Stereoisomerism and differential activity in excitation block by local anesthetics

The effects of the stereoisomers of propafenone and diprafenone in guinea-pig heart

1. Optically pure enantiomers of propafenone and diprafenone were prepared from their racemic mixtures and tested for their ability to block beta-adrenoceptors and to prolong functional refractory period in the guinea-pig heart. beta-Adrenoceptor affinity of the enantiomers was determined by the radioligand binding technique and in functional experiments. 2. Propafenone and diprafenone inhibited specific binding of the beta-adrenoceptor antagonist (-)-[3H]-CGP-12177 to guinea-pig myocardial membranes. beta-Adrenoceptor affinities of diprafenone enantiomers exceeded those of corresponding propafenone enantiomers by one order of magnitude. Displacement of (-)-[3H]-CGP-12177 by both antiarrhythmics was highly stereoselective, in that the (S)-enantiomers were 40-60 fold, i.e. 1.6-1.8 log units more potent than the (R)-enantiomers. 3. Propafenone and diprafenone antagonized the positive inotropic action of isoprenaline in isolated atria. beta-Adrenoceptor antagonist potencies of diprafenone enantiomers were about one order of magnitude higher than those of corresponding propafenone enantiomers. For both drugs the (S)-enantiomer was found to be considerably more potent (14-40 fold) than the (R)-enantiomer. 4. Propafenone and diprafenone prolonged functional refractory period of isolated auricles with equal potency and no difference in the antiarrhythmic activity of purified enantiomers was found. 5. It is concluded that the enantiomers of propafenone and diprafenone exert comparable antiarrhythmic activity, whereas only (S)-enantiomers block cardiac beta-adrenoceptors with high affinity, which explains the beta-adrenoceptor antagonist effects of the racemic drugs.

Stereoselective block of cardiac sodium channels by RAC109 in single guinea pig ventricular myocytes

The effects of the optical stereoisomers of the local anesthetic RAC109 (RAC109-I and RAC109-II) on sodium current in isolated guinea pig ventricular myocytes were investigated by use of the whole-cell variation of the patch-clamp technique. RAC109-I and RAC109-II produced similar levels of tonic block, but RAC109-I produced a significantly larger use-dependent block on repetitive pulsing to potentials positive to -60 mV. Definition of the time courses of block development at -20 mV and recovery at -140 and -160 mV indicated that RAC109-I had a higher affinity for activated and inactivated channels and dissociated more slowly at hyperpolarized potentials compared with RAC109-II. Removal of fast inactivation by alpha-chymotrypsin intensified tonic block but did not reduce use-dependent block by RAC109-I; this finding suggests that channel inactivation is not necessary for use-dependent block. The guarded-receptor model was used to calculate apparent rate constants of drug binding and unbinding. According to the model, RAC109-I and RAC109-II have significantly different unbinding rate constants for channels when they exist predominantly in rested, activated, or inactivated states, as well as significantly different binding rate constants when channels are activated. However, the apparent rates of drug binding to closed (rested and inactivated) channels are not significantly different for the two isomers; this finding indicates that drug binding to closed channels is not markedly stereospecific, in contrast to unbinding. The effects of RAC109 stereoisomers on cardiac sodium channels were also qualitatively similar to those previously reported in nerve; these findings suggest that the binding sites for local anesthetics in both tissue types have a similar structural topography.

Active transport of 3H-bretylium in the rat vas deferens in vitro

The uptake and efflux of 3H-bretylium in slices of the rat vas deferens were examined. The uptake of bretylium was temperature, Na+ and Cl- dependent, sensitive to 6-hydroxydopamine but insensitive to reserpine. The uptake was inhibited by desipramine, (+)-amphetamine, imipramine methiodide and cocaine, whereas the local anaesthetics millicaine and lidocaine were poor inhibitors. The spontaneous efflux was very slow at 0 degrees C and slow at 37 degrees. Desipramine accelerated the efflux probably by inhibiting the re-uptake of bretylium. (+)-Amphetamine, low external Na+, high external K+ and ouabain caused a pronounced increase in the efflux at 37 degrees C but was almost without effect at 0 degrees C. The efflux evoked by (+)-amphetamine, low Na+ and ouabain was inhibited by desipramine, imipramine methiodide and cocaine, whereas millicaine and lidocaine had very poor effects. The increase of bretylium efflux by hypertonic K+ was only slightly inhibited by desipramine and partially antagonized by omission of Ca2+ in the medium. The results obtained indicate that bretylium was taken up in noradrenergic neurones by the noradrenaline transport mechanism and that the efflux of bretylium produced by (+)-amphetamine, low external Na+, low external Cl- and by ouabain occurred by the same transport mechanism as the uptake but now working in the outward direction whereas the efflux induced by high K+ seemed to occur mainly with another mechanism.