Potassium channel toxins and transmitter release

Effects of Tityus stigmurus (Thorell 1876) (Scorpiones: Buthidae) venom in isolated perfused rat kidneys

ABSTRACT Scorpions belonging to the Tityus genus are of medical interest in Brazil. Among them, Tityus stigmurus is the main scorpion responsible for stings in the Northeast region. After a sting, the scorpion venom distributes rapidly to the organs, reaching the kidneys quickly. However, there are few studies concerning the renal pathophysiology of scorpion poisoning. In this study, we evaluated the effects of T. stigmurus venom (TsV) on renal parameters in isolated rat kidneys. Wistar rats (n = 6), weighing 250-300 g, were perfused with Krebs-Henseleit solution containing 6 g/100 mL bovine serum albumin. TsV at 0.3 and 1.0 μg/mL was tested, and the effects on perfusion pressure (PP), renal vascular resistance (RVR), urinary flow (UF), glomerular filtration rate (GFR), and electrolyte excretion were analyzed. Effects were observed only at TsV concentration of 1.0 μg/mL, which increased PP (controlPP40' = 92.7 ± 1.95; TsVPP40' = 182.0 ± 4.70* mmHg, *p < 0.05), RVR (controlRVR40' = 3.28 ± 0.23 mmHg; TstRVR40' = 6.76 ± 0.45* mmHg, *p < 0.05), UF (controlUF50' = 0.16 ± 0.04; TstUF50' = 0.60 ± 0.10* mL/g/min,*p < 0.05), GFR and electrolyte excretion, with histological changes that indicate renal tubular injury. In conclusion, T. stigmurus venom induces a transient increase in PP with tubular injury, both of which lead to an augmented electrolyte excretion.

Biochemistry of envenomation

Venoms and toxins are of significant interest due to their ability to cause a wide range of pathophysiological conditions that can potentially result in death. Despite their wide distribution among plants and animals, the biochemical pathways associated with these pathogenic agents remain largely unexplored. Impoverished and underdeveloped regions appear especially susceptible to increased incidence and severity due to poor socioeconomic conditions and lack of appropriate medical treatment infrastructure. To facilitate better management and treatment of envenomation victims, it is essential that the biochemical mechanisms of their action be elucidated. This review aims to characterize downstream envenomation mechanisms by addressing the major neuro-, cardio-, and hemotoxins as well as ion-channel toxins. Because of their use in folk and traditional medicine, the biochemistry behind venom therapy and possible implications on conventional medicine will also be addressed.

Comparative study between the protective effects of Saudi and Egyptian antivenoms, alone or in combination with ion channel modulators, against deleterious actions of Leiurus quinquestriatus scorpion venom

This study compared efficacy of two polyvalent antivenoms (Saudi Arabian and Egyptian), against lethality and pathophysiological changes of Leiurus quinquestriatus quinquestriatus (LQQ) scorpion venom in mice. Additionally, the study examined whether treatment with selected ion channel modulators, lidocaine, nimodipine or amiodarone would be effective, alone or combined with the antivenoms. The protein concentration of the Saudi antivenom was 1/3 of Egyptian, indicating lesser immunogenicity, while both preservative contents were within limits. In immunodiffusion experiments, both exhibited prominent precipitin bands indicating high concentrations of specific antibodies. Neutralizing capacities (60-70 LD(50)) stated on labels were confirmed. Both antivenoms significantly (P < 0.001) prolonged survival time (from 26.9 +/- 1.18 min, 100% dead with venom to 224-300 min, 0-30% dead) of envenomed mice, whether injected iv before or 5 min after venom. Injection of either antivenom plus ion channel modulators, gave comparable results to that observed in mice treated with antivenoms alone. The Na(+) channel blocker lidocaine and the Ca(2+) channel blocker nimodipine on their own significantly protected the animals (P < 0.05), but to a lesser extent. The two antivenoms, significantly ameliorated the venom-evoked changes in serum LDH (P < 0.001) and CKMB (P < 0.01) plus cardiac TNFalpha and nitrate/nitrite levels (P < 0.001). When combined with lidocaine or nimodipine, the effects were not greater than antivenom alone. Moreover, the antivenoms ameliorated characteristic venom-evoked changes in the isolated perfused Langendorff hearts. Lidocaine and amiodarone were more effective than nimodipine. In Conclusion both Saudi and Egyptian antivenoms protected mice from the pathological and lethal effects of LQQ scorpion. Sodium and calcium channel blockers, lidocaine and nimodipine, may be useful when antivenoms are not available.

