STRUCTURAL DETERMINANTS OF POTENCY AND STEREOSELECTIVE BLOCK OF HKV1.5 CHANNELS INDUCED BY LOCAL-ANESTHETICS

Block of hKv1.5 channels by bupivacaine is stereoselective, with (R)-( +)-bupivacaine being 7-fold more potent than (S)-(-)bupivacaine. The s tudy of the effects of chemically related enantiomers on these channel s may help to elucidate the structural determinants of stereoselective hKv1.5 channels block by local anesthetics. In this study, we analyze d 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 dec line superimposed to the slow inactivation during the application of d epolarizing pulses, which reached steady state at the end of 250-msec depolarizing pulses. The concentration-dependence block induced by (R) -(+)-ropivacaine yielded a K-D value of 32 +/- 1 mu M [i.e., 2.5-fold more potent than (S)-(-)-ropivacaine]. (R)-(+)-Ropivacaine block also was voltage dependent, with a fractional electrical distance (delta) o f 0.156 +/- 0.003 (n = 14) referred to the inner surface. Both (S)-(-) - and (R)-(+)-mepivacaine blocked hKv1.5 channels, with K-D values of 286.8 +/- 34.1 and 379.0 +/- 56.0 mu M, respectively [i.e., block was not stereoselective (p > 0.05)]. (S)-(-)-Mepivacaine and (R)-(+)-mepiv acaine 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 co nclude that (1) (R)-(+)-ropivacaine and mepivacaine enantiomers block the open state of hKv1.5 channels and (2) the length of their alkyl su bstituent at position 1 determines the potency and the degree of stere oselectivity.