Characterization, localization and pharmacological profile of a high-affinity [3H]lidocaine binding site

Effects of dipyridamole and aspirin on shear-induced platelet aggregation in whole blood and platelet-rich plasma

BACKGROUND

Shear-induced platelet aggregation (SIPA) is an important mechanism of thrombosis at arterial bifurcations or stenotic lesions.

METHODS

We investigated the in vitro effects of dipyridamole (DP) and acetylsalicyclic acid (ASA) on SIPA in whole blood and platelet-rich plasma (PRP).

RESULTS

In whole blood, DP 20 microM significantly inhibited SIPA, while DP 5 microM or ASA 5 or 20 microM did not. SIPA in whole blood was, however, significantly inhibited by the combination of 5 or 20 microM of DP and ASA. SIPA in PRP was not inhibited by any concentration of DP or ASA, alone or in combination.

CONCLUSIONS

These results suggest that DP has an effect on red blood cells and that ASA enhances the inhibitory effect of DP on SIPA in whole blood. These effects may be related to the additive effect of combination therapy with DP and ASA on stroke prevention.

Investigation of class I anti-arrhythmic drug actions on guinea-pig cardiac mitochondrial lactate dehydrogenase activity

1. The effects of the Class I anti-arrhythmic drugs quinidine, procainamide, lidocaine, phenytoin and tocainide on mitochondrial lactate dehydrogenase activity were compared in guinea-pig heart preparations. 2. All the tested drugs inhibited the enzyme activity in a concentration-dependent fashion, exhibiting varying profiles in their actions. Lidocaine exhibited inhibitory concentration 20% (IC20) and IC50 values of 0.52 +/- 0.02 mmol/L and 25.6 +/- 0.5 mmol/L, procainamide 6.0 +/- 0.2 mmol/L and 108 +/- 7.2 mmol/L, phenytoin 3.4 +/- 0.06 mumol/L and 0.34 +/- 0.02 mmol/L, quinidine 39.2 +/- 1.2 mumol/L and 9.8 +/- 0.8 mmol/L and tocainide 2.7 +/- 0.3 mmol/L and 44.6 +/- 2.5 mmol/L. 3. According to the IC50 values, this is the order of their inhibitory potencies: phenytoin > quinidine > lidocaine > tocainide > procainamide. This trend is in general agreement with the lipophilicity rank of the drugs. 4. It is concluded, therefore, that inhibition of mitochondrial lactate dehydrogenase is a property shared by most Class I anti-arrhythmic drugs which may depend on their lipophilicity and possibly their membrane stabilizing effects.

Effects of class I antiarrhythmic drugs on mitochondrial ATPase activity in guinea pig heart preparations

1. The effects of three class I antiarrhythmic drugs quinidine, lidocaine and lorcainide on undamaged myocardial mitochondrial ATPase [ATP: phosphohydrolase, EC 3.6.1.3] activity were evaluated in guinea pig heart preparations. 2. All three drugs inhibited the enzyme activity in a concentration-dependent fashion. 3. Lorcainide was the most potent, exerting inhibitory effects in the range of less than 1.0 nM-2.0 mM, with IC20 and IC50 values of 9.4 +/- 0.6 nM and 87.2 +/- 5.5 microM. However, in the range of approx. 10 nM-10 microM, the enzyme response decreased only slightly with increasing lorcainide concentrations. 4. Quinidine and lidocaine, on the other hand, inhibited the enzyme activity in the range of 1.0 microM-100 mM. 5. The IC20 and IC50 values for quinidine were 0.92 +/- 0.04 mM and 4.8 +/- 0.6 mM and for lidocaine were 115 +/- 6 microM and 2.3 +/- 0.3 mM. 6. The results show that all three drugs inhibit mitochondrial ATPase activity and that lorcainide is the most potent. 7. These inhibitory effects may be related to the lipophilicity and membrane stabilizing activity of this class of antiarrhythmic drugs.

EO-199, a specific antagonist of antiarrhythmic drugs: assessment by binding experiments and in vivo studies

EO-199, a demethylated analog of the novel class I antiarrhythmic drug EO-122 was found to antagonize the antiarrhythmic activity of EO-122 and that of procainamide (Class IA). EO-199 did not block significantly the activity of a class IB antiarrhythmic agent, lidocaine. EO-199 also displaced the specific binding of [3H]EO-122 to rat heart membranes similarly to procainamide whereas lidocaine did not. The correlation between binding experiments and pharmacological effects points to a possible subclassification of these drugs; the two chemical analogs EO-199 and EO-122, as well as procainamide (IA) but not lidocaine (IB), compete at the same site or the same state of the sodium channel. The availability of a specific antagonist might be useful for studying the mechanism of action of antiarrhythmic drugs as well as an antidote in cases of antiarrhythmics overdose intoxication.

Molecular pharmacological aspects of antiarrhythmic activity, II: Interaction of class I compounds with calmodulin

We have tested the calmodulin (CaM) inhibitory potency of class I antiarrhythmics in the phosphodiesterase (PDE) assay. The lipophilicity of the test compounds has been quantified by two experimental (log P, RM) and two calculative (sigma f, C log P) procedures. Five antiarrhythmics (asocainol, aprindine, lorcainide, propafenone, and ethmozine) exhibit IC50 values less than 250 microM for the inhibition of the CaM-stimulated PDE activity. Lipophilicity seems to be a prime, but not the sole descriptor of CaM inhibitory potency. The functional means of CaM inhibition by the test compounds for their antiarrhythmic properties remains to be clarified in further investigations.