Sea anemone toxins affecting potassium channels

The great diversity of K(+) channels and their wide distribution in many tissues are associated with important functions in cardiac and neuronal excitability that are now better understood thanks to the discovery of animal toxins. During the past few decades, sea anemones have provided a variety of toxins acting on voltage-sensitive sodium and, more recently, potassium channels. Currently there are three major structural groups of sea anemone K(+) channel (SAK) toxins that have been characterized. Radioligand binding and electrophysiological experiments revealed that each group contains peptides displaying selective activities for different subfamilies of K(+) channels. Short (35-37 amino acids) peptides in the group I display pore blocking effects on Kv1 channels. Molecular interactions of SAK-I toxins, important for activity and binding on Kv1 channels, implicate a spot of three conserved amino acid residues (Ser, Lys, Tyr) surrounded by other less conserved residues. Long (58-59 amino acids) SAK-II peptides display both enzymatic and K(+) channel inhibitory activities. Medium size (42-43 amino acid) SAK-III peptides are gating modifiers which interact either with cardiac HERG or Kv3 channels by altering their voltage-dependent properties. SAK-III toxins bind to the S3C region in the outer vestibule of Kv channels. Sea anemones have proven to be a rich source of pharmacological tools, and some of the SAK toxins are now useful drugs for the diagnosis and treatment of autoimmune diseases.

A new sea anemone peptide, APETx2, inhibits ASIC3, a major acid-sensitive channel in sensory neurons

From a systematic screening of animal venoms, we isolated a new toxin (APETx2) from the sea anemone Anthopleura elegantissima, which inhibits ASIC3 homomeric channels and ASIC3-containing heteromeric channels both in heterologous expression systems and in primary cultures of rat sensory neurons. APETx2 is a 42 amino-acid peptide crosslinked by three disulfide bridges, with a structural organization similar to that of other sea anemone toxins that inhibit voltage-sensitive Na+ and K+ channels. APETx2 reversibly inhibits rat ASIC3 (IC50=63 nM), without any effect on ASIC1a, ASIC1b, and ASIC2a. APETx2 directly inhibits the ASIC3 channel by acting at its external side, and it does not modify the channel unitary conductance. APETx2 also inhibits heteromeric ASIC2b+3 current (IC50=117 nM), while it has less affinity for ASIC1b+3 (IC50=0.9 microM), ASIC1a+3 (IC50=2 microM), and no effect on the ASIC2a+3 current. The ASIC3-like current in primary cultured sensory neurons is partly and reversibly inhibited by APETx2 with an IC50 of 216 nM, probably due to the mixed inhibitions of various co-expressed ASIC3-containing channels.

The effects of lignocaine on actions of the venom from the yellow scorpion "Leiurus quinquestriatus" in vivo and in vitro

Many toxins from scorpion venoms activate sodium channels, thereby enhancing neurotransmitter release. The aim of the present work was to determine if the in vivo and in vitro effects of Leiurus quinquestriatus venom (LQQ) could be ameliorated by lignocaine, a sodium channel blocker. In urethane anaesthetised rabbits, LQQ venom (0.5 mg kg(-1), i.v.) caused initial hypotension and bradycardia followed by hypertension, pulmonary oedema, electrocardiographic changes indicating conduction defects, ischaemia, infarction, and then hypotension and death. Lignocaine (1 mg kg(-1) i.v. bolus initially, followed by i.v. infusion of 50 microg kg(-1) min(-1)) significantly attenuated the majority of the venom-evoked effects and reduced mortality. Addition of LQQ venom (1, 3 and 10 microg ml(-1)) to chick biventer cervicis, guinea pig ileum, and rat vas deferens preparations, increased the height of electrically-induced twitches, elevated resting tension, and caused autorhythmic oscillations. Lignocaine (3 x 10(-4)-1.2 x 10(-3) M) greatly attenuated these venom-evoked actions in the three preparations. Antagonists of appropriate neurotransmitters were also tested to determine the contribution of released transmitters to LQQ effects. Atropine significantly decreased the venom-elicited effects on guinea pig ileum preparations, while prazosin and guanethidine significantly reduced the venom's actions on rat vas deferens. In chick biventer cervicis preparations, tubocurarine and hexamethonium significantly attenuated the venom-induced effects. This study supports the hypothesis that many effects of LQQ venom involve the release of neurotransmitters and may be ameliorated by treatment with lignocaine.

