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A UNISEX contraceptive which lets couples take joint responsibility for birth
control may be on the horizon.

Starving sperm of calcium turns them into lazy swimmers that are too sluggish
to break into an egg, researchers have found. They hope that a calcium-blocking
drug could one day be taken by both men and women as a hormone-free alternative
to the Pill.

David Clapham and his team at the Harvard Medical School in Boston were
looking for genes that control the flow of calcium ions in mouse cells. By
chance, they stumbled on one that is only expressed in the testes. After
knocking the gene out, the team was amazed by the huge effect on fertility.

When 13 males without the gene were let loose among 26 females, none became
pregnant, even after months together. Four normal males, however, managed to
impregnate all of the eight females in their cage almost at once.

The gene, which codes for a protein dubbed CatSper, is expressed only in the
tails of sperm. When team member Dejian Ren examined sperm without the gene
under the microscope, he found their tails were unable to generate the
“whiplash” motion sperm normally use to ram their way into an egg. Overall, the
mutants could only swim about a third as fast as normal sperm.

In test tubes, normal sperm penetrated four out of every five eggs. But the
sluggish mutant sperm failed to break into any.

The team aren’t sure whether
the gene directly controls the flow of calcium. But a contraceptive that targets
calcium ions in sperm tails could also work for women, since it should still
affect sperm after they’ve entered the woman’s reproductive system.

Claire Taynor, a spokeswoman for the London-based family planning
organisation Marie Stopes International, says any contraceptive that doesn’t
rely on hormones should be researched. “Some women get sick on hormonal
treatment. And if it’s for men it’s even better—anything that makes them
take responsibility is good.”

“That would be wonderful,” says Susan Benoff of the New York School of
Medicine. But she cautions that such a contraceptive could be years away, as
it’s very difficult at the moment to design a drug that targets only one type of
channel.