Effects of some antianginal and vasodilating drugs on sodium influx and on the binding of 3H-batrachotoxinin-A 20-alpha-benzoate and 3H-tetracaine

Long-duration ventricular fibrillation exhibits 2 distinct organized states

BACKGROUND

Previous studies showed that endocardial activation during long-duration ventricular fibrillation (VF) exhibits organized activity. We identified and quantified the different types of organized activity.

METHODS AND RESULTS

Two 64-electrode basket catheters were inserted, respectively, into the left ventricle and right ventricle of dogs to record endocardial activation from the endocardium during 7 minutes of VF (controls, n=6). The study was repeated with the K(ATP) channel opener pinacidil (n=6) and the calcium channel blocker flunarizine (n=6). After 2 minutes of VF without drugs, 2 highly organized left ventricular endocardial activation patterns were observed: (1) ventricular electric synchrony pattern, in which endocardial activation arose focally and either had a propagation sequence similar to sinus rhythm or arose near papillary muscles, and (2) stable pattern, in which activation was regular and repeatable, sometimes forming a stable re-entrant circuit around the left ventricular apex. Between 3 and 7 minutes of VF, the percent of time ventricular electric synchrony was present was control=25%, flunarizine=24% (P=0.44), and pinacidil=0.1% (P<0.001) and the percent of time stable pattern was present was control=71%, flunarizine=48% (P<0.001), and pinacidil=56% (P<0.001). The remainder of the time, nonstable re-entrant activation with little repeatability was present.

CONCLUSIONS

After 3 minutes, VF exhibits 2 highly organized endocardial activation patterns 96% of the time, one potentially arising focally in the Purkinje system that was prevented with a K(ATP) channel opener but not a calcium channel blocker and the other potentially arising from a stable re-entrant circuit near the apical left ventricular endocardium.

Cinnarizine has an atypical antipsychotic profile in animal models of psychosis

Cinnarizine, a drug known as a calcium channel blocker, is currently used for the treatment of migraine and vertigo. Induction of extrapyramidal signs by cinnarizine has been reported in the elderly, which is related to its moderate antagonistic properties at dopamine D2 receptors, resembling the mechanism of action of most antipsychotic drugs. Despite this effect, cinnarizine has never been tested as a putative antipsychotic drug. Here we evaluate the potential effect of cinnarizine in two pharmacological models of psychosis, namely amphetamine- and MK-801-induced hyperlocomotion, as well as its ability to induce catalepsy. Cinnarizine significantly counteracted MK-801 (0.25 mg/kg) and amphetamine (5mg/kg) locomotor effects at doses as low as 20mg/kg, having no incremental effect at 60 or 180 mg/kg. Regarding side-effects, cinnarizine induced no catalepsy in mice at the effective dose of 20 mg/kg, inducing only mild catalepsy at the doses of 60 and 180 mg/kg. Based on these results and on the antagonist effect of cinnarizine on dopamine D2 receptors, we suggest that it has a potential antipsychotic effect with an atypical profile that should be evaluated clinically.

Atypical antipsychotic profile of flunarizine in animal models

RATIONALE

Flunarizine is known as a calcium channel blocker commonly used in many countries to treat migraine and vertigo. Parkinsonism has been described as one of its side-effects in the elderly, which is in agreement with its recently characterized moderate D2 receptor antagonism.

OBJECTIVES

To perform a pre-clinical evaluation of flunarizine as a potential antipsychotic.

METHODS

We evaluated the action of orally administered flunarizine in mice against hyperlocomotion induced by amphetamine and dizocilpine (MK-801) as pharmacological models of schizophrenia, induction of catalepsy as a measure for extrapyramidal symptoms and impairment induced by dizocilpine on the delayed alternation task for working memory.

RESULTS

Flunarizine robustly inhibited hyperlocomotion induced by both amphetamine and dizocilpine at doses that do not reduce spontaneous locomotion (3-30 mg/kg). Mild catalepsy was observed at 30 mg/kg, being more pronounced at 50 mg/kg and 100 mg/kg. Flunarizine (30 mg/kg) improved dizocilpine-induced impairment on the delayed alternation test.

CONCLUSIONS

These results suggest a profile comparable to atypical antipsychotics. The low cost, good tolerability and long half-life (over 2 weeks) of flunarizine are possible advantages for its use as an atypical antipsychotic. These results warrant clinical trials with flunarizine for the treatment of schizophrenia.

