Two things return the membrane potential to its resting level. The sodium channels do not remain open forever at positive membrane potentials, but eventually close, a process known as inactivation. Secondly, the potassium channels open, so that potassium ions rush out of the cell, restoring the charge imbalance and sending the potential negative once more. It’s just as well that the potassium channels open later than the sodium channels because if they opened at the same time the sodium and potassium currents would cancel each other out and there would be no nerve impulse, and no thoughts or actions.
Terrible Stuff
The importance of sodium and potassium channels in generating the nerve impulse is demonstrated by the fact that a vast array of poisons from spiders, shellfish, sea anemones, frogs, snakes, scorpions and many other exotic creatures interact with these channels and thereby modify the function of nerve and muscle. Many are highly specific and target a single kind of ion channel. Which brings us back to Captain Cook and the puffer fish.
The tetrodotoxin contained in the liver and other tissues of this fish is a potent blocker of the sodium channels found in your nerves and skeletal muscles. It causes numbness and tingling of the lips and mouth within as little as thirty minutes after ingestion. This sensation of ‘pins and needles’ spreads rapidly to the face and neck, moves on to the fingers and toes, and is then followed by gradual paralysis of the skeletal muscles, resulting in loss of balance, incoherent speech, and an inability to move one’s limbs. Ultimately, the respiratory muscles are paralysed, which can be fatal. The heart is not affected, as it has a different type of sodium channel that is not targeted by tetrodotoxin. The toxin is also unable to cross the blood–brain barrier so that, rather horrifyingly, although unable to move and near death, the patient remains conscious. There is no antidote and death usually occurs within two to twenty-four hours. In 1845, the surgeon on board the Dutch brig Postilion, sailing off the Cape of Good Hope, observed that two seamen ‘died scarcely seventeen minutes after partaking of the liver of the fish’. However, victims can recover completely if they are given artificial respiratory support until the toxin has washed out of the body – which takes a few days.
Hiroshige’s ‘Amberjack and Fugu’. The puffer fish (fugu) is the smaller fish.
In Japan, the puffer fish is known as fugu, and is considered a great delicacy. Unfortunately, the fish is expensive in more ways than one, as unless it is carefully prepared the flesh can be toxic, and every year several people die from tetrodotoxin poisoning. Most incidents arise from fishermen eating their own catch. Restaurant casualties are far rarer because all fugu chefs must now be specially trained and licensed, which involves passing a rigorous test. Nevertheless, it occasionally happens. One of fugu’s most celebrated victims was the famous Japanese kabuki actor Bando Mitsugoro who died after eating it in 1975: he had demanded four servings of the liver, which is especially dangerous, and the restaurant had felt unable to refuse such a distinguished customer. Perhaps this is why fugu is forbidden to the Emperor of Japan. Properly prepared, the fish is supposed to cause a very mild intoxication and produce a stimulating, tingling sensation in the mouth. On the single occasion when I tried it myself, I found it rather insipid: it was the spice of danger that enlivened the dish.
Not all cases of fugu poisoning are due to the deliberate ingestion of the fish. In 1977, three people died in Italy after eating imported puffer fish mislabelled as anglerfish. Ten years later, two people in Illinois developed symptoms resembling those of tetrodotoxin poisoning after eating soup made from imported frozen ‘monkfish’. Analysis by the FDA confirmed the presence of the toxin and triggered a mass recall of all sixty-four crates of the imported product. Claims lawyers instantly leapt into action. Poisoning from commercially cooked shellfish is also worryingly common in China and Taiwan: between 1997 and 2001 three hundred people were intoxicated and sixteen died.
A wide variety of animals contain tetrodotoxin, from reef fish, crabs and starfish to marine flatworms, salamanders, frogs and toads. Most use it as a biological defence, but some, like the deadly blue-ringed octopus, package it in venom to poison their prey. It was a mystery why so many different kinds of animal should make tetrodotoxin until it was discovered that it is actually made by a bacterium (Psuedoalteromonas tetraodonia) that the animal eats or harbours within its intestine. Puffer fish reared in the absence of such bacteria do not contain tetrodotoxin. Whether such fish, in which the element of Russian roulette is removed, will be as highly prized by aficionados as the native fish is an interesting question.