Beta-agkistrodotoxin inhibits fast and Ca2+-activated K+ currents recorded from mouse motor nerve terminals

Beta-agkistrodotoxin (beta-AgTx), a polypeptide purified from the venom of Agkistrodon blomhoffii brevicaudus, is a presynaptic blocker acting on neurotransmitter release. In this work, perineural recording technique was employed to study the effects of beta-AgTx on sodium, potassium and calcium currents of mouse motor nerve terminals. The results showed that beta-AgTx selectively inhibited Ca2+-dependent (I(K,Ca)) and fast (I(K,f) K+ currents, but did not affect slow K+ current (I(K,s)), sodium and calcium currents. However there are other components in A. blomhoffii brevicaudus venom which inhibit perineural sodium current. The present data have provided additional evidence that the site of action of beta-AgTx is different from that of beta-bungarotoxin.

Outwardly rectifying currents in guinea pig outer hair cells

Studies of K+ conductances in hair cells report that big-conductance Ca(2+)-dependent K+ (BK) channels carry parts of the outwardly rectifying currents. Lin et al. (1995) suggested that in guinea pig outer hair cells (OHCs) a portion of these currents is carried via a voltage-dependent and Ca(2+)-independent K+ channel. The present study tests the hypothesis that there are two separable current components of the outwardly rectifying currents by using patch clamp methods in OHCs to characterize the voltage dependence and sensitivity of the outwardly rectifying currents to channel blockers. Lowering of external Ca2+ caused no change in the currents while charybdotoxin (ChTx; 100 nM), a BK K+ channel blocker, and Cd2+ (200 microM), and L-type calcium channel blocker, abolished about 50% of the currents. Both ChTx and Cd2+ caused a depolarizing shift in the half-activation voltage paralleled by a decrease in the voltage sensitivity. 4-Aminopyridine (4-AP, 0.01 mM), an A-type and delayed rectifier type channel blocker, abolished about 50% of the currents and caused a hyperpolarizing shift in the half-activation voltage together with an increase in the voltage sensitivity. The outwardly rectifying currents were more sensitive to block by 4-AP at membrane voltages around 40 mV compared to voltages around -20 mV. The differences in the current characteristics may be due to two separate channel types, one of which is similar to the delayed rectifier type channels while the other may be similar to the BK Ca(2+)-dependent K+ channels. In addition, the largest outwardly rectifying currents were present in long OHCs with the smallest present in short OHCs.

Apamin blocks the direct relaxant effect of melatonin on rat ileal smooth muscle

The present study investigated the mechanisms of melatonin-induced inhibition of the ileal smooth muscle contraction. Rat isolated ileal smooth muscle strips were stimulated in an organ bath using carbachol (CAR) or potassium chloride (KCl) depolarization. Under these conditions, melatonin produced a concentration-dependent inhibition of muscle contraction (mean inhibitory concentration, IC50: 17.3 x 10(-6) M), which was not blocked by either tetrodotoxin (10(-6) M), hexamethonium (10(-4) M), or phentolamine (10(-6) M). The inhibitory effect of melatonin during CAR stimulation was blocked in a concentration-dependent manner by the presence of apamin (4.8 x 10(-9) M), a K(+)-channel blocker. By contrast, other K(+)-channel blockers such as 4-aminopyridine (10(-4) M to 5 x 10(-3) M), tetraethylammonium (10(-4) to 10(-1) M), and glibenclamide (10(-5) M) were ineffective. Additionally, the Ca(2+)-channel antagonists nitrendipine (IC50: 2.4 x 10(-9) M) and verapamil (IC50: 1.1 x 10(-7) M) also blocked the inhibitory action of melatonin. These results suggest that melatonin may interact with an apamin-sensitive, possibly Ca(2+)-activated, K+ channel and thus cause an inhibition of ileal smooth muscle contractions.

Acetylcholine response in guinea pig outer hair cells. II. Activation of a small conductance Ca(2+)-activated K+ channel

The type of K+ channel involved in the acetylcholine (ACh) evoked response (Ksub; sub stands for suberyldicholine) in guinea pig outer hair cells (OHCs) is still uncertain. The present study tests the hypotheses that Ksub is one of the following: a big conductance Ca(2+)-dependent K+ channel (BK), a small conductance Ca(2+)-dependent K+ channel (SK), a KA type of K+ channel, or a Kn type of K+ channel. Patch-clamp technique in the whole-cell mode was used to record from single guinea pig OHCs. ACh (100 microM) was applied to voltage-clamped OHCs and the ACh-induced currents (IACh) were measured. Charybdotoxin (100 nM) had no effect on IACh, while apamin (1 microM) blocked more than 90% of IACh. Lowering the external Ca2+ concentration caused a hyperpolarizing shift of the IACh monitored as a function of the prepulse voltage. Increasing internal Mg2+ (Mgi2+) concentration caused a reduction in the outward IACh without affecting the inward IACh. The Ksub channel was found to be permeable to Cs+. In Cs+ solutions, IACh was 45% of the IACh in K+ solutions. The block of IACh by apamin, the dependence on extracellular Ca2+, the incomplete block of IACh by Cs+, and the ACh-induced Cs+ currents favor the hypothesis that Ksub belongs to the SK type of channels. An ionotropic/nicotinic nature of the ACh mechanism of action is favored. It is suggested that, in vivo, the amplitude of the ACh-induced hyperpolarization may depend on the Ca2+/Mg2+ ratio inside and outside the cell.

