Interactions of polyamines with neuronal ion channels

The involvement of spinal release of histamine on nociceptive behaviors induced by intrathecally administered spermine

The involvement of spinal release of histamine on nociceptive behaviors induced by spermine was examined in mice. Intrathecal spermine produced dose-dependent nociceptive behaviors, consisting of scratching, biting and licking. The nociceptive behaviors induced by spermine at 0.02 amol and 10 pmol were markedly suppressed by i.t. pretreatment with antiserum against histamine and were abolished in histidine decarboxylase-deficient mice. In histamine H₁ receptor-deficient mice, the nociceptive behaviors induced by spermine were completely abolished after treatment with 0.02 amol of spermine and significantly suppressed after treatment with 10 pmol of spermine. The i.t. pretreatment with takykinin NK₁ receptor antagonists eliminated the nociceptive behaviors induced by 0.02 amol of spermine, but did not affect the nociceptive behaviors induced by 10 pmol of spermine. On the other hand, the nociceptive behaviors induced by spermine at both 0.02 amol and 10 pmol were suppressed by i.t. pretreatment with antagonists for the NMDA receptor polyamine-binding site. The present results suggest that the nociceptive behaviors induced by i.t. administration of spermine are mediated through the spinal release of histamine and are elicited via activation of NMDA receptors.

Pharmacological response sensitization in nerve cell networks exposed to the antibiotic gentamicin

Gentamicin is an aminoglycoside antibiotic that is used in clinical, organismic, and agricultural applications to combat gram-negative, aerobic bacteria. The clinical use of gentamicin is widely linked to various toxicities, but there is a void in our knowledge about the neuromodulatory or neurotoxicity effects of gentamicin. This investigation explored the electrophysiologic effects of gentamicin on GABAergic pharmacological profiles in spontaneously active neuronal networks in vitro derived from auditory cortices of E16 mouse embryos and grown on microelectrode arrays. Using the GABA_A agonist muscimol as the test substance, responses from networks to dose titrations of muscimol were compared in the presence and absence of 100µM gentamicin (the recommended concentration for cell culture conditions). Spike-rate based EC₅₀ values were generated using sigmoidal fit concentration response curves (CRCs). Exposure to 100µM gentamicin exhibited a muscimol EC₅₀±S.E.M. of 80±6nM (n=10). The EC₅₀ value obtained in the absence of gentamicin was 124±11nM (n=10). The 35% increase in potency suggests network sensitization to muscimol in the presence of gentamicin. Action potential (AP) waveform analyses of neurons exposed to gentamicin demonstrated a concentration-dependent decrease in AP amplitudes (extracellular recordings), possibly reflecting gentamicin effects on voltage-gated ion channels. These in vitro results reveal alteration of pharmacological responses by antibiotics that could have significant influence on the behavior and performance of animals.

Autoimmune diseases and polyamines

Genetics and environmental factors have important roles in autoimmune diseases but neither has given us sufficient understanding of these mysterious diseases. Therefore, we are now looking closer at epigenetics, an interface between genetics and environmental factors. Epigenetics can be defined as reversible heritable changes to chromatin that can alter gene expression without altering the gene's DNA sequence. Methylation of DNA and histones are primary means of epigenetic control. By adding methyl groups to DNA and histones, it can limit accessibility of the underlying gene thereby altering the amount of gene expression. The methyl group is derived from an essential molecule in the cell, S-adenosylmethionine (SAM). However, a group of small molecules called polyamines also require SAM for their synthesis. Polyamines are essential for many cellular functions and polyamine activity is increased in many autoimmune diseases. Presented here is the "polyamine hypothesis" in which increased polyamine synthesis competes with cellular methylation (epigenetic control) for SAM. It is proposed that increased polyamine activity can cause disruption of cellular methylation, which can lead to abnormal expression of previously sequestered genes and disruption of other methylation-dependent cellular processes.

Contribution of peripheral vanilloid receptor to the nociception induced by injection of spermine in mice

Polyamines (putrescine, spermidine and spermine) are important endogenous regulators of ion channels, such as vanilloid (TRPV1), glutamatergic (NMDA or AMPA/kainate) and acid-sensitive (ASIC) receptors. In the present study, we have investigated the possible nociceptive effect induced by polyamines and the mechanisms involved in this nociception in vivo. The subcutaneous (s.c.) injection of capsaicin (as positive control), spermine, spermidine or putrescine produced nociception with ED(50) of 0.16 (0.07-0.39)nmol/paw, 0.4 (0.2-0.7) μmol/paw, 0.3 (0.1-0.9) μmol/paw and 3.2 (0.9-11.5) μmol/paw, respectively. The antagonists of NMDA (MK801, 1 nmol/paw), AMPA/kainate (DNQX, 1 nmol/paw) or ASIC receptors (amiloride, 100 nmol/paw) failed to reduce the spermine-trigged nociception. However, the TRPV1 antagonists capsazepine or SB366791 (1 nmol/paw) reduced spermine-induced nociception, with inhibition of 81 ± 10 and 68 ± 9%, respectively. The previous desensitization with resiniferatoxin (RTX) largely reduced the spermine-induced nociception and TRPV1 expression in the sciatic nerve, with reductions of 82 ± 9% and 67 ± 11%, respectively. Furthermore, the combination of spermine (100 nmol/paw) and RTX (0.005 fmol/paw), in doses which alone were not capable of inducing nociception, produced nociceptive behaviors. Moreover, different concentrations of spermine (3-300 μM) enhanced the specific binding of [(3)H]-RTX to TRPV1 receptor. Altogether, polyamines produce spontaneous nociceptive effect through the stimulation of TRPV1, but not of ionotropic glutamate or ASIC receptors.

Effect of naloxone and morphine on arcaine-induced state-dependent memory in rats

RATIONALE

Arcaine is a competitive antagonist of the polyamine binding site at the N-methyl-D-aspartic acid receptor which induces state-dependent recall. However, no study has addressed the involvement of other neurotransmitter/neuromodulators in arcaine-induced state dependency.

OBJECTIVES

The current study investigates whether the opioid system is involved in arcaine-induced state-dependent memory retrieval of the inhibitory avoidance task (IA) in rats.

RESULTS

The systemic administration of arcaine (30 mg/kg, intraperitoneally (i.p.)) or morphine (5 mg/kg, i.p.) 0, 3, 6, or 9 h post-training, reduced step-down latencies at testing. Arcaine (30 mg/kg, i.p.) or morphine (5 mg/kg, i.p.) injection 30 min before testing reversed the performance deficit induced by administration of arcaine or morphine 0, 3 or 6, but not 9 h post-training. The reversal of arcaine-induced impairment of IA performance was completely transferred to morphine and vice versa. The association of low and ineffective doses of morphine and arcaine (10 and 1.5 mg/kg, respectively) were additive and caused state dependency. Naloxone (2 mg/kg, 3 min post-training, or 1 mg/kg, 1 h pre-test, i.p.) reversed the amnesia and the state dependency induced by morphine and arcaine.

