Influence of lorcainide on microsomal Na+, K(+)-ATPase in guinea-pig isolated heart preparations

Inhibition of voltage-dependent K+ channels in rabbit coronary arterial smooth muscle cells by the class Ic antiarrhythmic agent lorcainide

Voltage-dependent K⁺ (Kv) channels play the role of returning the membrane potential to the resting state, thereby maintaining the vascular tone. Here, we used native smooth-muscle cells from rabbit coronary arteries to investigate the inhibitory effect of lorcainide, a class Ic antiarrhythmic agent, on Kv channels. Lorcainide inhibited Kv channels in a concentration-dependent manner with an IC₅₀ of 4.46 ± 0.15 μM and a Hill coefficient of 0.95 ± 0.01. Although application of lorcainide did not change the activation curve, it shifted the inactivation curve toward a more negative potential, implying that lorcainide inhibits Kv channels by changing the channels' voltage sensors. The recovery time constant from channel inactivation increased in the presence of lorcainide. Furthermore, application of train steps (of 1 or 2 Hz) in the presence of lorcainide progressively augmented the inhibition of Kv currents, implying that lorcainide-induced inhibition of Kv channels is use (state)-dependent. Pretreatment with Kv1.5 or Kv2.1/2.2 inhibitors effectively reduced the amplitude of the Kv current but did not affect the inhibitory effect of lorcainide. Based on these results, we conclude that lorcainide inhibits vascular Kv channels in a concentration and use (state)-dependent manner by changing their inactivation gating properties. Considering the clinical efficacy of lorcainide, and the pathophysiological significance of vascular Kv channels, our findings should be considered when prescribing lorcainide to patients with arrhythmia and vascular disease.

Class I antiarrhythmic drug effects on ouabain binding to guinea pig cardiac Na+-K+ATPase

The notion that the inhibition of the Mg2+-dependent ATP-hydrolytic function of the myocardial Na+-K+ATPase by class I antiarrhythmic agents occurs as a result of their binding to the same receptor sites as the digitalis glycosides was tested by performing competitive binding assays of [3H]ouabain (OUA) with eight drugs: disopyramide, encainide, lidocaine, lorcainide, phenytoin, procainamide, quinidine, and tocainide in guinea pig heart microsomal preparations. In the first set of experiments, 10-200 µM concentrations of the drugs were preincubated with the enzyme and displacement assays performed with 250 nM OUA. The drugs showed receptor occupancy of 19-32% at 50 µM, 25-44% at 100 µM, and 37-56% at 200 µM. Then, 10-500 nM concentrations of OUA were preincubated with the enzyme, and competitive assays were performed using 200 µM concentrations of the drugs. OUA occupied 39-51% of the receptor sites at 100 nM, 44-67% at 250 nM, and 62-82% at 500 nM, displacing the drugs in a concentration-dependent fashion. The results show that antiarrhythmic drugs interact with the same or similar receptor sites as ouabain on the Na+-K+ATPase, pointing to a possible contribution of these interactions to the mechanism for their inhibitory actions on the enzyme, and perhaps their arrhythmogenic effects.Key words: class I antiarrhythmic agents, proarrhythmias, Na+-K+ATPase, ouabain binding.

Actions of amiodarone on mitochondrial ATPase and lactate dehydrogenase activities in guinea pig heart preparations

The effects of amiodarone on mitochondrial ATPase (EC 3.6.1.3) and lactate dehydrogenase (LDH: EC 1.1.1.27) activities were studied in guinea pig mitochondrial preparations in order to test the hypothesis that amiodarone exerts some of its effects as a result of multiple actions on membrane-bound enzymes and receptors. Amiodarone inhibited the ATPase activity in the range of 10 pM to 10 mM (n = 10) with IC50 values of 56.4 +/- 7.2 microM. However, although the inhibitory action was very significant (P < 0.0001, compared to the control) in the concentration range of 100 pM to 10 microM, the differences in individual enzyme responses showed very weak correlation with drug concentration. In this region, the inhibitory effects were almost constant at approximately 37%. Below 100 pM and above this range however, the concentration-response relationships were steep, reaching total inhibition at approximately 2.5 mM. Amiodarone also exerted concentration-dependent inhibitory effects on lactate dehydrogenase activity. However, over the effective inhibitory concentration range (5-95%) of 7.5 microM to 2.5 mM (n = 8) and IC50 value of 108 +/- 6 microM, its inhibitory potency was twofold weaker than that of its ATPase inhibition. We propose that these actions contribute, at least in part, to the mechanism(s) of some of the pharmacological actions of amiodarone.

Effects of β-adrenoceptor blockers on mitochondrial ATPase activity in guinea pig heart preparations

The effects of three beta-adrenoceptor blockers atenolol, indenolol and nadolol on myocardial mitochondrial ATPase (ATP: phosphohydrolase EC 3.6.1.3) activity were evaluated and compared with that of propranolol in guinea pig heart preparations. Propranolol and indenolol inhibited ATPase activity with IC50 values of 4.4 +/- 0.5 and 5.3 +/- 0.4 mM, respectively. In contrast, however, nadolol and atenolol markedly enhanced mitochondrial ATPase activity. Atenolol increased the enzyme activity by approximately 5, 240 and 950%, while nadolol enhanced it by 13, 280 and 2800% at 100 microM, 1.0 mM and 10.0 mM, respectively. The results indicate that these drugs exhibit two modes of interaction with the mitochondrial ATPase: inhibition by propranolol and indenolol and stimulation by atenolol and nadolol. The inhibitory actions are probably related to the membrane-stabilizing effects and therefore antiarrhythmic actions of the two drugs, while the stimulatory effects of atenolol and nadolol are probably a result of interactions with some component of oxidative phosphorylation or the respiratory chain.

Variations in potassium concentration modify the inhibitory effect of lorcainide on myocardial Na(+)-K(+)-ATPase activity

1. The inhibitory action of lorcainide on the myocardial Na(+)-K(+)-ATPase (EC 3.6.1.3) activity was studied in guinea-pig heart preparations at medium K+ concentrations of 2.5, 5.0 and 10 mM. 2. Lorcainide exhibited characteristically similar concentration-dependent inhibitory effects at all three K+ concentrations tested. However, the inhibitory potencies were significantly increased at decreased K+ concentrations. 3. The IC50 values were 10.4 +/- 3.2 microM at 2.5 mM, 28.3 +/- 7.9 microM at 5.0 mM and 40.7 +/- 9.2 microM at 10.0 mM K+ respectively. Thus, reduction in the K+ concentration from the 'standard' 5.0 to 2.5 mM enhanced the inhibitory potency of lorcainide, the effective concentrations being shifted towards much lower ranges, while increasing it to 10 mM on the other hand produced opposite but less marked effects. 4. These results show that the inhibition of myocardial Na(+)-K(+)-ATPase activity by lorcainide depends on the K+ concentration of the incubation medium. These effects are probably related to the mechanism by which lorcainide interferes with the electrogenic Na+/K+ pump activity of the enzyme, and hence may contribute to some of its cardiac actions.