Ulens C, Daenens P, Tytgat J
Laboratory of Toxicology, Faculty of Pharmaceutical Sciences, University of Leuven, Belgium.
Cardiovasc Res. 1999 Dec;44(3):568-78. doi: 10.1016/s0008-6363(99)00258-8.
Norpropoxyphene (NP) is a major metabolite of propoxyphene (P), a relatively weak mu-opioid receptor agonist. Toxic blood concentrations ranging from 3 to 180 mumol/l have been reported and the accumulation of NP in cardiac tissue leads to naloxone-insensitive cardiotoxicity. Since several lines of evidence suggest that not only block of INa but also IK block may contribute to the non-opioid cardiotoxic effects of P and NP, we investigated the effects of P and NP on HERG channels. HERG presumably encodes IKr, the rapidly-activating delayed rectifier K+ current, which is known to have an important role in initiating repolarization of action potentials in cardiac myocytes.
Using the 2-microelectrode voltage clamp technique we investigated the interaction of P and NP with HERG channels, expressed in Xenopus oocytes.
Our experiments show that low drug concentrations (5 mumol/l) facilitate HERG currents, while higher drug concentrations block HERG currents (IC50-values of approx. 40 mumol/l) and dramatically shift the reversal potential to a more positive value because of a 30-fold increased Na(+)-permeability. P and NP also alter gating of HERG channels by slowing down channel activation and accelerating channel deactivation kinetics. The mutant S631C nullifies the effect of P and NP on the channel's K(+)-selectivity.
P and NP show a complex and unique drug-channel interaction, which includes altering ion-selectivity and gating. Site-directed mutagenesis suggests that an interaction with S631 contributes to the drug-induced disruption of K(+)-selectivity. No specific role of the minK subunit in the HERG block mechanism could be determined.
去甲丙氧芬（NP）是丙氧芬（P）的主要代谢产物，丙氧芬是一种相对较弱的μ-阿片受体激动剂。据报道，其毒性血药浓度范围为3至180μmol/L，NP在心脏组织中的蓄积会导致对纳洛酮不敏感的心脏毒性。由于有几条证据表明，不仅钠电流（INa）的阻断，钾电流（IK）的阻断也可能导致P和NP的非阿片类心脏毒性作用，我们研究了P和NP对人类ether-à-go-go相关基因（HERG）通道的影响。HERG可能编码快速激活延迟整流钾电流（IKr），已知其在启动心肌细胞动作电位复极化中起重要作用。
我们使用双微电极电压钳技术研究了P和NP与非洲爪蟾卵母细胞中表达的HERG通道的相互作用。
我们的实验表明，低药物浓度（5μmol/L）促进HERG电流，而较高药物浓度阻断HERG电流（半数抑制浓度（IC50）值约为40μmol/L），并由于钠通透性增加30倍而使反转电位显著向更正的值偏移。P和NP还通过减慢通道激活和加速通道失活动力学来改变HERG通道的门控。突变体S631C消除了P和NP对通道钾选择性的影响。
P和NP表现出复杂而独特的药物-通道相互作用，包括改变离子选择性和门控。定点诱变表明，与S631的相互作用导致药物诱导的钾选择性破坏。在HERG阻断机制中未确定小电导钾通道蛋白（minK）亚基的具体作用。
