Further Quinolizidine Derivatives as Antiarrhythmic Agents- 3.

Fourteen quinolizidine derivatives, structurally related to the alkaloids lupinine and cytisine and previously studied for other pharmacological purposes, were presently tested for antiarrhythmic, and other cardiovascular effects on isolated guinea pig heart tissues in comparison to well-established reference drugs. According to their structures, the tested compounds are assembled into three subsets: (a) N-(quinolizidinyl-alkyl)-benzamides; (b) 2-(benzotriazol-2-yl)methyl-1-(quinolizidinyl)alkyl-benzimidazoles; (c) N-substituted cytisines. All compounds but two displayed antiarrhythmic activity that was potent for compounds 4, 1, 6, and 5 (in ascending order). The last compound (N-(3,4,5-trimethoxybenzoyl)aminohomolupinane) was outstanding, exhibiting a nanomolar potency (EC50 = 0.017 µM) for the increase in the threshold of ac-arrhythmia. The tested compounds shared strong negative inotropic activity; however, this does not compromise the value of their antiarrhythmic action. On the other hand, only moderate or modest negative chronotropic and vasorelaxant activities were commonly observed. Compound 5, which has high antiarrhythmic potency, a favorable cardiovascular profile, and is devoid of antihypertensive activity in spontaneously hypertensive rats, represents a lead worthy of further investigation.

New arylsparteine derivatives as positive inotropic drugs.

Positive inotropic agents are fundamental in the treatment of heart failure; however, their arrhythmogenic liability and the increased myocardial oxygen demand strongly limit their therapeutic utility. Pursuing our study on cardiovascular activities of lupin alkaloid derivatives, several 2-(4-substituted-phenyl)-2-dehydrosparteines and 2-(4-substituted-phenyl)sparteines were prepared and tested for inotropic and chronotropic activities on isolated guinea pig atria. Four compounds (6b, 6e, 7b, and 7f) exhibited significant inotropism that, at the higher concentrations, was followed by negative inotropism or toxicity. Compound 7e (2-(4-tolyl)sparteine) exhibited a steep dose-depending inotropic activity up to the highest concentration tested (300 µM) with an Emax of 116.5 ± 3.4% of basal force, proving less potent but much more active in comparison to the highest concentrations tested of digoxin and milrinone having Emax of 87.5 ± 3.1% and 52.2 ± 1.1%, respectively. Finally, docking studies suggested that the relevant sparteine derivatives could target the sigma-1 receptor, whose involvement in cardiac activity is well documented.

In Vivo and In Vitro Activities and ADME-Tox Profile of a Quinolizidine-Modified 4-Aminoquinoline: A Potent Anti-P. falciparum and Anti-P. vivax Blood-Stage Antimalarial.

Natural products are a prolific source for the identification of new biologically active compounds. In the present work, we studied the in vitro and in vivo antimalarial efficacy and ADME-Tox profile of a molecular hybrid (AM1) between 4-aminoquinoline and a quinolizidine moiety derived from lupinine (Lupinus luteus). The aim was to find a compound endowed with the target product profile-1 (TCP-1: molecules that clear asexual blood-stage parasitaemia), proposed by the Medicine for Malaria Venture to accomplish the goal of malaria elimination/eradication. AM1 displayed a very attractive profile in terms of both in vitro and in vivo activity. By using standard in vitro antimalarial assays, AM1 showed low nanomolar inhibitory activity against chloroquine-sensitive and resistant P. falciparum strains (range IC50 16-53 nM), matched with a high potency against P. vivax field isolates (Mean IC50 29 nM). Low toxicity and additivity with artemisinin derivatives were also demonstrated in vitro. High in vivo oral efficacy was observed in both P.berghei and P. yoelii mouse models with IC50 values comparable or better than those of chloroquine. The metabolic stability in different species and the pharmacokinetic profile in the mouse model makes AM1 a compound worth further investigation as a potential novel schizonticidal agent.

Lupanine Improves Glucose Homeostasis by Influencing KATP Channels and Insulin Gene Expression.