The fictional British agent James Bond (007) appears to have a special attraction for tetrodotoxin, for he has been poisoned with it on no less than two occasions. From Russia with Love ends on a moment of high drama when the SMERF agent Rosa Klebb kicks him with a poison-filled spike mounted on the tip of her boot and he is left to die. Bond, of course, is invincible and the next novel (Dr No) begins with him recovering from what we learn is a near fatal dose of tetrodotoxin – ‘terrible stuff and very quick’. He survives only because his companion administered artificial respiration until medical help arrived. Bond also has an encounter with a blue-ringed octopus, whose bite is laced with tetrodotoxin, in the film Octopussy. As ever, 007 emerges from these incidents shaken but not stirred.
Red Tides and Suicide Potions
When conditions are right, spectacular blooms of the algae Alexandrium can occur that turn the sea the colour of blood. The deleterious effects of such red tides have been known for centuries and the biblical account of one of the great plagues of Egypt paints a vivid picture: ‘all the waters that were in the river were turned to blood. And the fish that was in the river died; and the river stank and the Egyptians could not drink of the water of the river.’ Such red tides are composed of millions of minute algae, known as dinoflagellates, which produce a number of virulent neurotoxins, including saxitoxin. Like tetrodotoxin, saxitoxin blocks sodium channels. Filter-feeding molluscs like mussels and clams may ingest the dinoflagellates, thereby concentrating the toxins they produce, and creatures that in turn feed on them may be poisoned. As much as 20,000 micrograms of saxitoxin per 100 grams of tissue (250 times the legally allowed limit) has been recorded in Alaskan mussels: at this level, consumption of a single mussel can kill you. Even more frighteningly, a single green shawl crab from the Great Barrier Reef can contain enough toxin to kill 3,000 people. Dinoflagellates are most abundant in spring and summer, due to the higher sunlight levels and warmer waters, which may be the origin of the old adage ‘Do not eat shellfish unless there is an R in the month’.
In developed countries shellfish poisoning is very rare, due to intensive surveillance programmes and stringent regulations which ensure that, once detected, the affected areas are quarantined and shellfish sales prohibited. In the last decade, seasonal outbreaks of paralytic shellfish poisoning (mainly due to saxitoxins) have led to temporary bans on the sale of shellfish from waters around the world. The Alaskan shellfish industry has been radically affected as the butter clam is toxic for large parts of the year. But while commercial seafood is safe, this is not necessarily the case for shellfish that people collect themselves. Between 1973 and 1992 there were 117 cases of paralytic shellfish poisoning in Alaska, 75 per cent of them between May and July. Fortunately only one person died, but many required hospitalization. The most dramatic outbreak in recent years happened in 1987 in Guatemala, when 187 people were affected by eating clams and 26 of them died.
Tetrodotoxin and saxitoxin are molecular mimics. Each physically plugs the external mouth of the sodium channel pore, is almost equally potent at inhibiting channel function, and produces similar physiological responses. Both are also valuable research tools because they block sodium channels rather specifically, leaving most other channels untouched. Tetrodotoxin is routinely used in scientific studies today to block sodium channels and enable other channels to be studied in isolation. Its action was discovered by Toshio Narahashi in 1962, working round t
he clock throughout Christmas and New Year with John Moore and William Scott. Narahashi recalls that the reviewer of their manuscript jotted down a request for some of the toxin at the bottom of his report. It was to be the first of many such requests.
By now you might be wondering why butter clams are not affected by the saxitoxin they contain and why puffer fish swim happily around, despite high tetrodotoxin levels. The answer is that the affinity of their own sodium channels for the toxin is dramatically reduced because evolution has changed one or more of the amino acids in the toxin-binding site. A similar mutation is found in the cardiac sodium channel, which helps explain why your heart continues to beat even when your respiratory muscles are totally paralysed.
