Effects of Bupivacaine and a Novel Local Anesthetic, IQB-9302, on Human Cardiac K 1 Channels TERESA GONZ ´ ALEZ, M ´ ONICA LONGOBARDO, RICARDO CABALLERO, EVA DELP ´ ON, JUAN TAMARGO, and CARMEN VALENZUELA Institute of Pharmacology and Toxicology, Consejo Superior de Investigaciones Cientı´ficas, School of Medicine, Universidad Complutense, Madrid, Spain Received July 31, 2000; accepted October 30, 2000 This paper is available online at http://jpet.aspetjournals.org ABSTRACT We have studied and compared the effects of bupivacaine with those induced by a new local anesthetic, IQB-9302, on human cardiac K 1 channels hKv1.5, Kv2.1, Kv4.3, and HERG. Both drugs have a close chemical structure, only differing in their N-substituent (n-butyl and cyclopropylmethyl, for bupivacaine and IQB-9302, respectively). Both drugs blocked Kv2.1, Kv4.3, and HERG channels similarly. Bupivacaine inhibited these channels by 48.6 6 3.4, 45.4 6 12.4, and 43.1 6 9.1%, re- spectively, and IQB-9302 by 48.1 6 3.3, 36.1 6 3.7, and 50.3 6 6.6%, respectively. However, bupivacaine was 2.5 times more potent than IQB-9302 to block hKv1.5 channels (EC 50 5 8.9 6 1.4 versus 21.5 6 4.7 mM). Both drugs induced a time- and voltage-dependent block of hKv1.5 and Kv2.1 channels. Block of Kv4.3 channels induced by either drug was time- and volt- age-dependent at membrane potentials coinciding with the activation of the channels. IQB-9302 produced an instanta- neous block of Kv4.3 and hKv1.5 channels at the beginning of the depolarizing pulse that can be interpreted as a drug inter- action with a nonconducting state. Bupivacaine and IQB-9302 induced a similar degree of block of HERG channels and in- duced a steep voltage-dependent decrease of the relative cur- rent. These results suggest that 1) bupivacaine and IQB-9302 block the open state of hKv1.5, Kv2.1, Kv4.3, and HERG chan- nels; and 2) small differences at the N-substituent of these drugs do not affect the drug-induced block of Kv2.1, Kv4.3, or HERG, but specifically modify block of hKv1.5 channels. Local anesthetics block the generation and conduction of nerve impulses by inhibiting the current through voltage- gated Na 1 channels in the membrane of nerve cells (Hille, 1977; Hondeghem and Katzung, 1977; Strichartz, 1987). Bu- pivacaine is a potent amide local anesthetic widely used for long-lasting epidural anesthesia (Strichartz, 1987). However, bupivacaine-like local anesthetics are not selective Na 1 channel blockers. In fact, at the same range of concentrations used in the clinical practice to block the generation and propagation of nerve action potentials, this type of local an- esthetic also blocks K 1 channels (Valenzuela et al., 1995a,b; Lipka et al., 1998). Moreover, at higher concentrations, they also block the L-type Ca 21 channels, thus decreasing cardiac contractility and conduction velocity through the atrioven- tricular node (Strichartz, 1987; Sanchez-Chapula, 1988). Bu- pivacaine-induced block of K 1 channels has been considered the molecular mechanism by which this drug induces a pro- longation of the QTc interval of the ECG in anesthetized dogs (Kasten and Martin, 1985; Wheeler et al., 1988) and human volunteers receiving high doses of this anesthetic (Scott et al., 1989). hKv1.5, Kv2.1, Kv4.3, and HERG channels are involved in the repolarization of the human cardiac action potential (Wang et al., 1993; Curran et al., 1995; Firek and Giles, 1995; Mays et al., 1995; Sanguinetti et al., 1995; Van Wagoner et al., 1997; Kaab et al., 1998). In fact, Kv1.5, Kv4.3, and HERG are considered to be the cloned counterparts of the I Kur , I TO , and I Kr , respectively (Fedida et al., 1993; Wang et al., 1993, 1999; Sanguinetti et al., 1995; Dixon et al., 1996; Feng et al., 1997). Moreover, the presence of mRNA encoding the expres- sion of Kv2.1 channels as well as the Kv2.1 protein in human atria has been demonstrated, although there is not a direct relationship of this channel with a native potassium current (Van Wagoner et al., 1997). The pharmacological effects of bupivacaine on different K 1 channels, including HERG, Kv1.4, Kv4.3, and hKvLQT11minK have been already stud- ied after the expression of the corresponding cRNA in Xeno- pus oocytes (Lipka et al., 1998). However, it has been re- ported that when studying the effects of lipophilic drugs (such as bupivacaine) that act from the inside of the cell This study was supported by Comision Interministerial de Ciencia y Tec- nologia SAF98-0058 (to C.V.), Comision Interministerial de Ciencia y Tecno- logia SAF99-0069 (to J.T.), CAM 08.4/0016198 (to E.D.), and U.S.-Spain Sci- ence and Technology Program 98131 (to C.V.) Grants. ABBREVIATIONS: I Kur , ultrarapid delay rectifier potassium current; I TO , transient outward potassium current; I Kr , rapid delay rectifier potassium current; IV, current-voltage relationship; t B , time constant of block; d; fractional electrical distance; E h , voltage at which 50% of the channels are open. 0022-3565/01/2962-573–583$3.00 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 296, No. 2 Copyright © 2001 by The American Society for Pharmacology and Experimental Therapeutics 3171/880562 JPET 296:573–583, 2001 Printed in U.S.A. 573