Effects on behaviour and EEG of single chain phospholipases A2 from snake and bee venoms injected into rat brain: search for a functional antagonism

Three phospholipase A2 (PLA2s), OS1 and OS1 purified from the taipan snake venom Oxyuranus scutellatus scutellatus and bee venom PLA2 were injected to rats by the intracerebroventricular route. OS1 showed no sign of neurotoxicity at doses at which OS2 and bee venom PLA2 produced multiform dose-dependent behavioural effects including motor disturbances (stereotyped movements), compulsive scratching, convulsions and breathing difficulties. EEG recordings showed at the very time when the animal was motionless the induction of several episodes of a low frequency hippocampal theta rhythm, index of long-term changes in synaptic neuroplasticity. Spike-wave discharges were also produced but the occurrence was not systematic. These seizures were often accompanied with behavioural convulsions. Blockers of NMDA receptors and drugs modifying the GABAergic transmission could not abolish the neurotoxic effects of PLA2s except for diazepam (10 mg/kg intraperitoneally) that prevented only OS2-induced disturbances. Blockers of L-type Ca2+ channels and K+ channel openers were also without effect. The toxicity of OS2 and bee venom PLA2 is probably due to their initial specific binding to their neuronal receptor sites.

Acquired neuromyotonia: evidence for autoantibodies directed against K+ channels of peripheral nerves

Acquired neuromyotonia is characterized by hyperexcitability of motor nerves leading to muscle twitching, cramps, and weakness. The symptoms may improve following plasma exchange, and injection of immunoglobulin G (IgG) from 1 neuromyotonia patient into mice increased the resistance of neuromuscular transmission to d-tubocurarine. Here we examine nerves and muscle in vitro from mice injected with plasma or purified IgG from 6 neuromyotonia patients or pooled control subjects, and cultured dorsal root ganglion cells after treatment with IgG. Three of the patients had antibodies against human voltage-gated potassium channels labeled with 125I-alpha-dendrotoxin. The quantal release of acetylcholine (quantal content) at end-plates in diaphragms from mice treated with neuromyotonia IgG preparations was increased by 21% relative to control values (p = 0.0053). With one IgG preparation, the duration of the superficial peroneal nerve compound action currents was increased by 93%. The dorsal root ganglion cells treated with this IgG showed a marked increase in repetitive firing of action potentials. All effects were similar to those obtained with aminopyridines. We conclude that at least some patients with acquired neuromyotonia have antibodies directed against aminopyridine- or alpha-dendrotoxin-sensitive K+ channels in motor and sensory neurons, and they are likely to be implicated in the disease process.

Structure of beta 2-bungarotoxin: potassium channel binding by Kunitz modules and targeted phospholipase action

BACKGROUND

beta-bungarotoxin is a heterodimeric neurotoxin consisting of a phospholipase subunit linked by a disulfide bond to a K+ channel binding subunit which is a member of the Kunitz protease inhibitor superfamily. Toxicity, characterized by blockage of neural transmission, is achieved by the lipolytic action of the phospholipase targeted to the presynaptic membrane by the Kunitz module.

RESULTS

The crystal structure at 2.45 A resolution suggests that the ion channel binding region of the Kunitz subunit is at the opposite end of the module from the loop typically involved in protease binding. Analysis of the phospholipase subunit reveals a partially occluded substrate-binding surface and reduced hydrophobicity.

CONCLUSIONS

Molecular recognition by this Kunitz module appears to diverge considerably from more conventional superfamily members. The ion channel binding region identified here may mimic the regulatory interaction of endogenous neuropeptides. Adaptations of the phospholipase subunit make it uniquely suited to targeting and explain the remarkable ability of the toxin to avoid binding to non-target membranes. Insight into the mechanism of beta-bungarotoxin gained here may lead to the development of therapeutic strategies against not only pathological cells, but also enveloped viruses.