Betaxolol, a beta(1)-adrenoceptor antagonist, reduces Na(+) influx into cortical synaptosomes by direct interaction with Na(+) channels: comparison with other beta-adrenoceptor antagonists

Betaxolol, a beta(1)-adrenoceptor antagonist used for the treatment of glaucoma, is known to be neuroprotective in paradigms of ischaemia/excitotoxicity. In this study, we examined whether betaxolol and other beta-adrenoceptor antagonists interact directly with neurotoxin binding to sites 1 and 2 of the voltage-sensitive sodium channel (Na(+) channel) in rat cerebrocortical synaptosomes. Betaxolol inhibited specific [(3)H]-batrachotoxinin-A 20-alpha-benzoate ([(3)H]-BTX-B) binding to neurotoxin site 2 in a concentration-dependent manner with an IC(50) value of 9.8 microM. Comparison of all the beta-adrenoceptor antagonists tested revealed a potency order of propranolol>betaxolol approximately levobetaxolol>levobunolol approximately carteolol>/=timolol>atenolol. None of the drugs caused a significant inhibition of [(3)H]-saxitoxin binding to neurotoxin receptor site 1, even at concentrations as high as 250 microM. Saturation experiments showed that betaxolol increased the K(D) of [(3)H]-BTX-B binding but had no effect on the B(max). The association kinetics of [(3)H]-BTX-B were unaffected by betaxolol, but the drug significantly accelerated the dissociation rate of the radioligand. These findings argue for a competitive, indirect, allosteric mode of inhibition of [(3)H]-BTX-B binding by betaxolol. Betaxolol inhibited veratridine-stimulated Na(+) influx in rat cortical synaptosomes with an IC(50) value of 28. 3 microM. Carteolol, levobunolol, timolol and atenolol were significantly less effective than betaxolol at reducing veratridine-evoked Na(+) influx. The ability of betaxolol to interact with neurotoxin site 2 of the Na(+) channel and inhibit Na(+) influx may have a role in its neuroprotective action in paradigms of excitotoxicity/ischaemia and in its therapeutic effect in glaucoma.

Inhibition by propofol of [3H]-batrachotoxinin-A 20-alpha-benzoate binding to voltage-dependent sodium channels in rat cortical synaptosomes

1. Propofol (2,6 di-isopropylphenol), an intravenous general anaesthetic, blocks voltage-dependent Na+ channels (Na+ channels). In this study the interaction between propofol and Na+ channels was analysed by examining its effects on neurotoxin binding to various receptor sites of the Na+ channel in rat cerebrocortical synaptosomes. 2. Propofol (10-200 microM) exhibited concentration-dependent inhibition of equilibrium binding of [3H]-batrachotoxinin-A 20-alpha-benzoate ([3H]-BTX-B) to receptor site 2 of the Na+ channel (mean IC50 = 26 microM; 6.5 microM free). Scatchard analysis revealed that propofol significantly increased the KD without affecting the Bmax for [3H]-BTX-B binding. 3. Kinetic studies of [3H]-BTX-B binding in the presence of various concentrations (25-200 microM) of propofol showed no significant changes in the association rate of [3H]-BTX-B. However, propofol at 200 microM significantly increased the rate of dissociation of [3H]-BTX-B, consistent with an indirect allosteric competitive mechanism of inhibition. 4. [3H]-saxitoxin binding to receptor site 1 and [3H]-brevetoxin-3 binding to receptor site 5 of the Na+ channel were not inhibited by propofol (10-200 microM). 5. Propofol (10-100 microM) exhibited concentration-dependent inhibition of veratridine-evoked Na+ influx either in the absence or presence of scorpion toxin with IC50 values of 46 microM (8.8 microM free) and 44 microM (8.5 microM free), respectively. 6. These results suggest that propofol inhibits voltage-dependent Na+ channels due to a preferential interaction with the inactivated state of the channel. Blockade of Na+ channels by propofol, which is known to inhibit glutamate release from synaptosomes, may contribute to its anaesthetic, anticonvulsant and neuroprotective properties.