CONCLUSION

These results suggest that state dependency induced by arcaine involves the opioid system.

Diabetes-induced inhibition of voltage-dependent calcium channels in the retinal microvasculature: role of spermine

PURPOSE

Although decentralized control of blood flow is particularly important in the retina, knowledge of the functional organization of the retinal microvasculature is limited. Here, the authors characterized the distribution and regulation of L-type voltage-dependent calcium channels (VDCCs) within the most decentralized operational complex of the retinal vasculature--the feeder vessel/capillary unit--which consists of a capillary network plus the vessel linking it with a myocyte-encircled arteriole.

METHODS

Perforated-patch recordings, calcium-imaging, and time-lapse photography were used to assess VDCC-dependent changes in ionic currents, intracellular calcium, abluminal cell contractility, and lumen diameter, in microvascular complexes freshly isolated from the rat retina.

RESULTS

Topographical heterogeneity was found in the distribution of functional VDCCs; VDCC activity was markedly greater in feeder vessels than in capillaries. Experiments showed that this topographical distribution occurs, in large part, because of the inhibition of capillary VDCCs by a mechanism dependent on the endogenous polyamine spermine. An operational consequence of functional VDCCs predominantly located in the feeder vessels is that voltage-driven vasomotor responses are generated chiefly in this portion of the feeder vessel/capillary unit. However, early in the course of diabetes, this ability to generate voltage-driven vasomotor responses becomes profoundly impaired because of the inhibition of feeder vessel VDCCs by a spermine-dependent mechanism.

CONCLUSIONS

The regulation of VDCCs by endogenous spermine not only plays a critical role in establishing the physiological organization of the feeder vessel/capillary unit, but also may contribute to dysfunction of this decentralized operational unit in the diabetic retina.

Regulation of cellular polyamine levels and cellular proliferation by antizyme and antizyme inhibitor

Polyamines are small aliphatic polycations present in all living cells. Polyamines are essential for cellular viability and are involved in regulating fundamental cellular processes, most notably cellular growth and proliferation. Being such central regulators of fundamental cellular functions, the intracellular polyamine concentration is tightly regulated at the levels of synthesis, uptake, excretion and catabolism. ODC (ornithine decarboxylase) is the first key enzyme in the polyamine biosynthesis pathway. ODC is characterized by an extremely rapid intracellular turnover rate, a trait that is central to the regulation of cellular polyamine homoeostasis. The degradation rate of ODC is regulated by its end-products, the polyamines, via a unique autoregulatory circuit. At the centre of this circuit is a small protein called Az (antizyme), whose synthesis is stimulated by polyamines. Az inactivates ODC and targets it to ubiquitin-independent degradation by the 26S proteasome. In addition, Az inhibits uptake of polyamines. Az itself is regulated by another ODC-related protein termed AzI (antizyme inhibitor). AzI is highly homologous with ODC, but it lacks ornithine-decarboxylating activity. Its ability to serve as a regulator is based on its high affinity to Az, which is greater than the affinity Az has to ODC. As a result, it interferes with the binding of Az to ODC, thus rescuing ODC from degradation and permitting uptake of polyamines.

Arcaine and MK-801 make recall state-dependent in rats

RATIONALE

The polyamines putrescine, spermidine, and spermine are a group of aliphatic amines that may act as physiological modulators of the N-methyl-D-aspartate (NMDA) receptor, a glutamate receptor implicated in memory formation and consolidation. Arcaine is a competitive antagonist of the polyamine binding site at the NMDA receptor, the post-training administration of which impairs memory of various tasks.

OBJECTIVES

In this study, we investigated whether the administration of arcaine and MK-801 alters the memory of the step-down inhibitory avoidance task, and whether the effects of these NMDA antagonists involve state-dependency mechanisms, in adult male Wistar rats.

RESULTS

The administration of arcaine (30 mg/kg, i.p.) or MK-801 (0.03 mg/kg, i.p.) immediately after training impaired inhibitory avoidance performance at testing. Arcaine- and MK-801-induced performance impairment was reversed by the administration of arcaine (30 mg/kg, i.p.) and MK-801 (0.03 mg/kg, i.p.), respectively, 30 min before testing. Response transfer also occurred if arcaine substituted MK-801 at testing, and vice-versa.

CONCLUSION

These results suggest that arcaine and MK-801 induce state-dependent recall and that, probably due to their ability to decrease NMDA receptor function, one drug can substitute for the other at testing, demonstrating a cross-state dependency between arcaine and MK-801.

Effect of polyamines on the structure, thermal stability and 2,2,2-trifluoroethanol-induced aggregation of alpha-chymotrypsin

Naturally occurring polyamines are known to interact with a variety of biomolecules and critically involve in some important physiological processes. They have also been shown to influence protein aggregation in vitro in some cases. The aim of the present study was to investigate how polyamines may influence the structure and thermal stability of alpha-chymotrypsin and modulate alcohol-induced aggregation of this protein. Various techniques, including turbidity measurements, tensiometry, DSC, intrinsic fluorescence and far- and near-UV circular dichroism spectroscopy were used to examine the effect of putrescine and spermidine on alpha-chymotrypsin. While slight changes in the secondary and tertiary structure of the protein was observed, a clear stabilizing effect against its thermal unfolding was achieved. Moreover, the polyamines were found to inhibit TFE-induced aggregation at 32% TFE and promote formation of non-native alpha-helices in the protein structure. Based on the observed increase in surface tension induced by polyamines, it is suggested that their effects on enhancing thermal stability and alcohol-induced alpha-helices formation may be due to their kosmotropic properties.

In Silico Chemical Library Screening and Experimental Validation of a Novel 9-Aminoacridine Based Lead-Inhibitor of Human S-Adenosylmethionine Decarboxylase

In silico chemical library screening (virtual screening) was used to identify a novel lead compound capable of inhibiting S-adenosylmethionine decarboxylase (AdoMetDC). AdoMetDC is intimately involved in the biosynthesis of polyamines, which are essential for tumor progression and are elevated in numerous types of tumors. Therefore, inhibition of this enzyme provides an attractive target for the discovery of novel anticancer drugs. We performed virtual screening using a computer model derived from the X-ray crystal structure of human AdoMetDC and the National Cancer Institute's Diversity Set (1990 compounds). Our docking study suggested several compounds that could serve as drug candidates since their docking modes and scores revealed potential inhibitory activity toward AdoMetDC. Experimental testing of the top-scoring compounds indicated that one of these compounds (NSC 354961) possesses an IC50 in the low micromolar range. A search of the entire NCI compound collection for compounds similar to NSC 354961 yielded two additional compounds that exhibited activity in the experimental assay but with significantly diminished potency relative to NSC 354961. In this report, we disclose the activity of NSC 354961 against AdoMetDC and its probable binding mode based on computational modeling. We also discuss the importance of virtual screening in the context of enzymes that are not readily amenable to high-throughput assays, thereby demonstrating the efficacy of virtual screening, combined with selective experimental testing, in identifying new potential drug candidates.

