Biochemical aspects of the mechanism of action of antiarrhythmic drugs on mitochondria. VII. Effect on energy-linked reactions and on membrane potential

Amiodarone effects on membrane organization evaluated by fluorescence polarization

The effects of amiodarone (0-100 microM) on the physical state of synthetic and native membranes were investigated by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH), probing the bilayer core, and of its anionic propionic acid derivative (DPH-PA), probing the outer regions of the bilayer. In the gel phase of dimyristoylphosphatidylcholine (DMPC) bilayers, amiodarone broadens the transition profile and shifts the phase transition midpoint to lower temperature values, as evaluated by both probes. On the other hand, the drug orders the fluid phase of the lipid either in hydrophobic core or in the outer regions of the bilayer, as detected by DPH and DPH-PA, respectively. The effects of amiodarone on the thermotropic behaviour of DPPC confirm and extend data in DMPC. Cholesterol concentration modulates to a great extent the effects of amiodarone in the fluid phase of DMPC. Thus, both probes, DPH and DPH-PA, detect either ordering effects of amiodarone for low cholesterol concentrations (< or = 20 mol%) or disordering amiodarone effects at higher cholesterol levels (> 20 mol%). In agreement with the results in models of synthetic lipids, the ordering effects of amiodarone in fluid native membranes of mitochondria and brain microsomes are depressed with the increase in intrinsic cholesterol. The ordering effects in mitochondria may induce bioenergetic dysfunctions and consequently disturbances in the electromechanic functioning of myocardium.

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 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.

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.