The effects of propafenone on the delayed rectifier K+ current were studied in guinea pig ventricular myocytes by using the patch-clamp technique. In these myocytes, this current consists of at least two components: a La(3+)-sensitive component activating rapidly with moderate depolarizations and a La(3+)-resistant current slowly activating at more positive potentials. In the absence of La3+ (when both components are present), propafenone inhibited the delayed outward current, its effects being more marked after weak than after strong depolarizations. Propafenone-induced block of the tail currents elicited on return to -30 mV was more marked after short than after long depolarizing pulses. In the presence of 1 mumol/L propafenone, the envelope-of-tails test was satisfied, thus indicating that at this concentration propafenone completely blocks the rapidly activating component. In the presence of La3+ (when only the slow component is present), the steady state inhibition induced by 5 mumol/L propafenone on both the maximum activated and the tail currents was independent of the test pulse voltage. Development of propafenone-induced block on the slowly activating component was very fast and linked to channel opening. In addition, the blockade appeared to be use dependent, with the rate constant of the onset kinetics at 2 Hz being 0.44 +/- 0.1 pulse-1. The recovery process from propafenone-induced block exhibited a time constant of 2.5 +/- 0.4 s. These results indicated that propafenone preferentially inhibits the rapidly activating component of the delayed rectifier and that it blocks in a voltage-independent and time-dependent manner the slow component of this current.

Background Zatebradine is a bradycardic agent that inhibits the hyperpolarization-activated current (I f ) in the rabbit

1 The electromechanical effects of two enantiomers, S-16257-2 (S57) and S-16260-2 (R60), were studied and compared in guinea-pig isolated atria and ventricular papillary muscles. The possible stereoselectivity of the interaction on the cardiac Na' channel was analysed by comparing the effects of the two enantiomers on the onset and recovery kinetics of the frequency-dependent V,,.S block. 2 In spontaneously beating right atria, S57 and R60 (10-8 M-10-4 M) exerted a negative chronotropic effect (pIC50 = 5.07 ± 0.19 and 4.76 ± 0.18, respectively) and prolonged the sinus node recovery time, this effect being more marked with S57. In electrically driven left atria, S57 decreased (P<0.05) contractile force only at 10-4 M and R60 at concentrations > 5 x 10-5 M, whereas in papillary muscles the negative inotropic effect appeared at concentrations > 10-5 M. 3 In papillary muscles driven at 1 Hz, S57 and R60 at concentrations higher than 5 x 10-6 M produced a concentration-dependent decrease in the maximum upstroke velocity (V,,,.,) and amplitude of the cardiac action potential without altering the resting membrane potential or the action potential duration. S57 and R60 had no effect on the characteristics of the slow action potentials elicited by isoprenaline in ventricular muscle fibres depolarized in high K+ (27 mM) solution. 4 At 5 x 10-5 M, S57 and R60 produced a small tonic V,,, block. However, in muscles driven at rates between 0.5 and 3 Hz both enantiomers produced an exponential decline in V,,,, (frequency-dependent V,,,<. block) which augmented at higher rates of stimulation. The onset and offset rates of the frequencydependent Vm... block were similar for both drugs. Both S57 and R60 prolonged the recovery time constant from the frequency-dependent block from 20.1 ± 2.9 ms to 2-3 s.5 At 5 x 10-1 M, S57 and R60 shifted the membrane responsiveness curve in a hyperpolarizing direction. 6 It can be concluded that S57 and R60, the two enantiomers of the new bradycardic agent, produced a similar frequency-dependent V,,,, block which indicated that the interaction with the Na+ channel was not stereospecific. The analysis of the onset and offset kinetics of the frequency-dependent V,.,, block suggested that both enantiomers can be considered as Na+ channel blockers with intermediate kinetics, e.g., class IA antiarrhythmic drugs.