Modulation of neuronal voltage-activated calcium and sodium channels by polyamines and pH

The endogenous polyamines spermine, spermidine and putrescine are present at high concentrations inside neurons and can be released into the extracellular space where they have been shown to modulate ion channels. Here, we have examined polyamine modulation of voltage-activated Ca(2+) channels (VACCs) and voltage-activated Na(+) channels (VANCs) in rat superior cervical ganglion neurons using whole-cell voltage-clamp at physiological divalent concentrations. Polyamines inhibited VACCs in a concentration-dependent manner with IC(50)s for spermine, spermidine, and putrescine of 4.7 +/- 0.7, 11.2 +/- 1.4 and 90 +/- 36 mM, respectively. Polyamines caused inhibition by shifting the VACC half-activation voltage (V(0.5)) to depolarized potentials and by reducing total VACC permeability. The shift was described by Gouy-Chapman-Stern theory with a surface charge density of 0.120 +/- 0.005 e(-) nm(-2) and a surface potential of -19 mV. Attenuation of spermidine and spermine inhibition of VACC at decreased pH was explained by H(+) titration of surface charge. Polyamine-mediated effects also decreased at elevated pH due to the inhibitors having lower valence and being less effective at screening surface charge. Polyamines affected VANC currents indirectly by reducing TTX inhibition of VANCs at high pH. This may reflect surface charge induced decreases in the local TTX concentration or polyamine-TTX interactions. In conclusion, polyamines inhibit neuronal VACCs via complex interactions with extracellular H(+) and Ca. Many of the observed effects can be explained by a model incorporating polyamine binding, H(+) binding and surface charge screening.

Screening of a combinatorial library of synthetic polyamines displaying selectivity in multiple ion-pairing interactions with model polyanionic compounds in aqueous organic solutions

The biological activity of natural polyamines is due in large part to their ability to form ion-pairing interactions with polyanionic biomolecules, such as proteins, oligonucleotides, and sulfated oligosaccharides. Unfortunately, the diversity of biogenic polyamines is compromised by their limitation to only just a few internitrogen spacers. As a proof-of-principle study, a synthetic split-pool library of linear triamines was screened in an on-bead assay against a selection of model trisulfonated azo dyes (1, 2, and 3) and a short glutamate-rich nonameric peptide (4) to demonstrate its use in the discovery of selective ligands via multivalent ion pairing. From screening a 196-membered split-pool library against the dyes in aqueous organic solutions, with or without spermidine as competing ligand, it was found that the most frequent residues possessed internitrogen distances that were very similar to the sulfonate distances on the dyes. The results from these screening assays were used in the design of two polyamine sequences (8, 8Aoc(R)-8Aoc(R), and 12, 2Acc(R)-epsilonAhx(R)) for follow-up studies in solution phase. These triamines demonstrated the same selectively toward dyes 2 and 3 as observed by the solid-phase approach. In addition, resin-supported triamines, synthesized as discrete compounds, were able to selectively extract either dye 2 or 3 from a mixture of the two, further verifying the observations made from the library screening efforts. With peptide 4, containing three glutamate residues, a preference was found for rather long residues (12 and 8 carbons long), which is suggestive of a linear peptide, rather than a helical motif under the conditions of the screening.

Autoimmune disorders result from loss of epigenetic control following chromosome damage

Multiple sclerosis, systemic lupus erythematosus, and rheumatoid arthritis share common features in typical cases such as: adult onset, central nervous system problems, female predominance, episodes triggered by a variety of stresses, and an autoimmune reaction. At times, the different disorders are found in the same patient or close relatives. These disorders are quite complex but they may share a common mechanism that results in different, tissue-specific consequences based on the cell types in which the mechanism occurs. Here, it is hypothesized that DNA damage can lead to loss of epigenetic control, particularly when the damaged chromatin is distributed unevenly to daughter cells. Expression of genes and pseudogenes that have lost their epigenetic restraints can lead to autoimmune disorders. Loss of control of genes on the X chromosome and loss of control of polyamine expression are discussed as examples of this mechanism.

Time-dependent modulation of inhibitory avoidance memory by spermidine in rats

The polyamines, spermine, spermidine, and putrescine, are a group of aliphatic amines that may act as physiological modulators of N-methyl-D-aspartate (NMDA) receptors. Although the modulatory role of polyamines in NMDA receptor function has long been known, the effects of polyamines on learning and memory only recently began to be unraveled. In the present study, we investigated the effect of bilateral infusions of spermidine (0.02-2 nmol), a polyamine agonist, into the CA1 region of the rat dorsal hippocampus on inhibitory avoidance learning 30 min pre-training, immediately post-training, 6 h post-training, or 10 min pre-test. Bilateral microinjections of 0.2 nmol spermidine prolonged step-down latencies compared to the respective control group when administered 30 min pre-training or immediately post-training. These results provide evidence that the modulatory effects of spermidine on the acquisition and/or early consolidation of memory of inhibitory avoidance tasks in the hippocampus occur within a limited time window.

Solid-Phase Synthesis of Rigid Acylpolyamines Using Temporary N-4,4‘-Dimethoxytrityl Protection in the Presence of Trityl Linkers

An N-protection protocol employing the 4,4'-dimethoxytrityl (Dmt) group in combination with borane reduction of resin-bound polyamides was shown to be an efficient methodology that enables synthesis of novel analogues of natural acylpolyamine toxins. Thus, three philanthotoxins containing polyamine chains with piperidyl and cyclohexyl structural elements, which introduce conformational rigidity, increased lipophilicity, and altered proteolytic properties, were obtained in 39-44% overall yield.

Maintained regulation of polyamines in spinal cord from patients with amyotrophic lateral sclerosis

Levels of the polyamines putrescine, spermidine, and spermine were investigated in postmortem spinal cord from seven patients with amyotrophic lateral sclerosis (ALS) and seven control subjects. The method consisted of precolumn derivatization of the polyamines, followed by high-performance liquid chromatography (HPLC) analysis and fluorescence detection. The stability of the polyamines was examined in rat spinal cord during the interval of 0-36 h postmortem. The levels of putrescine, spermidine, and spermine increased by 32%, 15%, and 2%, respectively. Polyamine levels did not differ significantly between the ALS group and the control group, suggesting a maintained regulation of polyamines in the end stage of the disease. However, an effect of gender on the levels of spermidine and spermine was observed. Levels of spermidine and spermine in the ventral horn region of female ALS patients were significantly higher in comparison with the same region of the male ALS group (p<0.05). The female ALS group also presented significantly higher levels of spermidine in comparison with female controls (p<0.05).

Determination of rat hepatic polyamines by electron-capture gas chromatography

INTRODUCTION

An efficient and reproducible electron-capture gas chromatographic protocol that allows the simultaneous detection and quantification of the polyamines putrescine (1,4-diaminobutane), cadaverine (1,5-diaminopentane), and spermidine (N-[3-aminopropyl]-1,4-diaminobutane) was developed.