The glucose-lowering effects of lupin seeds involve the combined action of several components. The present study investigates the influence of one of the main quinolizidine alkaloids, lupanine, on pancreatic beta cells and in an animal model of type-2 diabetes mellitus. In vitro studies were performed with insulin-secreting INS-1E cells or islets of C57BL/6 mice. In the in vivo experiments, hyperglycemia was induced in rats by injecting streptozotocin (65 mg/kg body weight). In the presence of 15 mmol/L glucose, insulin secretion was significantly elevated by 0.5 mmol/L lupanine, whereas the alkaloid did not stimulate insulin release with lower glucose concentrations. In islets treated with l-arginine, the potentiating effect of lupanine already occurred at 8 mmol/L glucose. Lupanine increased the expression of the Ins-1 gene. The potentiating effect on secretion was correlated to membrane depolarization and an increase in the frequency of Ca(2+) action potentials. Determination of the current through ATP-dependent K⁺ channels (KATP channels) revealed that lupanine directly inhibited the channel. The effect was dose-dependent but, even with a high lupanine concentration of 1 mmol/L or after a prolonged exposure time (12 h), the KATP channel block was incomplete. Oral administration of lupanine did not induce hypoglycemia. By contrast, lupanine improved glycemic control in response to an oral glucose tolerance test in streptozotocin-diabetic rats. In summary, lupanine acts as a positive modulator of insulin release obviously without a risk for hypoglycemic episodes.

Assessing the genotoxicities of sparteine and compounds isolated from Lupinus mexicanus and L. montanus seeds by using comet assay.

The genus Lupinus is widely distributed. Its seeds are used for animal and human food, and Lupinus possesses pharmacological potential because of its high content of quinolizidine alkaloids and flavonoids; however, there is little available information about its genotoxicity. We used the comet assay and staminal nuclei of Tradescantia (clone 4430) to evaluate the in vitro genotoxicity of 4 concentrations (0.01, 0.1, 0.5, and 1.0 mM) of alkaloid extracts of Lupinus mexicanus and Lupinus montanus, flavonoids of L. mexicanus, and commercial sparteine; nitrosodiethylamine was used as a positive control and untreated nuclei were used as a negative control. All concentrations of L. mexicanus and L. montanus showed significant genotoxic activity (P ≤ 0.05). A similar behavior was observed for flavonoid extracts of L. montanus except the 1.0 mM concentration. Sparteine showed genotoxic activity only at 0.5 mM. The order of genotoxicity of the compounds studied was as follows: L. mexicanus > L. montanus > flavonoids of L. montanus > sparteine. There is evident genotoxic activity in the compounds that were studied, particularly at lower concentrations (0.01 and 0.1 mM). Given the limited information about the genotoxicity of the compounds of L. mexicanus and L. montanus, further studies are necessary.

Synthesis and comparison of antiplasmodial activity of (+), (-) and racemic 7-chloro-4-(N-lupinyl)aminoquinoline.

Recently the N-(-)-lupinyl-derivative of 7-chloro-4-aminoquinoline ((-)-AM-1; 7-chloro-4-{N-[(1S,9aR)(octahydro-2H-quinolizin-1-yl)methyl]amino}quinoline) showed potent in vitro and in vivo activity against both Chloroquine susceptible and resistant strains of Plasmodium falciparum. However, (-)-AM-1 is synthesized starting from (-)-lupinine, an expensive alkaloid isolated from Lupinus luteus whose worldwide production is not sufficient, at present, for large market purposes. To overcome this issue, the corresponding racemic compound, derived from synthetic (±)-lupinine was considered a cheaper alternative for the development of a novel antimalarial agent. Therefore, the racemic and the 7-chloro-4-(N-(+)-lupinyl)aminoquinoline ((±)-AM-1; (+)-AM-1) were synthesized and their in vitro antimalarial activity and cytotoxicity compared with those of (-)-AM-1. The (+)-lupinine required for the synthesis of (+)-AM-1 was obtained through a not previously described lipase catalyzed kinetic resolution of (±)-lupinine. In terms of antimalarial activity, (±)-AM1 and (+)-AM1 demonstrated very good activity in vitro against both CQ-R and CQ-S strains of P. falciparum (range IC(50) 16-35 nM), and low toxicity against human normal cell lines (therapeutic index >1000), comparable with that of (-)-AM1. These results confirm that the racemate (±)-AM1 could be considered as a potential antimalarial agent, ensuring a decrease of costs of synthesis compared to (-)-AM1.

Evaluation of in vitro susceptibility to sparteine in four strains of Mycobacterium tuberculosis. / Evaluación de la susceptibilidad in vitro a esparteína, en cuatro cepas de Mycobacterium tuberculosis.

