badfish,badbird-全文預(yù)覽
【正文】 29 After handling the bird and later touching his mouth with his hand, Dr. Westwood noticed that his fingers and lips were going numb. His mind immediately flashed back to the disastrous trip to Indonesia and he began to panic. Luckily, the symptoms faded. 30 His friend Bill used a key to identify the animal as a pitohui. The pitohui are small, social songbirds that live in Papua New Guinea. Their encounter was the first time any scientist had realized the birds were toxic. 31 Dr. Westwood collected feather and tissue samples to bring back to the lab. After returning to Montana, he isolated the toxic pound that he was active in the feathers of the pitohui. It appeared that the active ingredient was a homobatrachotoxin. 32 ? Homobatrachotoxin is a steroidal alkaloid that is similar to batrachotoxin, the toxic principle of the Central American poison arrow frog Phyllobates aurotaenia. ? Batrachotoxin and homobatrachotoxin are both known to act on voltagesensitive sodium channels in excitable tissues. 33 CQ11: What symptoms did Dr. Westwood experience after both his pufferfish meal and after handling the new bird species? A. Nausea B. Numbness C. Paralysis D. Difficulty breathing E. More than one of the above 34 CQ12: The toxin found in the bird feathers: A. Is used by the birds to deter predators. B. Is found in birds that have eaten poison arrow frogs. C. Is the same toxin found in pufferfish flesh. D. Affects voltage sensitive ion channels. E. Affects voltage sensitive potassium channels. 35 Dr. Westwood has asked you to help elucidate the mechanism of action of this new toxic pound. ? In your first experiment, you generated action potentials in axons of large neurons obtained from squid in the presence of this new toxin. You found after depolarizing, the membrane potential remained positive for an extended length of time and the repolarization was often extremely delayed. ? Draw a graph showing membrane potential vs. time to illustrate this effect. 36 CQ13: Choose the graph below that most closely matches the graph you just created: 70 55 0 30 Membrane potential (mV) Time 70 55 0 30 Membrane potential (mV) Time 70 55 0 30 Membrane potential (mV) Time 70 55 0 30 Membrane potential (mV) Time A B C D 37 CQ14: You experiment with higher concentrations of toxin and find cases when the cell could not repolarize at all or, if it began to repolarize, it immediately depolarized again. This tells you that the toxin: A. Prevents voltagegated K+ channels from opening. B. Increases the speed that Na+ ion channels open. C. Prevents Na+ ion channels from closing. D. Makes Na+ ion channels close too soon. E. Doesn’t affect the Na+ ion channels at all, it affects the sodiumpotassium pump instead. CQ15: In this case study you have learned: A. Poisons that affect nerves interfere with ion channels. B. Never eat pufferfish. C. Science depends on accidental discoveries. D. Electrical potentials across membranes can be manipulated with ion
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