METHODS

Hepatic tissue from male Sprague-Dawley rats was used for analysis. The polyamines and the internal standard (sertraline) were extracted and derivatized with pentafluorobenzoyl chloride (PFBC) under basic aqueous conditions prior to analysis on a gas chromatograph equipped with a capillary column (narrow-bore fused silica column; 25 mm x 0.32 mm) and an electron-capture detector.

RESULTS

PFBC reacts with the amine functions of the polyamines examined here to produce PFB derivatives with high sensitivity on electron-capture detection. The method permitted the quantitative analyses of all three amines in rat hepatic tissue; the concentration of putrescine, but not spermidine, was increased significantly following a 14-day administration of the diamine oxidase (DAO) inhibitor aminoguanidine. Cadaverine was also present at increased concentrations in hepatic homogenates from aminoguanidine-treated rats.

DISCUSSION

Extractive derivatization with PFBC followed by gas chromatographic analysis using electron-capture detection results in a rapid and reproducible assay that permits the simultaneous detection and quantification of putrescine, cadaverine, and spermidine in biological tissue.

Reversible changes in goldfish brain polyamine concentrations and synthetic enzymes after cold exposure

Exposure of goldfish to the cold (5 degrees C) caused a sharp increase in brain putrescine level during the first week. Such increase continued at a minor rate for the whole period of exposure (2 months). In contrast, the content of spermidine and spermine remained unchanged. Putrescine increase was concomitant with a remarkable rise in ornithine decarboxylase activity (ODC), which reached a maximum stimulation after 1 week of cold exposure, and declined thereafter, remaining significantly higher than the control for the entire period of study. Cold exposure caused also a reduction of S-adenosylmethionine decarboxylase (AdoMetDC) activity and an increase of ornithine level, whereas methionine content was unchanged. When fish exposed to cold temperature were returned to 20 degrees C, the modifications observed on brain polyamine metabolism were completely reversed. Supported by previous observations, our results suggest that the changes in the polyamine metabolism induced in goldfish brain by cold exposure could represent an homeostatic mechanism carried out by the goldfish to minimize the possible effects of thermal changes.

Targeting ionotropic receptors with polyamine-containing toxins☆

This review summarises current knowledge of polyamine-containing spider toxins and their interactions with ionotropic receptors of invertebrate and vertebrate excitable cells. Their diverse actions on ionotropic glutamate and acetylcholine receptors, which include potentiation, closed channel block and open channel block, are discussed in the context of toxin and target structures. Factors that complicate attempts to identify and pharmacologically characterise the binding sites for these toxins include their ability to permeate channels of some ionotropic receptors and their apparent accumulation in a cellular compartment, possibly the membrane bilayer.

Characterisation of a novel class of polyamine-based neuroprotective compounds

Prolonged cerebral ischaemia initiates complex intra- and inter-cellular signalling cascades ultimately resulting in neuronal death. Well-characterised mediators of ischaemic cell death are glutamate, free radicals and nitric oxide. Many drugs that block these mechanisms are neuroprotective in vitro, but have unfavourable side-effect profiles in man. We have recently demonstrated that the compound L-arginyl-3,4-spermidine (L-Arg3,4) is neuroprotective in vitro through an interaction with several of these mechanisms, and prevents ischaemic neurodegeneration in vivo with no gross side effects. In this study, we have used solid-phase combinatorial chemistry, to synthesise a number of analogues of L-Arg3,4, and investigate the structure-activity relationship using an in vitro, organotypic hippocampal slice culture model of cerebral ischaemia. A number of molecular features were identified which were essential for the neuroprotective activity including the requirement for a positive charge and an amino acid in the L-configuration. Relatively minor alterations to both the terminal arginine and polyamine moieties significantly attenuated neuroprotective efficacy. Our data implies that these compounds are neuroprotective through a currently undefined mechanism rather than non-specific ionic interactions described previously for other polyamine-containing compounds.

Regulation of voltage-sensitive Ca2+ channels in bipolar cells by divalent cations and polyamines

Ca2+ plays a key role in intracellular signal transduction in neurons but in excess it can lead to cell death. Thus its entry into cells is highly regulated by both extrinsic and intrinsic mechanisms. Little is known of the regulation of Ca2+ entry into retinal neurons. Here we describe the role of divalent cations and polyamines as intrinsic modulators of Ca2+ entry into retinal bipolar cells. Cone-dominant (small) bipolar cells of the white bass retina were studied using whole cell patch clamp techniques. With biophysical and pharmacological tools it was determined that these cells expressed a Ca2+ current similar to an L-type current. This current was very susceptible to blockage by divalent cations including Ca2+. In addition, when tested with the polyamines, spermine, spermidine and putrescine, only spermine effectively inhibited the current. When the dose response curve was fit with the Hill function we found an EC50 of 28 microM and a Hill-coefficient of about 2. Our results indicate that divalent cations and the polyamine, spermine, are effective modulators of calcium entry into cone-dominated bipolar cells. The in vivo regulation of the concentrations of these molecules provides an exquisitely sensitive mechanism for regulating Ca2+ entry into bipolar cells under different conditions.

Synthesis and high performance liquid chromatography/electrospray mass spectrometry single-bead decoding of split-pool structural libraries of polyamines supported on polystyrene and polystyrene/ethylene glycol resins

Natural polyamines are ubiquitous biomolecules present in all living cells. These cationic compounds play essential roles in both cell growth and differentiation and are known to interact in complex ways with polyanionic biomolecules. Consequently, there is significant interest in expanding nature's polyamine diversity using combinatorial synthesis and screening strategies. This article describes an efficient split-pool solid-phase synthetic strategy toward the generation of encoded libraries of polyamines via the exhaustive borane-promoted reduction of trityl-linked, resin-bound polyamides. Model structural libraries of tetra- and pentaamines were designed from a set of geometrically diverse amino acid building blocks. To encode the libraries, a partial termination synthesis approach was employed at the polyamide stage, allowing each library to be analyzed from single beads by HPLC/ESMS under two sets of conditions featuring both pH extremes. Determination of the sequence of polyamine residues was simply achieved by the mass differences observed between the full oligomers and the terminated ones. Both polystyrene- and TentaGel-supported libraries, including a library of 4913 pentaamines, were prepared and successfully decoded. For the TentaGel-supported libraries, suitable for on-bead aqueous screening of biomolecules, a novel trityl-derivatized resin was prepared in which the trityl group is anchored to the poly(ethylene glycol) chains via a methylene group. The resulting resin is much more resistant than other commercially available polystyrene-poly(ethylene glycol) trityl resins to the harsh borane reduction conditions required. Two workup conditions for the cleavage of the resultant borane-amine adducts were evaluated on the TentaGel bound polyamide 14. Although the two methods showed a comparable efficiency when using the polystyrene support, with 14 it was found that the piperidine-exchange method afforded polyamines of higher purity than the iodine-based oxidative method previously developed in our laboratory.