Sparteine is an alkaloid with bacteriostatic activity on the genus Mycobacterium. The aim of this study was to evaluate the antimicrobial activity of sparteine on the growth of 4 ATCC strains of Mycobacterium tuberculosis (susceptible, resistant to isoniazid, resistant to rifampicin and multidrug-resistant) in vitro. Validation of bactericidal activity of sparteine sulfate was carried out through an adaptation of the Microscopic-Observation Drug-Susceptibility (MODS) method according to the guidelines of the Peruvian National Health Institute. The results demonstrate that at concentrations of 25; 50 and 100 Mm of sparteine sulfate, there is no development of colony-forming units in any of the 4 evaluated strains. Our results demonstrate the potential in vitro antimicrobial effect of sparteine on multidrug-resistant tuberculosis.

La esparteína es un alcaloide con actividad bacteriostática sobre el género Mycobacterium. El objetivo de este trabajo fue evaluar la acción antimicrobiana de la esparteína en el crecimiento de cuatro cepas ATCC de Mycobacterium tuberculosis (susceptible, resistente a isoniazida, resistente a rifampicina y multidrogorresistente) in vitro. La evaluación de la actividad bactericida del sulfato de esparteína se realizó a través de una adaptación del método de ensayo de cultivo y susceptibilidad a medicamentos antituberculosos mediante observación microscópica (MODS, por sus siglas en inglés), según el protocolo descrito en el manual técnico elaborado por el Instituto Nacional de Salud. Los resultados demuestran que a concentraciones de 25; 50 y 100 mM de sulfato de esparteína, no se desarrollan unidades formadoras de colonia en las cuatro cepas evaluadas de Mycobacterium tuberculosis. Los resultados demuestran el potencial efecto antimicrobiano in vitro de la esparteína en la tuberculosis multidrogorresistente.

Development and application of a method to investigate drug-metabolizing enzyme inhibitors using sparteine for probe of cytochrome P450 2D6 and tris(2,2'-bipyridine)ruthenium(II)-electrogenerated chemiluminescence detection.

We studied the detection of drug-metabolizing enzyme inhibitiors using column-switching high performance liquid chromatography with tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)(3)(2+))-electrogenerated chemiluminescence detection. This can be applied to evaluate the genetic diversity concerning the ability of cytochrome P450 (CYP) 2D6 to metabolize drug in vitro. We demonstrated the ability of CYP2D6 to enable us to examine drugs metabolizing enzyme inhibition with high performance and sensitivity. This method can be applied to investigate metabolite inhibitors of CYP2D6 in vitro and in vivo. Thus, Metixene was found to be a potential CYP2D6 inhibitor.

The anticonvulsant effect of sparteine on pentylenetetrazole-induced seizures in rats: a behavioral, electroencephalographic, morphological and molecular study.

Abnormal synchronous activity in neurons generates epileptic seizures. Antiepileptic drugs (AEDs) are effective in 70% of patients, but this percentage is drastically lower in developing countries. Sparteine is a quinolizidine alkaloid synthesized from most Lupine species and has a probable anticonvulsive effect. For this reason, the objective of the present work was to study the anticonvulsant effect of sparteine using a dose-effect curve and to determine its effectiveness against seizures using behavioral, electroencephalographic, morphological and molecular data. Wistar rats were grouped into control [saline solution (0.9%), pentylenetetrazole (90 mg/kg), and sparteine (13, 20 and 30 mg/kg), intraperitoneal (i.p.)] and experimental (sparteine + pentylenetetrazole) groups. The rats were implanted with surface electrodes to register electrical activity, and convulsive behavior was evaluated according to Velisek's scale. The rats were perfused to obtain brain slices for cresyl violet staining and cellular density quantification as well as for immunohistochemistry for NeuN and GFAP. Other animals were used to determine the hippocampal mRNA expression of the M2 and M4 acetylcholine receptors by qPCR. Sparteine exhibited a better anticonvulsant effect at a dose of 30 mg/kg (i.p.) than at the other doses used. This anticonvulsant effect was characterized by a decrease in the severity of convulsive behavior, 100% survival, an inhibitory effect on epileptiform activity 75 min after pentylenetetrazole administration, and the conservation of the cellular layers of CA1, CA3 and the dentate gyrus (DG); however, astrogliosis was observed after 30 mg/kg sparteine treatment. In addition, sparteine treatment increased the mRNA expression of the M4 receptor three hours after administration. According to our findings, the effective dose of sparteine as an anticonvulsant agent by i.p. injection is 30 mg/kg. The astrogliosis that was observed after sparteine administration may be a compensatory mechanism to diminish excitability and maintain neuronal homeostasis, possibly through redistributing potassium and glutamate. The increase in the mRNA expression of the M4 receptor may suggest the participation of the M4 receptor in the anticonvulsive effect of sparteine, as the activation of this receptor may inhibit acetylcholine release and facilitate the subsequent release of GABA. However, the precise mechanisms by which sparteine produces these effects are not known, and therefore, further experiments are necessary.