[3H](+)-pentazocine binding to rat brain sigma 1 receptors

[3H](+)-Pentazocine binding has been characterized in the rat brain. It binds to a single population of binding sites with affinity of about 7 nM and density of 280 fmol/mg protein. [3H](+)-Pentazocine binding is not enriched in the crude synaptic membrane, being about 1/6 of what we found in the crude membrane preparation. The binding, like that for other sigma ligands, was enriched in the microsomal and nuclear fractions. The inhibition by haloperidol, proadifen and d-fenfluramine was the same in the crude synaptic membrane, nuclear and microsomal fractions, suggesting that [3H](+)-pentazocine binds to a homogeneous protein in the different subcellular fractions. Our pharmacological characterization using 45 different drugs suggests that the [3H](+)-pentazocine binding site in rat brain differs from other sigma ligands, like N-propyl-3-(3-hydroxyphenyl)piperidine ([3H](+)-3PPP), N,N'-di(o-tolyl)guanidine ([3H]DTG) and (+)-N-allylnormetazocine ([3H](+)-SKF10,047). [3H](+)-Pentazocine binding in rat brain is inhibited by sigma compounds and some cytochrome P450 ligands, like proadifen and 1-[2-[bis(4-fluoro-phenyl) methoxy]ethyl]-4-[3-phenylpropyl] piperazine (GBR 12909), although with considerably lower potency than reported for other sigma ligands. Other inhibitors are some serotonin uptake blockers or their metabolites and phenylalkylamines.

Novel sites for phenylalkylamines: characterisation of a sodium-sensitive drug receptor with (-)-[3H]emopamil

(-)-Emopamil ((S)-emopamil, (2S)-2-isopropyl-5-(methylphenethylamino)- 2-phenylvaleronitrile hydrochloride) is a Ca(2+)-antagonistic phenylalkylamine which also blocks serotonin (5-HT2) receptors and has antiischemic properties. The (-)-[3H]emopamil tissue distribution profile of specific binding is in striking contrast to that observed for (+)-[3H]PN 200-110 or (-)-[3H]desmethoxyverapamil: (-)-[3H]emopamil labels membrane fractions from guinea-pig liver much greater than adrenal gland greater than kidney approximately lung approximately ductus deferens approximately brain approximately skeletal muscle. Binding to liver membrane was saturable (KD = 12.8 nM, Bmax = 35 pmol/mg of protein), stereoselective, reversible (K-1 = 0.22 min-1 at 25 degrees C) and inhibited by tetraethylammonium (IC50: 1.8 mM) greater than Li+ (IC50: 12.5 mM) approximately Na+ (IC50: 13.6 mM) and [NH4+] (IC50: 79.3 mM) but not by Rb+, Cs+ or K+. The high-affinity liver membrane binding sites have a pharmacological profile that is distinct from the phenylalkylamine receptor domain of the voltage-dependent L-type Ca2+ channel. Similar sites exist in brain and other tissues, albeit with a lower density. Amiodarone, butoprozine and amiloride derivatives bind with high affinity whereas 1,4-dihydropyridines do not interact at all. It is suggested that the novel phenylalkylamine site is linked to a sodium-dependent carrier or transport system.

Extracellular ions during veratridine-induced neurotoxicity in hippocampal slices: neuroprotective effects of flunarizine and tetrodotoxin

Veratridine, by blocking Na+ channel inactivation and shifting activation to more negative membrane potentials, causes Na(+)-influx and a persistent tendency for depolarization. Veratridine is neurotoxic to cultured neurones, and this neurotoxicity can be blocked by the class IV calcium antagonist, flunarizine. We were interested to know whether similar effects could be found in a functional differentiated tissue containing adult neurones and glial cells. We examined this in hippocampal slices using extracellular potential recordings and ion-selective microelectrodes sensitive to [Na+]o, [Ca2+]o and [K+]o. Veratridine blocked synaptic transmission in CA1, and induced several episodes of spreading depression (SD). This was followed by a long-lasting increase in [K+]o and a continuous decrease in [Ca+]o. Following veratridine exposure to hypoxia only revealed a small negative DC shift and small shifts in extracellular ions; indicating that the cells had lost the ability to maintain ion homeostasis before the hypoxia, and that veratridine had been neurotoxic. In hippocampal slices obtained from guinea pigs which had been pretreated with 40 mg/kg x 2 flunarizine orally the time before the first SD induced by veratridine was doubled. Although the ion shifts during the first SD were similar to controls, flunarizine reduced the time of recovery of [Ca2+]o, [K+]o and DC potential. The increase in [K+]o baseline and the massive decrease in [Ca2+]o baseline seen following the SDs in the solvent group were smaller in the flunarizine-treated slices. During the subsequent hypoxic period the negative DC shift was 8x larger in the flunarizine group, and the shifts in [K+]o, [Na+]o and [Ca2+]o were bigger. Tetrodotoxin also delayed the first SD during veratridine and increased the size of the DC shift during the subsequent hypoxic period. Both flunarizine and tetrodotoxin therefore protected adult brain tissue containing glia from the neurotoxicity of veratridine. These findings suggest that persistent Na(+)-influx and the consequent Ca2(+)-influx produce neurotoxicity, and that the ability to attenuate this neurotoxicity may be important in the mechanism of action of cerebroprotective drugs from different pharmacological classes.