Spermine-induced negative inotropic effect in isolated rat heart, is mediated through the release of ATP

Putrescine, spermidine and spermine are natural compounds found in up to millimolar concentrations in eukaryotic and prokaryotic cells. At physiologic pH, the polyamines are protonated (+2, +3 and +4 charges), their polycationic properties lead to the assumption that they could affect physiological systems by binding to anionic sites of the cellular membrane and/or by modulating ion channels. At the cardiovascular level, their effects are not completely understood. However, these compounds may be able to exert the induction of synthesis and release of cellular mediators. In an attempt to explore this possibility, we used the isolated and perfused rat heart, Langendorff, model in order to evaluate the inotropic effects of these polyamines, putrescine, spermidine and spermine. Dose-response curves (0.1-0.6 mM) for putrescine, spermidine and spermine were constructed; with the finding that spermine had the largest negative effect. The obtained effects were not blocked by nitric oxide synthesis inhibitors (L-NAME), H(1) and H(2) receptor antagonists (Brompheniramine and Cimetidine) or by Glibenclamide, an antagonist of ATP-sensitive K(+) channels. We found that spermine-induced and increased ATP concentration in cardiac effluents. Reactive Blue, a P(2y) purinoreceptor antagonist and Aminophylline, an unspecific adenosine receptor antagonist, blocked the spermine-induced effects. These results showed that ATP, at least in part, is responsible of the spermine cardiovascular effects. Adenosine was shown to also play an important role on those effects.

Involvement of voltage- and ligand-gated Ca2+ channels in the neuroexcitatory and synergistic effects of putative uremic neurotoxins

BACKGROUND

Renal failure has been viewed as a state of cellular calcium toxicity due to the retention of small fast-acting molecules. We have tested this hypothesis and identified potentially neuroexcitatory compounds among a number of putative uremic neurotoxins by examining the acute in vitro effects of these compounds on cultured central neurons. The in vitro neuroexcitatory and synergistic effects of guanidinosuccinate and spermine were also examined in vivo.

METHODS

The acute effects of 17 candidate uremic neurotoxins on murine spinal cord neurons in primary dissociated cell culture were investigated using the tight-seal whole-cell recording technique. The compounds studied comprised low-molecular-weight solutes like urea, indoles, guanidino compounds, polyamines, purines and phenoles, homocysteine, orotate, and myoinositol. Currents evoked by these compounds were further examined using various ligand- and voltage-gated ion channel blockers. The acute in vivo effects of guanidinosuccinate and spermine were behaviorally assessed following their injection in mice.

RESULTS

It was shown that 3-indoxyl sulfate, guanidinosuccinate, spermine, and phenol evoked significant whole-cell currents. Inward whole-cell current evoked by 3-indoxyl sulfate was not blocked by any of the applied ligand- or voltage-gated ion channel blockers, and the compound appeared to influence miscellaneous membrane ionic conductances, probably involving voltage-gated Ca2+ channels as well. Phenol-evoked outward whole-cell currents were at least partly due to the activation of voltage-gated K+ channels, but may also involve a variety of other ionic conductances. On the other hand, inward whole-cell currents evoked by guanidinosuccinate and spermine were shown to be due to specific interaction with voltage- and ligand-gated Ca2+ channels. Guanidinosuccinate-evoked current was caused by activation of N-methyl-d-aspartate (NMDA) receptor-associated ion channels. Low (micromol/L) concentrations of spermine potentiated guanidinosuccinate-evoked current through the action of spermine on the polyamine binding site of the NMDA receptor complex, whereas current evoked by high (mmol/L) concentrations of spermine alone involved direct activation of voltage-gated Ca2+ channels. Finally, intracerebroventricular administration of 0.25 micromol/L spermine potentiated clonic convulsions induced by guanidinosuccinate. These neuroexcitatory and synergistic effects of guanidinosuccinate and spermine could take place at pathophysiologic concentrations.

CONCLUSION

The observed in vitro and in vivo effects of uremic retention solutes suggest that the identified compounds could play a significant role in uremic pathophysiology. Some of the compounds tested displayed in vitro and in vivo neuroexcitatory effects that were mediated by ligand- and voltage-gated Ca2+ channels. The findings suggest a mechanism for the involvement of calcium toxicity in the central nervous system complications in renal failure with particular reference to guanidinosuccinate and spermine.

PnTx3-6 a spider neurotoxin inhibits K+-evoked increase in [Ca2+](i) and Ca2+-dependent glutamate release in synaptosomes

The present experiments investigated the effect of a neurotoxin purified from the venom of the spider Phoneutria nigriventer. This toxic component, P. nigriventer toxin 3-6 (PnTx3-6), abolished Ca(2+)-dependent glutamate release with an IC(50) of 74.4nM but did not alter Ca(2+)-independent secretion of glutamate when brain cortical synaptosomes were depolarized by KCl (33mM). This effect was most likely due to interference with the entry of calcium through voltage activated calcium channels (VACC), reducing the increase in the intrasynaptosomal free calcium induced by membrane depolarization with an IC(50) of 9.5nM. We compared the alterations induced by PnTx3-6 with the actions of toxins known to block calcium channels coupled to exocytosis. Our results indicate that PnTx3-6 inhibition of glutamate release and intrasynaptosomal calcium involves P/Q type calcium channels and this toxin can be a valuable tool in the investigation of calcium channels.

Monovalent cation (MC) current in cardiac and smooth muscle cells: regulation by intracellular Mg2+ and inhibition by polycations

1 Previously we have described a monovalent cation (MC) current that could be unmasked by the removal of extracellular divalent cations in vascular smooth muscle cells (SMC) and cardiac myocytes, but specific and potent inhibitors of MC current have not been found, and the mechanism of its intracellular regulation remains obscure. 2 Here we show that small MC current is present in intact cells and could be dramatically up-regulated during cell dialysis. MC current in dialyzed cells strongly resembled monovalent cation current attributed to Ca(2+) release-activated Ca(2+)-selective (CRAC) channels, but its activation did not require depletion of Ca(2+) stores, and was observed when the cells were dialyzed with, or without BAPTA. 3 Intracellular free Mg(2+) inhibits MC current with K(d)=250 microM. 4 Extracellular (but not intracellular) spermine effectively blocked MC current with K(d) =3-10 microM, while store-operated cations (SOC) channels and capacitative Ca(2+) influx were not affected. 5 Spermine effectively inhibited MC current-induced SMC depolarization, and prevented Ca(2+) paradox-induced vascular contracture. 6 Both, MC and SOC currents were inhibited by 2-aminoethoxydiphenyl borate (2-APB). 7 It is concluded that MC current could be regulated by intracellular Mg(2+), and low concentrations of extracellular spermine could be used to discriminate it from SOC current, and to assess its role in cellular function.

