Inhibitory Effect of Tricyclic Antidepressant Doxepin on Voltage-Dependent K+ Channels in Rabbit Coronary Arterial Smooth Muscle Cells

Doxepin, tricyclic antidepressant, is widely used for the treatment of depressive disorders. Our present study determined the inhibitory effect of doxepin on voltage-dependent K⁺ (Kv) channels in freshly isolated rabbit coronary arterial smooth muscle cells using a whole-cell patch clamp technique. Vascular Kv currents were inhibited by doxepin in a concentration-dependent manner, with a half-maximal inhibitory concentration (IC₅₀) value of 6.52 ± 1.35 μM and a Hill coefficient of 0.72 ± 0.03. Doxepin did not change the steady-state activation curve or inactivation curve, suggesting that doxepin does not alter the gating properties of Kv channels. Application of train pulses (1 or 2 Hz) slightly reduced the amplitude of Kv currents. However, the inhibition of Kv channels by train pulses were not changed in the presence of doxepin. Pretreatment with Kv1.5 inhibitor, DPO-1, effectively reduced the doxepin-induced inhibition of the Kv current. However, pretreatment with Kv2.1 inhibitor (guangxitoxin) or Kv7 inhibitor (linopirdine) did not change the inhibitory effect of doxepin on Kv currents. Inhibition of Kv channels by doxepin caused vasoconstriction and membrane depolarization. Therefore, our present study suggests that doxepin inhibits Kv channels in a concentration-dependent, but not use-, and state-dependent manners, irrespective of its own function.

Differential neurotoxicity of tricyclic antidepressants and novel derivatives in vitro in a dorsal root ganglion cell culture model

BACKGROUND AND OBJECTIVE

Tricyclic antidepressants are commonly employed orally to treat major depressive disorders and have been shown to be of substantial benefit in various chronic pain conditions. Among other properties they are potent Na+ channel blockers in vitro and show local anaesthetic properties in vivo. The present study aimed to determine their differential neurotoxicity, and that of novel derivatives as prerequisite for their potential use in regional anaesthesia.

METHODS

To directly test neurotoxicity in adult peripheral neurons, the culture model of dissociated adult rat primary sensory neurons was employed. Neurons were incubated for 24 h with amitriptyline, N-methyl-amitriptyline, doxepin, N-methyl-doxepin, N-propyl-doxepin, desipramine, imipramine and trimipramine at 100 mumol, and at concentrations correlating to their respective potency in blocking sodium channels.

RESULTS

All investigated substances showed considerable neurotoxic potency as represented in significantly decreased neuron numbers in cultures as compared to controls. Specifically, doxepin was more neurotoxic than amitriptyline, and both imipramine and trimipramine were more toxic than desipramine or amitriptyline. Novel derivatives of tricyclic antidepressants were, in general, more toxic than the parent compound.

CONCLUSIONS

Tricyclic antidepressants and novel derivatives thereof show differential neurotoxic potential in vitro. The rank order of toxicity relative to sodium channel blocking potency was desipramine < amitriptyline < N-methyl amitriptyline < doxepin < trimipramine < imipramine < N-methyl doxepin < N-propyl doxepin.

Is doxepin a safer tricyclic for the heart?

BACKGROUND

Many clinicians believe that doxepin is the safest tricyclic with respect to cardiovascular effects. This belief has persisted for two decades despite the absence of rigorous prospective evaluation.

METHOD

To address this issue, the authors studied the cardiovascular effects of doxepin in 32 depressed patients with preexisting left ventricular impairment, ventricular arrhythmias, and/or conduction disease.

RESULTS

Doxepin (1) did not have a robust effect on heart rate, (2) did not adversely affect left ventricular function, (3) did have a significant antiarrhythmic effect, (4) slowed cardiac conduction, and (5) caused a significant increase in orthostatic hypotension. Five (16%) of the 32 patients dropped out due to cardiovascular side effects. The overall dropout rate was 41%.

CONCLUSIONS

The cardiovascular effects of doxepin in depressed patients with heart disease are comparable to those documented for imipramine and nortriptyline. Doxepin afforded no greater margin of cardiovascular safety; in fact, the drug was poorly tolerated by this patient population.

Cardiotoxic actions of doxepin and barium chloride in conscious rabbits

Infusion of doxepin (Dx) i.v. to conscious rabbits induced dose dependently cardiac dysrhythmias of varying severity. Attempts to reduce sympathetic tone with diazepam or clonidine failed to modify Dx effects. Activation of the heart with aminophylline sensitized the animals to Dx effects. Infusion of barium chloride (Ba) caused severe ventricular dysrhythmias which were promptly abolished or reduced by verapamil. A subtoxic dose of Dx, but not diazepam, clonidine or taurine, counteracted Ba effects. Both the toxic and "therapeutic" effects of Dx might depend on its "quinidine-like" membrane effects.