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

Electrophysiological findings support the existence of voltage-dependent, sodium channel-associated receptors for class I antiarrhythmics. We have tried to identify such receptors with tritiated lidocaine. High-affinity binding sites were discovered in heart and brain membranes, but liver and kidney particulate fractions had the highest density of sites. The dissociation constants were 75 nM in bovine heart and 29 nM in guinea-pig liver membranes. Binding was reversible (t 1/2: 102 s at 2 degrees C), optimal at pH 9-10 and was only partly destroyed by heat treatment. Subcellular fractionation experiments excluded a plasmalemmal association of the lidocaine site in heart. The competition profile of 16 antiarrhythmics indicated chemical comparability of the sites in heart and liver. These data greatly challenge the applicability of labeled lidocaine as sodium channel probe. The pharmacological significance of the site described here remains to be clarified.

The antiarrhythmic properties of beta-adrenoceptor antagonists

The antiarrhythmic properties of beta-adrenoceptor antagonists have long been recognized and, according to the Vaughan Williams classification of antiarrhythmic drugs, these drugs are regarded as a separate entity, namely class II. In this review, Ad IJzerman and Willem Soudijn focus on the nature and the molecular basis of the antiarrhythmic action of beta-adrenoceptor antagonists. Structure-activity relationship studies demonstrate that the antiarrhythmic profile of different beta-adrenoceptor antagonists is not fixed and some compounds have class I or class III properties which may be clinically relevant.

Effects of amiloride and its analogues on [3H]batrachotoxinin-A 20-alpha benzoate binding, [3H]tetracaine binding and 22Na influx

The ability of amiloride and its analogues to inhibit [3H]batrachotoxinin-A 20-alpha benzoate [( 3H]BTX-B) and [3H]tetracaine binding to rat synaptosomes and to a rat heart membrane preparation was tested. Their ability to inhibit 22Na influx was determined with rat synaptosomes. 5-N-substituted analogues were generally more potent in inhibiting [3H]BTX-B and [3H]tetracaine binding than compounds substituted on the guanidine group. However, the inhibition was not competitive. Amiloride and some of its analogues were as active or more active in inhibiting [3H]tetracaine binding than they were in inhibiting [3H]BTX-B binding. 22Na influx was inhibited with the same relative potencies as [3H]BTX-B binding and a good correlation was found between the two inhibitions. These results show an effect of amiloride and its analogues on the voltage-sensitive Na+ channels, which could partly explain the inotropic effects of these drugs.

Inhibitory effects of some cyclohexylaralkylamines related to perhexiline on sodium influx, binding of [3H]batrachotoxinin A 20-alpha-benzoate and [3H]nitrendipine and on guinea pig left atria contractions

The antagonist activities of some cyclohexylaralkylamines derived from perhexiline on the fast Na+ channel and slow Ca2+ channel in rat brain and rat heart were examined and compared to the antagonist activities of nifedipine, verapamil, prenylamine and perhexiline. Prenylamine, perhexiline and the cyclohexylaralkylamine derivatives inhibited the [3H]batrachotoxinin A 20-alpha-benzoate binding more than the [3H]nitrendipine binding in rat brain. The nature of the interaction of the cyclohexylaralkylamines with the binding of [3H]batrachotoxinin and [3H]nitrendipine was non-competitive. The synaptosomal 22Na uptake induced by protoveratrine B, a Na+ channel agonist, was also inhibited. Prenylamine, perhexiline and perhexiline derivatives were more potent on the fast Na+ channel than on the Ca2+ channel in contrast to nifedipine and verapamil. The inhibition of Na+ and Ca2+ channels was also shown in guinea pig left atria. Perhexiline, prenylamine and the perhexiline derivatives inhibited the protoveratrine B-induced contraction more than they inhibited that induced by CaCl2, in contrast with nifedipine and verapamil. Our results showed that prenylamine, perhexiline and its related cyclohexylaralkylamines inhibited the fast Na+ channel far more than the slow Ca2+ channel in rat brain, rat heart and guinea pig atria.