Transcriptomic Signatures of Experimental Alkaloid Consumption in a Poison Frog.

In the anuran family Dendrobatidae, aposematic species obtain their toxic or unpalatable alkaloids from dietary sources, a process known as sequestering. To understand how toxicity evolved in this family, it is paramount to elucidate the pathways of alkaloid processing (absorption, metabolism, and sequestering). Here, we used an exploratory skin gene expression experiment in which captive-bred dendrobatids were fed alkaloids. Most of these experiments were performed with Dendrobates tinctorius, but some trials were performed with D. auratus, D. leucomelas and Allobates femoralis to explore whether other dendrobatids would show similar patterns of gene expression. We found a consistent pattern of up-regulation of genes related to muscle and mitochondrial processes, probably due to the lack of mutations related to alkaloid resistance in these species. Considering conserved pathways of drug metabolism in vertebrates, we hypothesize alkaloid degradation is a physiological mechanism of resistance, which was evidenced by a strong upregulation of the immune system in D. tinctorius, and of complement C2 across the four species sampled. Probably related to this strong immune response, we found several skin keratins downregulated, which might be linked to a reduction of the cornified layer of the epidermis. Although not conclusive, our results offer candidate genes and testable hypotheses to elucidate alkaloid processing in poison frogs.

Discovery of Natural Compounds Promoting Cardiomyocyte Differentiation.

The commitment of pluripotent stem cells to the cardiac lineage has enormous potential in regenerative medicine interventions for several cardiac diseases. Thus, it is necessary to understand and regulate this differentiation process for potential clinical application. In this study, we developed defined conditions with chemical inducers for effective cardiac lineage commitment and elucidated the mechanism for high-efficiency differentiation. First, we designed a robust reporter-based platform to screen chemical inducers of cardiac differentiation in the mouse P19 teratocarcinoma cell line. Using this system, we identified two natural alkaloids, lupinine and ursinoic acid, which enhanced cardiomyocyte differentiation of P19 cells in terms of beating colony numbers with respect to oxytocin, and confirmed their activity in mouse embryonic stem cells. By analyzing the expression of key markers, we found that this enhancement can be attributed to the early and rapid induction of the Wnt signaling pathway. We also found that these natural compounds could not only supersede the action of the Wnt3a ligand but also had a very quick response time, allowing them to act as efficient cardiac mesoderm inducers that subsequently promoted cardiomyocyte differentiation. Thus, this study offers a way to develop chemical-based differentiation strategy for high-efficiency cardiac lineage commitment, which has an advantage over currently available methods with complex medium composition and parameters. Furthermore, it also provides an opportunity to pinpoint the key molecular mechanisms pivotal to the cardiac differentiation process, which are necessary to design an efficient strategy for cardiomyocyte differentiation.

Angustifoline inhibits human colon cancer cell growth by inducing autophagy along with mitochondrial-mediated apoptosis, suppression of cell invasion and migration and stimulating G2/M cell cycle arrest.

PURPOSE: The prime objective of the present study was to investigate the anticancer properties of angustifoline against COLO-205 human colon cancer cells. Its effects on cell autophagy, apoptosis, cell invasion and cell migration, and cell cycle arrest were also evaluated in the current study. METHODS: WST-1 assay was used to study cytotoxic effects of the compound on the cell viability. Effects on apoptosis and cell cycle arrest were evaluated by flow cytometry. In vitro wound healing assay and matrigel assay were carried out to study the effects of angustifoline on cell migration and cell invasion respectively. To confirm autophagy, we evaluated the expression of several autophagy-associated proteins using Western blot assay along with transmission electron microscopy (TEM). RESULTS: The findings indicated that angustifoline induced dose- and time-dependent cytotoxicity in COLO-205 human colon cancer cells along with inhibiting cancer cell colony formation. Angustifoline-treated cells exhibited cell shrinkage along with distortion of the normal cell morphology. Angustifoline-treated cells were also arrested in the G2/M phase of the cell cycle, showing strong dose-dependence. The compound also led to inhibition of cell migration and cell invasion. The results showed that treatment of these cells led to generation of autophagic cell vesicles. Furthermore, it was observed that the expression of Beclin-1 and LC3-II proteins was significantly upregulated in the angustifoline-administered COLO-205 cells. CONCLUSIONS: In brief, the present study hints towards the potent anticancer potential of the natural product angustifoline against COLO-205 human colon cancer cells with in depth mechanistic studies.