1 The effects of S-16257-2 (S57) and S-16260-2 (R60), the two enantiomers of a new bradycardic agent, were studied on human cloned K+ channels (hKvl.5) stably expressed in a mouse L cell line using the whole-cell configuration of the patch-clamp technique. 2 S57 and R60 did not modify the sigmoidal activation time course of the current but reduced the amplitude and increased the rate of the decay of the current during the application of depolarizing pulses. Both, S57 and R60 produced a concentration-dependent block of hKvl.5 channels with apparent KD values of 29.0 + 1.9 pM and 40.9 +4.0 pM, respectively. Thus, S57 was 1.4 fold more potent than R60 in blocking hKv1.5 channels. 3 The blockade produced by S57 and R60 was voltage-dependent and increased steeply between -30 and 0 mV, which corresponded with the voltage range for channel opening. This result indicated that both enantiomers block the hKvl.5 channels, preferentially, when they are in the open state. Between 0 and + 60 mV the blockade exhibited a shallow voltage-dependence which was described by an electrical distance of 0.18 + 0.002 and 0.19 + 0.004 for S57 and R60, respectively. 4 S57 and R60 also increased the rate of decline of the current during the application of depolarizing pulses. The time constant of such decline (TBI,) was faster in the presence of R60 than in the presence of S57 (16.2+ 1.5 ms vs. 24.0+2.6 ms; P<0.01). The apparent association rate constants (k) were similar for S57 and R60 ((0.52+0.13) x 106 M-1 s-l and (0.66+0.13) x 106 M-l s'l, respectively), whereas the dissociation rate constant (1) was faster for R60 than for S57 (25.8+ 1.8 s-' and 13.0+2.4 sl', respectively). 5 Both enantiomers slowed the deactivation of the tail currents elicited upon repolarization to -40 mV, thus inducing a 'crossover' phenomenon. These results suggested that drug unbinding is required before hKvl.5 channels can close. 6 It is concluded that R60 and S57 produced a similar time-voltage-and state-dependent block of hKvl.5 channels that can be interpreted as open channel block by the charged form of each enantiomer. The main difference between R60 and S57 were linked to the apparent dissociation rate constants.

Imipramine is a tricyclic antidepressant drug that also exhibits antiarrhythmic effects and whose clinical spectrum of activity is similar to that of quinidine. It has been previously demonstrated that imipramine inhibits the aggregate time-dependent outward K' current (IK). IK is composed of at least two components: a slowly activating La3"-resistant delayed rectifying current (IKS) and a rapidly activating sensitive current (IK,r). To assess the effects of imipramine on IK,r and IK,S, single guinea pig ventricular myocytes were studied using the nystatin-perforated patch-clamp technique in the absence and in the presence of La3`. Imipramine inhibited IK,, and IK,S in a concentration-dependent manner.The effects of imipramine on the aggregate time-dependent outward current were more marked than those on lK, alone.Thus, 1 ,umol/L imipramine decreased the tail currents elicited on return to -30 mV after long depolarizing pulses (5 seconds, from -40 to +50 mV) in the absence and in the presence of La3+ by 27±4% and 15±3% (n=6), respectively. Moreover, the inhibition induced by imipramine was greater after short (0.5-second) pulses than after 5-second depolarizing pulses, both in the absence and in the presence of La3+ (53+±3% and 30+5%, respectively; n=6; P<.05). Imipramine did not significantly modify either the activation midpoint or the slope factor of the aggregate IK and K,s activation curves. The reduction of 'K,S by imipramine was voltage dependent and was more marked at negative membrane potentials. In the presence of 1 ,umol/L imipramine, the ratio of tail current to time-dependent current remained constant at 0.37+0.03, regardless of the test pulse duration at +50 mV. Thus, the envelope-of-tails test was satisfied in the presence of 1 ,umol/L imipramine, which indicates that imipramine, at this concentration, blocks IK,r. Imipramine (1, 5, and 10 ,umol/L) had no effect on the kinetics of the later phase of 'K activation but delayed the beginning of the activation of lK, by 62±22, 74+23, and 155±53 milliseconds in the presence of 1, 5, and 10 ,umol/L imipramine, respectively. These results suggest that imipramine preferentially blocks rapidly activating K+ channels. In addition, experiments performed in the presence of 30 gmol/L La3+ suggest that the drug preferentially binds, but maybe not exclusively, to a closed state of the slowly activating