Effects of prenatal exposure to tricyclic antidepressants on adrenergic responses in progeny

Studies were undertaken to examine the effects on cardiovascular function in progeny of rats treated maternally with the tricyclic antidepressants, doxepin and imipramine. Doxepin was administered once daily during the first trimester (days 1-7), second trimester (days 8-14) or third trimester (days 15-21) of pregnancy, while imipramine was administered during the third trimester only. Exposure to doxepin during the first or second trimester increased infant mortality rate, while third trimester exposure to imipramine, but not doxepin, enhanced infant mortality. Additionally, third trimester imipramine, but not doxepin, exposure reduced birth weight and growth rate of progeny. Tests of cardiac and aortic response to adrenergic stimulation revealed that third trimester exposure to either of these tricyclic antidepressants enhance the responsiveness of the beta-adrenergic system in the aorta, while doxepin exposure during the other trimesters was without effect on aortic beta-response. Prenatal exposure to these antidepressants did not alter blood pressure or the contractile response of aortic tissues to alpha-receptor stimulation. Collectively, these studies reveal that at doses of tricyclic antidepressants which do not cause dysmorphic effects on the fetus, subtle alterations occur in selective adrenergic functions which can persist into adulthood in the rat.

Prostaglandins and the cardiotoxic effects of doxepin in rabbits and guinea-pigs

Intravenous administration of tricyclic antidepressants to rabbits leads to dysrhythmias largely attributable to their non-specific membrane effects. Further experiments with doxepin (Dx) were conducted to assess an eventual contribution by prostaglandins (PG) to these events. Dx infused intravenously to conscious or anaesthetized guinea-pigs proved as cardiotoxic as amitriptyline but more toxic than protriptyline, thus confirming our previous results in rabbits. PG F2 alpha given intravenously before or during the Dx infusion failed to antagonize cardiotoxicity. In conscious rabbits, pretreatment with drugs (aminophenazon, indomethacin, tolfenamic acid) known to inhibit PG synthesis failed to modify the Dx cardiotoxicity as also did PG F2 alpha which elevated blood pressure, however. PG F2 alpha also failed to counteract the ouabain cardiotoxicity in anaesthetized guinea-pigs and major Ba++-induced dysrhythmias in conscious rabbits. Our results suggest that a) at least Dx induced dysrhythmias are unresponsive to PGs, and that b) previous speculations about PGs as antiarrhythmic agents may be exaggerated.

Relationship between surface activity and toxicity to Chang liver cultures of tricyclic antidepressants

Chang liver cell cultures were exposed to the tricyclic antidepressants, chlorimipramine (CIM), nortriptyline (NT), amitriptyline (AT), imipramine (IM), and dosepin (DOX). Loss of enzymes into surrounding media and cytopathic changes were used to quantitate cytotoxicity. Time- and concentration-related cytotoxic effects were evident for all drugs. The order of cytotoxic potency was CIM greater than NT greater than AT greater than IM greater than DOX. All tricyclic antidepressants tested lowered the surface tension of the salt solution contained in the tissue culture media and the order of their surface activity was identical to that of their cytotoxicity. It is postulated that the cellular toxicity induced by tricyclic antidepressants in vitro is related to a function of their surface activity.

Enhancement of propoxyphene-induced analgesia by doxepin

The activity of the centrally acting analgesic, propoxyphene, either alone or combined with the tricyclic antidepressant, doxepin, has been studied. Doses of doxepin, in themselves lacking any analgesic effect, remarkably enhanced the analgesic activity of propoxyphene, either by the oral or intraperitoneal route. On the other hand, oral toxicity data prove that doxepin does not alter significantly propoxyphene acute toxicity.

Effect of beta-adrenoceptor blocking drugs, physostigmine, and atropine on the toxicity of doxepin in mice

Large doses of doxepin given intravenously to animals cause tachyarrhythmias, and still higher doses lead to a progressive and, finally, lethal bradycardia. The effect of pretreatment with five different beta-adrenoceptor blocking drugs (propranolol, alprenolol, practolol, metoprolol or tolamolol), p physostigmine, or atropine on these toxic actions of doxepin was investigated. Mice were sedated with diazepam. Doxepin was injected i.v. 0.1 mg every 15 sec until death. ECG was recorded at 10 sec after every injection. All five beta-blockers injected i.p. 30 min before doxepin inhibited the doxepin-induced tachyarrhythmias. None of the drugs prevented or postponed the death of mice. Large doses of beta-blockers dose-dependently enhanced the doxepin-induced bradycardia and accelerated death. The cardioselective beta-blocking drugs practolol and metoprolol proved less active in enhancing bradycardia than the third cardio-selective drug, tolamolol, and non-selective propranolol and alprenolol. This difference may have resulted from properties other than beta-blockade since practolol and metoprolol lack the "cardiodepressant" and local anaesthetic properties. Since physostigmine and atropine did not modify the doxepin effects the anticholinergic property may not be important in the severe cardiotoxic effects of doxepin.