On the mechanism of drug-induced blockade of Na+ currents: interaction of antiarrhythmic compounds with DPI-modified single cardiac Na+ channels.

Abstract

In patch-clamped membranes from neonatal rat cardiocytes, elementary Na + currents were recorded at 19° C for study of the inhibitory influence of several antiarrhythmic drugs including lidocaine, diprafenone, propafenone, and prajmalium on DPI-modified cardiac Na L ocal anesthetics and related antiarrhythmic drugs compose a structurally heterogeneous group of organic compounds capable of interacting with Na + channels in excitable membranes, including cardiac cells. Several lines of evidence strongly support the assumption that a channel-associated binding site exists 1 -3 whose interaction or binding with these drugs will finally block the channel and hinder sodium ions from passing through the pore. The resultant decrease in excitability may be an antiarrhythmic principle in heart From the Physiological Institute of the University Freiburg Freiburg/Br., Federal Republic of Germany.Supported by a grant of the Deutsche Forschungsgemeinschaft (Ko 778/2-1), Bonn, Federal Republic of Germany.Address for correspondence: Professor Dr. M. Kohlhardt, Physiological Institute of the University, Hermann-Herder-Str. 7, D-78 Freiburg/Br., Federal Republic of Germany.Received January 20, 1988; accepted September 28, 1988. to suppress irregular impulses. The remarkably different manifestation of Na + current depression, which attracted considerable theoretical interest and was well documented in numerous biophysical studies during the past years, reflects the combined influence of such factors as voltage, driving rate, and drug hydrophobicity in determination of the strength of blockade of Na + currents. The rather complex block phenomenology finds elegant and conclusive explanations by the models of Hille 2 and Hondeghem and Katzung 3 on the one hand and Starmer et al< on the other hand. The HilleHondeghem-Katzung model postulates a modulated receptor whose drug affinity depends on the channel state, rested or activated/inactivated, while the Starmer model assumes a guarded receptor, that is, drug access is controlled by the channel state. Such modeling of block proved valuable and indispensable in the understanding of some elementary properties of voltage-dependent Na + channels.by guest on May 12, 2018

Responsiveness of cardiac Na+ channels to antiarrhythmic drugs: The role of inactivation

“…In fact, noninactivating Na § channels consistently become flicker-blocked by local anesthetics and related compounds regardless of the tool experimentally applied to modify them, (-)-DPI, batrachotoxin [24] or, as shown in neuroblastoma cells after N-bromoacetamide treatment [38], covalently binding protein re-agents. Evidence in support of the hypothesis that local anesthetics penetrate in the Na § pore comes from the voltage dependence of the flicker-block reaction kinetics proven for propafenone [17] and several local anesthetics including cocaine, bupivacaine and the lidocaine derivative QX-314 [37]. The proposed existence of a binding site in the interior of the Na § pore is also stressed by the modulating influence of the external Na + concentration on the flicker-block reaction kinetics [17,37].…”

“…Evidence in support of the hypothesis that local anesthetics penetrate in the Na § pore comes from the voltage dependence of the flicker-block reaction kinetics proven for propafenone [17] and several local anesthetics including cocaine, bupivacaine and the lidocaine derivative QX-314 [37]. The proposed existence of a binding site in the interior of the Na § pore is also stressed by the modulating influence of the external Na + concentration on the flicker-block reaction kinetics [17,37].…”

“…1; left part). Analogous to their response at 19~ [17], treating the cytoplasmic membrane surface with propafenone (5-10 txmol/ liter) increased, by chopping long-lasting openings, the number of transitions between the conducting and a nonconducting state. In the experiment depicted in Fig Gating of Na + channels and the underlying conformation changes of the c~-subunit of the channel protein exhibit almost the same temperature sensitivity as many enzyme reactions and become decelerated with temperature coefficients of 2 or more on lowering the temperature from 19 to 9~ The resultant prolongation of the conducting state is paralleled by an increase of the fraction of long openings.…”

“…Since NPo responded with an extreme decline to 10% of the control value, block attenuation as achieved by a 20-mV hyperpolarization of the holding potential was necessary in order to collect a sufficiently large ensemble of single-channel events. The reliability of this experimental protocol resides in the experimentally established fact that class 1 antiarrhythmic agents interfere with open DPI-modifled Na + channels with a differential voltage dependence: flicker blockade has been shown to be highly insensitive to changes in holding potential [17]. "/'open amounted to 1.86 -+ 0.01 before and 1.77 +-0.21 msec after propafenone treatment.…”

“…Another, second type of block having a quite different biophysical phenomenology and not being involved in the conventional IN~ blockade operates additionally in kinetically modified Na + channels. After pharmacological or chemical removal of Na + inactivation, they become blocked repetitively during their open state [17,37,38]. This microscopic or flicker blockade visualizes the arrival and departure of a blocking molecule and may be considered to reflect the reaction kinetics of the drug-receptor interaction that underlies the channel blockade.…”