No abstract

1 The electrophysiological effects of CRE-1087, a new antiarrhythmic drug, were studied in guinea-pig papillary muscles. 2 CRE-1087 (10-7 M-O-4 M) produced a concentration-dependent decrease in the action potential amplitude and Vmax of the upstroke without altering the action potential duration or the resting membrane potential. 3 At 5 x 10-6 M, CRE-1087 produced a 5.0 ± 1.0% tonic Vmax block and this value was not modified at the different rates of stimulation tested. In the presence of CRE-1087, trains of stimuli at rates between 0.5 and 3 Hz led to an exponential decline in Vmax (frequency-dependent Vx,, block) which augmented at higher rates of stimulation. At 2 Hz the onset kinetics of the frequency-dependent Vmax block was fitted by a monoexponential function being the K value 0.099 ± 0.012 AP-'. The time constant for the recovery of Vmax from the frequency-dependent block was prolonged to 18.3 ± 1.5 s. 4 CRE-1087 (5 X 10-6 M) did not shift the membrane responsiveness curve. 5 CRE-1087 had no effect on the characteristics of the slow action potentials elicited by isoprenaline in ventricular fibres depolarized by 27 mM KCl, which suggested that CRE-1087 did not exhibit class IV (Ca antagonist) antiarrhythmic actions. 6 The slow onset and the slow offset kinetics of frequency-dependent Vmax block during repetitive activity suggested that in guinea-pig ventricular muscle fibres CRE-1087 exhibited class Ic antiarrhythmic actions.

1 The electrophysiological effects of prazosin, a highly specific ax-adrenoceptor antagonist, on transmembrane action potential characteristics were studied in guinea-pig papillary muscles. 2 At concentrations between 10-6 M and 10-5 M, prazosin produced a concentration-dependent decrease in the maximum upstroke velocity (V,,,.) and a progressive lengthening of the action potential duration at 50% (APDm) and 90% (APD90) of repolarization. The prolongation of the APD5o and APD91 values was independent of the frequency of stimulation. The prolongation of the APD9o was accompanied by a parallel lengthening of the effective refractory period (ERP) and thus, the ERP/APD9( ratio remained unaltered at all drug concentrations tested. 3 In the presence of prazosin, 5 x 10-6 M, the percentage of 1,, block increased with the frequency of stimulation, the inhibitory effect being more marked at fast driving rates (frequency-dependent V,,,C,, block). At 3 Hz,, the onset kinetics of the frequency-dependent Vmax block was better fitted by a biexponential function, the K values of the fast (K1) and slow components (K2) being 0.254 ± 0.037 APà nd 0.045 ± 0.010 AP-', respectively. However, prazosin did not produce tonic V,.. block. 4 The recovery time constant (Tre) from the frequency-dependent V.,.z block was prolonged from 19.6 ± 2.5 ms to 24.4 ± 5.5 s. This result indicated that prazosin can be considered as a slow kinetics Na channel blocker.5 Prazosin, 5 x 10-6 M, shifted the membrane responsiveness curve in a hyperpolarizing direction, which indicated that the blockade of sodium channels increased at less negative potentials (voltagedependent . block). 6 It is concluded that in guinea-pig papillary muscles, prazosin inhibited the V,,,, and lengthened the duration of the action potentials, thus exhibiting both class I and class III antiarrhythmic actions, respectively, that were possibly unrelated to blockade of myocardial al-adrenoceptors.