Saxitoxin was exploited by US agents engaged in covert government operations both as a suicide and an assassination agent. It has the advantage that it is highly poisonous so that only tiny amounts (which can be easily concealed) are needed, and it is faster and more effective than cyanide. Because it is stable, water soluble and about a thousand times more toxic than synthetic nerve gases such as sarin, saxitoxin (known as agent SS or TZ) was also stockpiled by the US government as a chemical weapon. It was extracted from thousands of butter clams laboriously collected by hand in Alaska. In 1969/70, President Nixon halted the US biological weapons programme and ordered existing stocks to be destroyed, in accordance with a United Nations agreement. But five years later, Senator Frank Church, Chair of a Select Committee on Intelligence investigating the CIA, discovered that a middle-level official had failed to do so. About 10 grams of the toxin, enough to kill several thousand people, still remained in downtown Washington in direct violation of the presidential order. It had been packed into two one-gallon cans and stored in a small freezer under a workbench, which must have caused some consternation to its discoverer.
This information interested Murdoch Ritchie, of Yale University School of Medicine, as he realized that the toxin could be of considerable value to scientists studying how nerves work. He immediately wrote to Church requesting that it should not be incinerated. To his surprise, the CIA offered Ritchie the entire supply, with the proviso that he organize its distribution to the scientific community. Ritchie quickly realized that safeguarding the stockpile would be an enormous responsibility. Moreover, as the supply was limited and the demand was likely to be considerable, he might ‘be forced to ration it, or even deny some applications, and would surely make enemies’. Wisely, Ritchie recommended it be given to the National Institutes of Health for distribution. The outcome was a happy bonus for ion channel research.
Saxitoxin has always been hard to obtain because of its colourful history, and the first laboratory synthesis of saxitoxin (in 1977) led to even more stringent controls. It is now listed in Schedule 1 of the Chemical Weapons Convention. In contrast, for many years tetrodotoxin could be routinely ordered from suppliers of laboratory chemicals. Since 11 September 2001, however, things have tightened up worldwide. Researchers can only hold tiny amounts of the toxin, and all stocks must be registered. They are also carefully monitored, as I discovered recently myself when we received an unsolicited visit from the anti-terrorist branch of the British police to check on our tetrodotoxin supplies.
The Queen of Poisons
Not all sodium channel toxins work by blocking flux through the pore. Some produce equally devastating effects by locking the channels open, resulting in overstimulation of nerve and muscle fibres. One of the most potent is aconite, which has been used as a murder weapon for centuries. A recent victim was Lakhvinder Cheema, who came home, took some leftover vegetarian curry out of the fridge and heated it up for himself and his fiancée, Gurjeet. They sat down to eat their dinner, chatting happily about their forthcoming wedding. But not for long. Ten minutes or so later, Lakhvinder found his face becoming numb and very quickly both he and Gurjeet started to go blind, became dizzy, and lost control of their arms and legs. They called for an ambulance, but Lakhvinder died within the hour and Gurjeet was left fighting for her life. She survived only because she had eaten less curry. The dish had been laced with aconite by Lakhvinder’s jealous ex-mistress Lakhvir Singh, who had slipped into his flat when he was out.
Aconite, or more correctly aconitine, is colloquially called the Queen of Poisons and it comes from monkshood (wolfsbane), a pretty plant with a tall spike of blue helmet-shaped flowers often grown in gardens. In Greek mythology it is said to have sprung from the saliva that dripped from the ravening jaws of Cerberus, the three-headed dog that guards the gates of Hell. Its poisonous properties have intrigued writers for centuries and it features in numerous literary works, including Oscar Wilde’s story ‘Lord Arthur Savile’s Crime’ and James Joyce’s novel Ulysses, in which Rudolph Bloom dies as a ‘consequence of an overdose of monkshood (aconite) self-administered in the form of a neuralgic liniment’. Similarly, Ovid relates how Medea tried to kill Theseus by lacing his wine with aconite. People have also died from accidental ingestion of aconite, one of the best-known victims being the Canadian actor Andre Noble. Because the toxin is absorbed through the skin, even picking the plant without wearing gloves may cause symptoms. As Keats advises, one should not ‘twist Wolfs-bane, tight-rooted, for its poisonous wine’.
