At a stimulation rate of 1 Hz, GE 70 decreased V max in guinea pig heart muscle preparations at the same concentration range as propafenone . Block of V max was use-and frequency-dependent, which is in agreement with results obtained with propafenone . The steady state inactivation curve was not shifted by GE 70, in contrast to reports of (R)- and (S)-propafenone , indicating an absence or much lower affinity of GE 70 to the inactivated sodium channel compared with propafenone. Reported time constant of recovery from the use-dependent block by propafenone in guinea pig papillary muscles and ventricular myocytes varies from 5.3 to 15.5 s. The time constant of recovery of V max from the use-dependent block by GE 70 in guinea pig papillary muscles of 6.6±0.6 s was similar to that for propafenone. Published data for propafenone and the presented data for GE 70 suggest similarities in use- and frequency-dependent block of V max and time course of recovery from that block, but differences in the voltage dependence of V max block.
Effects on single channel current: The effect of 3 |J.mol/L GE 70 on single channel kinetics was studied in (S)-DPI 201-106 treated cell-attached patches (n=4). Figure 4 shows recordings from a typical experiment. The amplitude of the single channel current remained unchanged, while average open probability was reduced from 0.70±0.08 to 0.27±0.11 (P<0.05, n=4). The number of sweeps without activity was increased twofold by 3 |J.mol/L GE 70 from 15±5% nulls in control to 33±7% (n=4). Histograms of mean open and closed times (t) were best fitted with two exponentials. The mean open times were decreased (control: T1=3.9±0.5 ms, T2=38.3±4.7 ms; 3 |J.mol/L GE 70: T1=1.7±0.4 ms, P<0.05; T2=12.7±2.9 ms, P<0.05) and mean closed times were increased (control: T1=4.5±1.2 ms, T2=63.8±17.5 ms; 3 |J.mol/L GE 70: T1=5.5±1.6 ms, P>0.1; T2=154.7±22.3 ms, P<0.05) (Figure 5). Continue reading
Effects on the macroscopic sodium current: GE 70 (1, 3 and 10 |J.mol/L) was added to the bathing solution in all experiments. The drug concentration dependently decreased sodium current (peak sodium current and mean current per record) with a 50% inhibitory concentration of 2.3±0.2 |j,mol/L. The percentage reduction of the ensemble-averaged current (n=5) was almost the same at any measured holding potential (-140, -120, -110, -100, -90 and -80 mV). The membrane potential required for half-maximal steady state inactivation of the cardiac sodium channels (peak sodium current, 2 Hz) was not shifted by GE 70 (3 |j,mol/L: n=5) (Figure 2). Continue reading
RESULTS Data acquisition and analysis were controlled by pCLAMP (Axon Instruments) and ASCD (Droogmans, Laboratorium voor Fysiologie, KU Leuven, Belgium).
Statistical analysis: Quantitative results are given as arithmetic means ± SEM of n experiments. P<0.05 was considered statistically significant (Student’s t test, paired observations). Continue reading
Single channel recording: Single ventricular myocytes were enzymatically dissociated . Experiments were performed at room temperature (22±1°C). A bathing solution was used containing (in mmol/L) potassium aspartate 140, MgCl2 2, EGTA 10, ATP 2 and HEPES 10, titrated with KOH to pH 7.4. Cells were depolarized by this extracellular solution to about 0 mV. The pipette solution contained (in mmol/L) NaCl 140, HEPES 10, CsCl 10, MgCl2 2 and CaCl2 1.8, titrated with NaOH to pH 7.4. Single channel currents were measured in the cell-attached mode of the patch clamp technique . Continue reading
ANIMALS AND METHODS
Recording of action potentials: Guinea pigs of either sex (320 to 490 g) were killed by a blow to the neck. After excision of the heart, papillary muscles were dissected from the right ventricle. The isolated preparations were fixed in a lucite chamber (volume 1.5 mL), which was continuously superfused (1.6 to 2.1 mL/min) with a gassed (95% oxygen and 5% carbon dioxide) bathing solution of the following composition (in mmol/L): NaCl 136.9, KCl 5.4, MgCl2 1.05, NaHCO3 11.9, NaH2PO4 0.42, CaCl2 1.8, glucose 5.55; pH 7.2 to 7.4. Continue reading
GE 70 (rac-1-[3-(phenylethyl)-2-benzofuryl]-2-(isopropyl-amino)-ethanol hydrochloride) is a newly synthesized antiarrhythmic compound in a series of substances structurally related to propafenone (Figure 1). Racemic propafenone is classified as a Ic antiarrhythmic agent. Besides these properties propafenone posesses a weak calcium antagonistic effect and nonselective beta-adrenoceptor blocking activity, which may be clinically relevant in humans. The (+)-(S)-enantiomer is the beta-adreno-ceptor blocking moiety of propafenone, whereas both (R)-and (S)-propafenone exert equal class I antiarrhythmic activity. Propafenone blocks sodium channels during the activated and the inactivated states. The drug provoked a flickering block in DPI-modified sodium channels.