Voltage-dependent blockade of connexin40 gap junctions by spermine

The effects of spermine and spermidine, endogenous polyamines that block many forms of ion channels, were investigated in homotypic connexin (Cx)-40 gap junctions expressed in N2A cells. Spermine blocked up to 95% of I(j) through homotypic Cx40 gap junctions in a concentration- and transjunctional voltage (V(j))-dependent manner. V(j) was varied from 5 to 50 mV in 5-mV steps and the dissociation constants (K(m)) were determined from spermine concentrations ranging from 10 micro M to 2 mM. The K(m) values ranged from 4.9 mM to 107 micro M for 8.6 < or = V(j) < or = 37.7 mV, within the physiological range of intracellular spermine for V(j) > or = 20 mV. The K(m) values for spermidine were > or = 5 mM. Estimates of the electrical distance (delta) for spermine (z = +4) and spermidine (z = +3) were 0.96 and 0.76 respectively. Cx40 single channel conductance was 129 pS in the presence of 2-mM spermine and channel open probability was significantly reduced in a V(j)-dependent manner. Similar concentrations of spermine did not block I(j) through homotypic Cx43 gap junctions, indicating that spermine selectively blocks Cx40 gap junctions. This is contrary to our previous findings that large tetraalkylammonium ions, also known to block several forms of ion channels, block junctional currents (I(j)) through homotypic connexin Cx40 and Cx43 gap junctions.

L-arginyl-3,4-spermidine is neuroprotective in several in vitro models of neurodegeneration and in vivo ischaemia without suppressing synaptic transmission

1. Stroke is the third most common cause of death in the world, and there is a clear need to develop new therapeutics for the stroke victim. To address this need, we generated a combinatorial library of polyamine compounds based on sFTX-3.3 toxin from which L-Arginyl-3,4-Spermidine (L-Arg-3,4) emerged as a lead neuroprotective compound. In the present study, we have extended earlier results to examine the compound's neuroprotective actions in greater detail. 2. In an in vitro ischaemia model, L-Arg-3,4 significantly reduced CA1 cell death when administered prior to induction of 60 min of ischaemia as well as when administered immediately after ischaemia. Surprisingly, L-Arg-3,4 continued to prevent cell death significantly when administration was delayed for as long as 60 min after ischaemia. 3. L-Arg-3,4 significantly reduced cell death in excitotoxicity models mediated by glutamate, NMDA, AMPA, or kainate. Unlike glutamate receptor antagonists, 300 microM L-Arg-3,4 did not suppress synaptic transmission as measured by evoked responses in acute hippocampal slices. 4. L-Arg-3,4 provided significant protection, in vitro, in a superoxide mediated injury model and prevented an increase of superoxide production after AMPA or NMDA stimulation. It also decreased nitric oxide production after in vitro ischaemia and NMDA stimulation, but did so without inhibiting nitric oxide synthase directly. 5. Furthermore, L-Arg-3,4 was significantly neuroprotective in an in vivo model of global forebrain ischaemia, without any apparent neurological side-effects. 6. Taken together, these results demonstrate that L-Arg-3,4 is protective in several models of neurodegeneration and may have potential as a new therapeutic compound for the treatment of stroke, trauma, and other neurodegenerative diseases.

Regulation of Ca2+ channel and phosphatase activities by polyamines in intestinal and vascular smooth muscle--implications for cellular growth and contractility

Polyamines added extracellularly to intestinal and vascular smooth muscle cells cause relaxation through inhibition of Ca2+ channel activity. Intracellularly applied polyamines also affect Ca2+ channel properties. Polyamines do not readily pass over the plasma membrane because of their positive charges but in permeabilized smooth muscle preparations they have free access to the cytoplasm. In this system they increase sensitivity of the contractile machinery to Ca2+ through inhibition of myosin phosphatase activity. The magnitude of Ca2+ channel and phosphatase inhibition depends on the number of positive charges on the polyamine molecule. Polyamines have an obligatory, but yet undefined, role in regulation of cell growth and proliferation. Several groups of protein kinases, such as tyrosine and mitogen activated protein (MAP)-kinases transmit the growth signal from the plasma membrane to the cell nucleus where mitosis and protein synthesis are initiated. The data reviewed here show that polyamines may affect such signal transmission via inhibition of phosphatase activity.

Short- and long-chain natural polyamines play specific roles in adult cricket neuroblast proliferation and neuron differentiation in vitro

In the house cricket (Acheta domesticus) mushroom bodies, neurogenesis still occurs during adulthood. Using in vitro approaches, the respective roles of natural polyamines in neurogenesis were examined. Mushroom body neuroblast proliferation was assayed in organotypic culture using 5-bromo, 2'-deoxyuridine labeling. The number of labeled cells was significantly increased when putrescine was added to culture medium, whereas spermidine and spermine supplementation did not alter cell proliferation. Conversely, in vitro morphometric studies on mushroom body neurons cultured in a defined medium showed that putrescine addition failed to alter any morphological character of these interneurons, whereas addition of the long-chain polyamines, spermidine and spermine, stimulated neuron differentiation. These two polyamines significantly increased total neurite length; moreover, spermidine-treated cells exhibited more branches than the controls. The present data demonstrate that putrescine has a mitogenic effect on mushroom body neuronal precursors, and that spermidine and spermine, which failed to induce neuroblast proliferation, act on neuronal differentiation, inducing neurite outgrowth. Our results indicate that short- and long-chain polyamines play specific roles during neurogenesis, and provide a basis for further studies on neuronal precursor proliferation and differentiation.

Effect of two polyamine toxins on the bacterial porin OmpF

Spermine, a polyamine based on a 12-carbon motif, is an effective inhibitor of E. coli OmpF porin. Here we study the inhibition of porin by two polyamine toxins commonly used as modulators of polyamine-sensitive channels: Philanthotoxin-433 (PhTX) from wasp venom and Joro spider toxin (JSTX). Both are highly asymmetric molecules, with at one end a 12-carbon chain polyamine targeting the molecule to the porin constriction zone, and at the other end large aromatic groups conferring to this extremity a size in the order of the OmpF constriction zone. Here we report that PhTX, but not Joro toxin, induces a high degree of flickering in the OmpF-mediated current. The effect is concentration and voltage-dependent, and greatly diminished in a mutant lacking D113 on the constriction loop, a residue previously shown to be required for spermine sensitivity. Possible reasons for the distinct sensitivity of OmpF to PhTX and Joro toxin are discussed.

Cytoplasmic polyamines as permeant blockers and modulators of the voltage-gated sodium channel

We report that voltage-gated Na+ channels (Na(V)) from rat muscle (mu1) expressed in HEK293 cells exhibit anomalous rectification of whole-cell outward current under conditions of symmetrical Na+. This behavior gradually fades with time after membrane break-in, as if a diffusible blocking substance in the cytoplasm is slowly diluted by the pipette solution. The degree of such block and rectification is markedly altered by various mutations of the conserved Lys(III) residue in Domain III of the Na(V) channel selectivity filter (DEKA locus), a principal determinant of inorganic ion selectivity and organic cation permeation. Using whole-cell and macropatch recording techniques, we show that two ubiquitous polyamines, spermine and spermidine, are potent voltage-dependent cytoplasmic blockers of mu1 Na(V) current that exhibit relief of block at high positive voltage, a phenomenon that is also enhanced by certain mutations of the Lys(III) residue. In addition, we find that polyamines alter the apparent rate of macroscopic inactivation and exhibit a use-dependent blocking phenomenon reminiscent of the action of local anesthetics. In the presence of a physiological Na+/K+ gradient, spermine also inhibits inward Na(V) current and shifts the voltage dependence of activation and inactivation. Similarities between the endogenous blocking phenomenon observed in whole cells and polyamine block characterized in excised patches suggest that polyamines or related metabolites may function as endogenous modulators of Na(V) channel activity.