17 Oxo Sparteine and Lupanine, Obtained from Cytisus scoparius, Exert a Neuroprotection against Soluble Oligomers of Amyloid-ß Toxicity by Nicotinic Acetylcholine Receptors.

Alzheimer's disease (AD) is a neurodegenerative pathology, which is characterized by progressive and irreversible cognitive impairment. Most of the neuronal perturbations described in AD can be associated with soluble amyloid- ß oligomers (SO-Aß). There is a large amount of evidence demonstrating the neuroprotective effect of Nicotine neurotransmission in AD, mainly through nicotinic acetylcholine receptor (nAChR) activation and antiapoptotic PI3K/Akt/Bcl-2 pathway signaling. Using HPLC and GC/MS, we isolated and characterized two alkaloids obtained from C. scoparius, Lupanine (Lup), and 17- oxo-sparteine (17- ox), and examined their neuroprotective properties in a cellular model of SO-Aß toxicity. Our results showed that Lup and 17- ox (both at 0.03µM) prevented SO-Aß-induced toxicity in PC12 cells (Lup: 64±7%; 17- ox: 57±6%). Similar results were seen in hippocampal neurons where these alkaloids prevented SO-Aß neurotoxicity (Lup: 57±2%; 17- ox: 52±3%) and increased the frequency of spontaneous calcium transients (Lup: 60±4%; 17- Ox: 40±3%), suggesting an enhancing effect on neural network activity and synaptic activity potentiation. All of the neuroprotective effects elicited by both alkaloids were completely blocked by α-bungarotoxin. Additionally, we observed that the presence of both Lup and 17- ox increased Akt phosphorylation levels (52±4% and 35±7%, respectively) in cells treated with SO-Aß (3 h). Taken together, our results suggest that the activation of nAChR by Lup and 17- ox induces neuroprotection in different cellular models, and appears to be an interesting target for the development of new pharmacological tools and strategies against AD.

Bactericidal and fungicidal activities of Calia secundiflora (Ort.) Yakovlev.

Calia secundiflora (Ortega) Yakovlev (Fabaceae) is considered a medicinal plant in Mexico but has scarcely been used because of the toxicity of its quinolizidine alkaloids. Several quinolizidine alkaloids have shown bactericidal, nematicidal, and fungicidal activities. The purpose of this study was to identify the alkaloids in the seeds and evaluate the activity of the organic extract on several phytopathogenic fungi and bacteria. An in vitro bioassay was conducted with species of the following phytopathogenic fungi: Alternaria solani, Fusarium oxysporum and Monilia fructicola; and of the following bacteria Pseudomonas sp., Xanthomonas campestris and Erwinia carotovora. Cytisine, lupinine, anagyrine, sparteine, N-methylcytisine, 5,6-dehydrolupanine, and lupanine were identified by liquid chromatography-mass spectrometry in the extract of seeds; the most abundant compound of the extract was cytisine. It was observed that the crude extract of Calia secundiflora was moderately active on bacteria and more potent on phytopathogenic fungi. In contrast cytisine showed the opposite effects.

C2-Modified Sparteine Derivatives Are a New Class of Potentially Long-Acting Sodium Channel Blockers.

The lupin alkaloid sparteine is a well-known chiral diamine with a range of applications in asymmetric synthesis, as well as a blocker of voltage-gated sodium channels (VGSCs). However, there is only scarce information on the VGSC-blocking activity of sparteine derivatives where the structure of the parent alkaloid is retained. Building on the recent renewed availability of sparteine and derivatives we report herein how modification of sparteine at position 2 produces irreversible blockers of VGSCs. These compounds could be clinically envisaged as long-lasting local anesthetics.

Sparteine as an anticonvulsant drug: Evidence and possible mechanism of action.

