +/- 56.0 microM, respectively [i.e., block was not stereoselective (p > 0.05)]. (S)-(-)-Mepivacaine and (R)-(+)-mepivacaine block displayed no apparent time-dependence due to a very fast dissociation rate constant. However, block by mepivacaine enantiomers was voltage dependent, with delta values of 0.154 +/- 0.015 and 0.160 +/- 0.008 for the (S)-(-)- and (R)-(+)-enantiomers, respectively. We conclude that (1) (R)-(+)-ropivacaine and mepivacaine enantiomers block the open state of hKv1.5 channels and (2) the length of their alkyl substituent at position 1 determines the potency and the degree of stereoselectivity.