Glutamate receptors and nociception: implications for the drug treatment of pain

Evidence from the last several decades indicates that the excitatory amino acid glutamate plays a significant role in nociceptive processing. Glutamate and glutamate receptors are located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Glutamate acts at several types of receptors, including ionotropic (directly coupled to ion channels) and metabotropic (directly coupled to intracellular second messengers). Ionotropic receptors include those selectively activated by N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate. Metabotropic glutamate receptors are classified into 3 groups based on sequence homology, signal transduction mechanisms and receptor pharmacology. Glutamate also interacts with the opioid system, and intrathecal or systemic coadministration of glutamate receptor antagonists with opioids may enhance analgesia while reducing the development of opioid tolerance and dependence. The actions of glutamate in the brain seem to be more complex. Activation of glutamate receptors in some brain areas seems to be pronociceptive (e.g. thalamus, trigeminal nucleus), although activation of glutamate receptors in other brain areas seems to be antinociceptive (e.g. periaqueductal grey, ventrolateral medulla). Application of glutamate, or agonists selective for one of the several types of glutamate receptor, to the spinal cord or periphery induces nociceptive behaviours. Inhibition of glutamate release, or of glutamate receptors, in the spinal cord or periphery attenuates both acute and chronic pain in animal models. Similar benefits have been seen in studies involving humans (both patients and volunteers); however, results have been inconsistent. More research is needed to clearly define the role of existing treatment options and explore the possibilities for future drug development.

Spermidine: A predictor for neurological outcome and infarct size in focal cerebral ischemia?

BACKGROUND AND PURPOSE

Polyamines are mainly restricted to the intracellular space. During focal cerebral ischemia, polyamines are released from the intracellular compartment. Experimental studies have implicated a marked elevation in brain tissue and blood. The aim of our study was to investigate whether the elevation of polyamines in the blood of patients with focal cerebral ischemia correlates with the clinical outcome and the infarct volume.

METHODS

Polyamines were measured in 16 patients with focal cerebral ischemia and in 8 healthy control subjects. Blood samples for polyamine measurement were taken at admission and at fixed time points for the next 28 days. Polyamines were analyzed in red blood cells by a high-pressure liquid chromatography system. Clinical findings were recorded with the NIH Stroke Scale score. Volume of infarction was analyzed from cranial CT at admission and on days 4 to 6 after ischemia.

RESULTS

A significant increase of the spermidine level in the peripheral blood could be observed in all patients with focal cerebral ischemia as compared with control subjects (P:<0.01), starting with the admission. Spermidine values correlated positively with the clinical outcome at several time points in the first 48 hours (r=0.90 to 0.40; P:<0.01) and with the infarct volume in cranial CT on days 4 to 6 (r=0.91; P:<0.01).

CONCLUSIONS

As hypothesized from experimental data, polyamine levels in blood increase in patients after focal cerebral ischemia. The results indicate that the peripheral spermidine level is closely associated with the clinical outcome as well as with the infarction volume. Therefore, polyamines may be used as a novel predictor for the prognosis of patients with focal cerebral ischemia.

Voltage-dependent conductance changes in the store-operated Ca2+ current ICRAC in rat basophilic leukaemia cells

Tight-seal whole-cell patch-clamp experiments were carried out in order to investigate the effects of different holding potentials on the rate of development and amplitude of the Ca2+ release-activated Ca2+ current ICRAC in rat basophilic leukaemia (RBL-1) cells. ICRAC was monitored at -80 mV from fast voltage ramps, spanning 200 mV in 50 ms. At hyperpolarised potentials, the macroscopic CRAC conductance was lower than that seen at depolarised potentials. The conductance increased almost 5-fold over the voltage range -60 to +40 mV and was seen when the stores were depleted either by the combination of IP3 and thapsigargin in high Ca2+ buffer, or passively with 10 mM EGTA or BAPTA. The voltage-dependent conductance of the CRAC channels could not be fully accounted for by Ca2+-dependent fast inactivation, nor by other slower inhibitory mechanisms. It also did not seem to involve intracellular Mg2+ or the polycations spermine and spermidine. Voltage step relaxation experiments revealed that the voltage-dependent conductance changes developed and reversed slowly, with a time constant of several seconds at -60 mV. In the presence of physiological levels of intracellular Ca2+ buffers, ICRAC was barely detectable when cells were clamped at -60 mV and dialysed with IP3 and thapsigargin, but at 0 mV the current in low Ca2+ buffer was as large as that seen in high Ca2+ buffer. Our results suggest that CRAC channels exhibit slow voltage-dependent conductance changes which can triple the current amplitude over the physiological range of voltages normally encountered by these cells. The role of this conductance change and possible underlying mechanisms are discussed.

Glutamate-mediated transmission, alcohol, and alcoholism

Glutamate-mediated neurotransmission may be involved in the range of adaptive changes in brain which occur after ethanol administration in laboratory animals, and in chronic alcoholism in human cases. Excitatory amino acid transmission is modulated by a complex system of receptors and other effectors, the efficacy of which can be profoundly affected by altered gene or protein expression. Local variations in receptor composition may underlie intrinsic regional variations in susceptibility to pathological change. Equally, ethanol use and abuse may bring about alterations in receptor subunit expression as the essence of the adaptive response. Such considerations may underlie the regional localization characteristic of the pathogenesis of alcoholic brain damage, or they may form part of the homeostatic change that constitutes the neural substrate for alcohol dependence.

Neuroethological analysis of the effects of spider venom from Scaptocosa raptoria (Lycosidae: Araneae) microinjected in the lateral ventricle of Wistar rats

The inhibition of excitatory mechanisms by components of low molecular weight, isolated from spiders and solitary wasps, such as, the acyl-polyamines, has demonstrated, on the one hand, neuroprotection potential, and on the other hand, it is well known that some arthropod venom components have convulsant activity. While many sophisticated experiments are conducted to determine the mechanisms and effects of arthropod venoms, relatively little attention has been paid to the behavioral changes that occur in mammals after being administered given doses of them. The precise detection of these behavioral changes can be used as a sensitive indicator of central nervous system dysfunction. This study investigated the behavioral effects of crude venom from the spider Scaptocosa raptoria after intracerebroventricular injection in male Wistar rats. The venom induced behavioral changes quantified using a neuroethological method, which allows the evaluation of the following parameters: frequency, duration, and strength of statistical association (chi-square) between pairs of behaviors. The rats exhibited a period of freezing, which was always followed by procursive-type seizures (running, gyrating, atonic falling, and jumping).