Another potent sodium channel opener is batrachotoxin, which is secreted from glands on the backs of the vividly marked yellow and black ‘poison dart’ frogs of South and Central America. It is collected by the Chocó Amerindians, who use it to tip their blow darts. Like aconitine, there is no known antidote. Batrachotoxin is not made by the frog itself but instead acquired from the beetles on which they feast – although whether the beetles make the poison themselves, or, in turn, get it from something they eat is still uncertain. Poison dart frogs are not the only creatures to steal beetle batrachotoxin to use as a defence. The New Guinea hooded pitohui, resplendent in showy red and black plumage, does so too. Its feathers and skin are laced with batrachotoxin as the biologist John Dumbacher discovered to his cost when he sucked his fingers after being scratched by a bird he was trying to free from a net: both his fingers and lips quickly developed a tingling sensation and then went temporarily numb. His local guides informed him that pitohui were ‘rubbish birds’ and well known to be poisonous.
Equally fascinating is grayanotoxin, which also locks sodium channels open. It is produced by some species of rhododendron and becomes concentrated in the honey of bees that feed on the flowers’ nectar. Consumption of affected honey causes ‘mad honey syndrome’ and has been poisoning us for centuries. One of the earliest accounts is by Xenophon, who relates that during an expedition near Trabzon (on the Black Sea) in 401 BC, ‘The number of bee hives was extraordinary, and all the soldiers that ate of the honey combs, lost their senses, vomited, and were affected with purging, and none of them were able to stand upright; such as had eaten only a little were like men greatly intoxicated, and such as had eaten much were like mad-men, and some like persons at the point of death. They lay upon the ground, in consequence, in great numbers, as if there had been a defeat; and there was general dejection.’ Something of an understatement, one imagines. But no one died and by the next day all had recovered their senses. The symptoms suggested to military commanders in classical times that mad honey might prove an effective biological weapon, if it were strewn in the path of the opposition, and in 67 BC three Roman cohorts (about 1,440 soldiers) under the command of General Pompey were slaughtered by enemy forces while incapacitated from consuming mad honey. Today, mad honey poisoning rarely occurs, for only certain varieties of plants make the toxin and commercial honey is blended so that any toxin present is diluted out. A few cases occasionally occur in the Black Sea region of Turkey where, rather improbably, mad honey is thought to enhance sexual performance. Fortunately, it is usually not fatal.
Sodium channel toxins can make valuable insecticides if they can be targeted to ion channels found specifically in insect nerves. One of the most famous is dichlorodiphenyltrichloroethane, otherwise known as DD
T. It opens sodium channels in insect nerves, but not those in mammalian nerves, which are genetically and structurally distinct. Activation of the sodium channels causes insect axons to fire impulses spontaneously, which leads to muscle spasms and eventually to death. DDT played an important part in controlling the spread of typhus and malaria during and after World War II, but its efficacy gradually diminished as insects became resistant to its effects. This was because strong evolutionary pressure led to genetic mutations that altered the binding site for DDT on the sodium channel (thus preventing its action) becoming widespread. Use of DDT also became increasingly controversial after the publication of Rachel Carson’s book Silent Spring, which linked DDT and other pesticides to a marked decline in songbird numbers in the United States. Although DDT does not open avian or mammalian sodium channels, it has other effects: in birds, for example, it causes thinning of the eggshell, leading to breakage and fewer hatchlings.
Sodium Rules
It is evident from this string of cautionary tales and grisly stories that Nature has evolved a vast number of toxins that target sodium channels – far more than interact with other types of ion channel. One reason for this may be that sodium channels are specialized for fast conduction of nerve and muscle impulses. Block them, and your prey will be swiftly paralysed and more easily caught.
The many toxins that target ion channels have also been of considerable value to scientists struggling to understand how nerves work. As they are often highly specific in their targets they can be used to dissect out the contribution of an individual channel to the electrical activity of a given cell. Today, toxins can be simply bought from a specialist company. In the past, it was a different matter and the scientists not only had to purify the toxins themselves, but often had to collect the animals that produced the venom as well.
The Spark of Life: Electricity in the Human Body Page 8
