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Science: When sperm meets egg, it's electrifying

When sperm meets egg, the chemistry is highly charged. Scientists in Israelhave found a ‘trigger’ that sperm use to induce an electrical current toflow inward across the egg’s wall. This not only activates fertilisation butalso alters the cell wall so that other sperm are prevented from entering.The discovery could have important implications for human fertility andcontraception because it might enable sperm to be helped or hindered inentering an egg. (Science, vol 261, p 484).

Daphne Atlas and Yuval Kupitz of the Hebrew University in Jerusalem exposedeggs from the frog Xenopus laevis to adenosine triphosphate (ATP), theenergy-containing molecule of all living things. They found that themolecule caused receptors on the surface of the eggs to release a stream ofsodium ions. Sperm are laden with ATP, so the researchers suspected thatthey might stimulate eggs in a similar way. ‘Preliminary evidence on humanfertilisation shows that the mechanism is analogous to that in frogs,’ saysAtlas.

Antagonists such as guanosine triphosphate that block the ATP receptors onthe egg could be used to prevent fertilisation. On the other hand,conception could be made more probable by enhancing the receptor’sactivity, so helping some infertile couples.

Plants and animals use ATP to supply the energy they need for metabolicprocesses. Directed by enzymes, ATP rapidly loses phosphate groups,releasing the energy needed to contract muscles, manufacture biochemicalsand transport molecules and ions, such as sodium, across cell walls.

Atlas and Kupitz guessed that ATP might also be involved in fertilisationbecause sperm carry such a large amount of it. In many species of animal,the stream of sodium ions released across the egg’s cell wall is its firstresponse upon meeting a sperm.

To test their idea, the researchers treated isolated frogs’ eggs with a 0.5millimolar solution of ATP and measured the current. They found that within1 second a current of between 0.1 and 2.5 microamps began to flow inward,reaching a peak in less than 30 seconds. They believe this timing is whatwould be expected if channels in the cell wall were opening.

The researchers repeated the experiment, but excluded sodium ions. Thecurrent was far weaker, confirming that sodium ions carry most of thecurrent.

Atlas and Kupitz then tested the effects of sperm ATP on the eggs. Theyexposed different groups of eggs to Xenopus sperm, both with and withoutmolecules that were known to block ATP receptors. They observed the changesin the eggs with a microscope. Fertilised eggs should divide – the firstsigns of fertilisation – after about two hours.

Atlas and Kupitz found that eggs treated with sperm and antagonists showedno signs of fertilisation, while the others had begun to divide. ‘We haveantagonists that block the ATP receptor, and could have a bearing oncontraception,’ says Atlas.

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