Molecular basis for the polyamine-ompF porin interactions: inhibitor and mutant studies

By testing the sensitivity of Escherichia coli OmpF porin to various natural and synthetic polyamines of different lengths, charge and other molecular characteristics, we were able to identify the molecular properties required for compounds to act as inhibitors of OmpF in the nanomolar range. Inhibitors require at least two amine groups to be effective. For diamines, the optimum length of the hydrocarbon spacer was found to be of eight to ten methylene groups. Triamine molecules based on a 12-carbon motif were found to be more effective that spermidine, an eight-carbon trivalent derivative. But differences in inhibition efficiencies were also found for trivalent compounds depending on the relative position of the internal secondary amine group with respect to the terminal groups. Finally, quaternary ammonium derivatives had no effect, suggesting that the nature of the terminal amine is important for the interaction. From these observations, we deduce that inhibition efficiency in the nanomolar range requires a 12-carbon chain triamine with terminal primary amine groups and replacement of the eighth methylene by a secondary amine. The need for this type of molecular architecture suggests that inhibition is governed by interactions between specific amine groups and protein residues, and that this is not simply due to the accumulation of charges into the pore. Together with previous observations from site-directed mutagenesis studies and inspection of the crystal structure of OmpF, these results allowed us to propose three residues (D113, D121 and Y294) as putative sites of interaction between the channel and spermine. Alanine substitution at each of these three residues resulted in a loss of inhibition by spermine, while mutations of only D113 and D121 affected inhibition by spermidine. Based on these observations, we suggest a model for the molecular determinants involved in the porin-polyamine interaction.

Structure-activity relationship and site of binding of polyamine derivatives at the nicotinic acetylcholine receptor

Several wasp venoms contain philanthotoxins (PhTXs), natural polyamine amides, which act as noncompetitive inhibitors (NCIs) on the nicotinic acetylcholine receptor (nAChR). Effects of varying the structure of PhTXs and poly(methylene tetramine)s on the binding affinity have been investigated. Using the fluorescent NCI ethidium in a displacement assay Kapp values of these compounds have been determined. We found that an increase in size of the PhTX's hydrophobic head group significantly increased the binding affinity, while inserting positive charge almost completely destroyed it. Elongating the PhTX polyamine chain by introducing an additional aminomethylene group decreased the binding affinity, whereas a terminal lysine improved it. In general, poly(methylene tetramine)s showed higher binding affinities than PhTX analogues. The stoichiometry of PhTX binding was determined to be two PhTX molecules per receptor monomer. PhTXs appeared to bind to a single class of nonallosterically interacting binding sites and bound PhTX was found to be completely displaced by well-characterized luminal NCIs. To elucidate the site of PhTX binding, a photolabile, radioactive PhTX derivative was photocross-linked to the nAChR in its closed channel conformation resulting in labeling yields for the two alpha and the beta, gamma and delta subunits of 10.4, 11.1, 4.0 and 7.4%, respectively. Based on these findings we suggest that PhTXs and poly(methylene tetramine)s enter the receptor's ionic channel from the extracellular side. The hydrophobic head groups most likely bind to the high-affinity NCI site, while the positively charged polyamine chains presumably interact with the negatively charged selectivity filter located deep in the channel lumen.

Effects of dietary L-arginine on the reactivity of canine coronary arteries

Experiments were designed to determine the effects of supplemental dietary L-arginine on the endothelial and smooth muscle function of canine coronary arteries. One group of dogs was fed the standard laboratory chow while another group was supplemented with 250 mg/kg per day L-arginine. All dogs had undergone bilateral reversed interposition saphenous vein grafting and received 325 mg/day oral aspirin. After 5 weeks of arginine feeding, left circumflex coronary arteries were removed, cut into rings, and suspended for the measurement of isometric force in organ chambers. Concentration-response curves were obtained to L-arginine, UK-14,304 (alpha2-adrenergic agonist) and A23187 (calcium ionophore) in the absence and presence of N(G)-monomethyl-L-arginine (L-NMMA) and tetraethylammonium (TEA) alone or in combination. Serum concentrations of L-arginine increased by about 20% following 2 weeks of arginine feeding and remained elevated throughout the study. In rings with and without endothelium contracted with prostaglandin F2alpha, L-arginine caused concentration-dependent contractions in rings from control animals but no significant change in tension in rings from arginine-fed animals. Contractions to L-arginine in control animals were reduced by either L-NMMA or TEA. Endothelium-dependent relaxations to the alpha2-adrenergic agonist were decreased with arginine feeding while relaxations to the calcium ionophore and the endothelium-derived factor nitric oxide were similar among groups. Relaxations to UK-14,304 were reduced by L-NMMA in both groups but by TEA only in rings from control animals. These results suggest that dietary supplementation with L-arginine modifies reactivity of endothelium and smooth muscle by at least two mechanisms: one associated with activation of potassium channels and the other with receptor-coupled release of nitric oxide.

Analogues of neuroactive polyamine wasp toxins that lack inner basic sites exhibit enhanced antagonism toward a muscle-type mammalian nicotinic acetylcholine receptor

Philanthotoxin-433 (PhTX-433), a natural polyamine wasp toxin, is a noncompetitive antagonist of certain ionotropic receptors. Six analogues of PhTX-343 (a synthetic analogue of the natural product), in which the secondary amino groups are systematically replaced by oxygen or methylene groups, have been synthesized by coupling of N-(1-oxobutyl)tyrosine with 1,12-dodecanediamine, 4,9-dioxa-1, 12-dodecanediamine, or appropriately protected di- and triamines, the latter being obtained by multistep syntheses. The resulting PhTX-343 analogues were purified and characterized, and their protolytic properties (stepwise macroscopic pK(a) values) were determined by (13)C NMR titrations. All analogues are fully protonated at physiological pH. The effects of these compounds on acetylcholine-induced currents in TE671 cells clamped at various holding potentials were determined. All of the analogues noncompetitively antagonized the nicotinic acetylcholine receptor (nAChR) in a concentration-, time-, and voltage-dependent manner. The amplitudes of acetylcholine-induced currents were compared at their peaks and at the end of a 1 s application in the presence or absence of the analogues. Most of the analogues were equipotent with or more potent than PhTX-343. The dideaza analogue PhTX-12 [IC(50) of 0.3 microM (final current value)] was the most potent, representing the highest potency improvement (about 50-fold) yet achieved by modification of the parent compound (PhTX-343). Thus, the presence of multiple positive charges in the PhTX-343 molecule is not necessary for antagonism of nAChR. In contrast, the compounds were much less potent than PhTX-343 at locust muscle ionotropic glutamate receptors sensitive to quisqualate (qGluR). The results demonstrate that the selectivity for different types of ionotropic receptors can be achieved by manipulating the polyamine moiety of PhTX-343.