Sparteine is a quinolizidine alkaloid extracted from Lupinus that has numerous pharmacological properties both in humans and animal models. In the central nervous system, sparteine reduces locomotor activity, has light analgesic effects, also has no effects on short-term memory or spatial learning and does not induce changes in behavior or electroencephalographic (EEG) activity. However, the anticonvulsant profile of sparteine is not fully characterized in experimental animals and there are no data in humans. Therefore, the present review focuses on the experimental evidence supporting the anticonvulsant action of sparteine in models of acute seizures and status epilepticus (SE), as well as its possible mechanisms of action. The evidence that supports the anticonvulsant effect of (-)-Sparteine sulfate includes the inhibition of seizures induced by maximal electro-stimulation, a delay in the onset of convulsive behavior and the prolongation of survival time in mice treated with pentylenetetrazole (PTZ). Additionally, sparteine delays the onset of convulsive behavior and decreases the severity and mortality of rats treated with PTZ and pilocarpine. Sparteine decreases amplitude and frequency or blocks the epileptiform activity induced by PTZ, pilocarpine and kainic acid. Sparteine may decrease hyperexcitability through the activation of the M2 and M4 subtypes of mAChRs, which is a probable mechanism of action that together with its systemic effects may favor its anticonvulsant effects against seizures and SE.

Methoxyphenamine O-demethylase and 5-hydroxylase: a GLC-ECD assay to study their activities and their inhibition by debrisoquine and sparteine.

A GLC-ECD method is described for the determination of the O-desmethyl, N-desmethyl and aromatic 5-hydroxy metabolites of methoxyphenamine in liver homogenates. The O-desmethyl and 5-hydroxy metabolites are deficient in poor metabolizers of debrisoquine and sparteine and the Dark Agouti rat model of this human phenotype. The present analytical method can be useful in determining methoxyphenamine O-demethylase and 5-hydroxylase activities as well as identifying those substrates which inhibit these and are worthy of further study.

Oscillatory contractions in tail arteries from genetically hypertensive rats.

This study characterizes a cellular mechanism for oscillatory contractions induced by norepinephrine in vascular smooth muscle from spontaneously hypertensive stroke prone rats (SHRSP). Helically cut strips of tail arteries from SHRSP and normotensive Wistar-Kyoto rats (WKY) were mounted in a muscle bath for measurement of isometric force generation. Norepinephrine-induced responses of arteries from SHRSP were characterized by fluctuations in contractile activity, whereas those in arteries from WKY remained constant with time. The magnitude of the oscillatory contractile activity (frequency X mean amplitude) varied directly with norepinephrine concentration (5.9 X 10(-9) to 1.8 X 10(-7) M). The oscillatory contractile activity varied inversely with the potassium concentration (3-20 mM) of the buffer solution and directly with the calcium concentration (0.1-5.0 mM) of the buffer solution. The oscillatory activity was converted to maintained contraction by barium (10(-4) M), quinidine (3 X 10(-6) M), sparteine (10(-3) M), D-600 (10(-7) M), and nifedipine (10(-8) M). Tetraethylammonium and 3,4-diaminopyridine, inhibitors of voltage-dependent potassium channels, did not alter the oscillatory contractile activity induced by norepinephrine. These observations suggest that oscillatory contractile activity in tail arteries from SHRSP is caused by an abnormal variation in potassium efflux during stimulation with norepinephrine. The altered potassium efflux appears to be related to calcium entry, which is sensitive to inhibition by channel blockers. This altered membrane property may contribute to changes in vascular sensitivity in hypertension.

Differential effects of alkaloids on sodium currents of isolated single skeletal muscle fibers.

The effects of the alkaloids ajmaline, lupanine, sparteine, serpentine, strychnine, and yohimbine were studied with the loose patch clamp technique on sodium currents of isolated single skeletal muscle fibers. The IC50 values for half-maximal blocking of the sodium currents were 6.6 microM for ajmaline, 55.7 microM for quinidine, 168.8 microM for sparteine, and 1.2 mM for lupanine. The observed Na+ channel inhibition is in accordance with the use of ajmaline, quinidine and sparteine as antiarrhythmic drugs. The interference of alkaloids with Na+ channels can also be interpreted as a means to strongly interfere with neuronal transmission in herbivores. Alkaloids thus serve as chemical defense compounds for the plants producing them.

Analogues of sparteine. 5. Antiarrhythmic activity of selected N,N'-disubstituted bispidines.

A series of seven N,N'-disubstituted bispidines, structurally analogous to the inner (B and C) rings of sparteine (1) and encompassing a range of lipophilicity in which 1 was centered, has been compared to 1 in regard to antiarrhythmic potency and acute toxicity. Several of the bispidines were of comparable potency, and all but one were somewhat less toxic than 1. The ability of the mononitrate salts of 1 and bispidines 6 and 7 to bind calcium and magnesium cations in Me2SO-d6 solvent has been evaluated by proton magnetic resonance analysis. No binding could be demonstrated under these conditions, which suggested that pharmacologic effects of these compounds may be due to properties other than direct binding of these cations.