A Birth-Control Pill for Men?
Jan. 5, 2000 (Boston) -- Thanks to a litter of infertile mice, a new contraceptive pill for men may soon be in the works, say British and Swiss researchers in the Jan 6. issue of the journal Nature.
Unlike other male versions of "the pill" currently on the drawing board, which block the action of the male hormone testosterone on the sperm-producing cells within the testicles, the new birth-control method would maintain normal sperm production. However, it would block the release of sperm into semen, thereby significantly decreasing the likelihood of pregnancy following sexual intercourse, say Richard J. Evans, PhD, and others from the University of Leicester, England, and the Glaxo-Wellcome Geneva Biomedical Research Institute in Switzerland.
Reversing the mechanism to increase the number of sperm in semen might also help some men with infertility problems, the scientists add.
They discovered the birth-control mechanism by accident, while attempting to study the role that an important energy-carrying molecule called ATP plays in muscle contraction. ATP acts as a kind of storage battery for energy, releasing it to body tissues as it is needed. Researchers had previously found that certain smooth muscle tissues such as the vas deferens, the duct through which matured sperm are carried into the penis, have special "receptors" that accept ATP molecules in the same way that a door lock accepts a key. When the ATP key is inserted into a type of receptor called P2X1, it causes the vas deferens to contract and expel sperm into the other fluids in semen, much the way that squeezing one end of the tube causes toothpaste to be squirted out of the other end.
To see what would happen to normal body functions if the P2X1 receptor for ATP was missing, the scientists bred a strain of mice with a genetic mutation that caused the receptor to be deleted. But when they tried to breed the mutant mice with others of their kind, they found that the little critters copulated normally but couldn't reproduce. "We came across this by accident. When we made this strain of mouse it just turned out to have this characteristic," says researcher Catrin A. Pritchard, PhD, from the department of biochemistry at the University of Leicester.
Only male mice missing both copies of the gene for P2X1 appeared to be affected by the mutation; female mice missing one or both copies reproduced normally, as did male mice who had only a single copy of the gene. But when male mice with the full-blown mutation were mated with normal females, the matings resulted in pregnancy only 13.7% of the time, compared to a 100% fatherhood rate for males with a single copy of the gene. Other than an inability to father pups, the only abnormality the researchers could find in the affected male mice was a slight increase in blood pressure at rest.
The finding points the way to a possible birth-control pill for men, using a drug that would prevent the release of sperm into semen by effectively locking ATP out of the P2X1 receptor, Pritchard tells WebMD. "What we're hoping is that we can stimulate some drug company interest in studying this further in terms of maybe looking at some drugs that are already available ... that could maybe go into some test to see how effective they would be." Alternatively, a drug that stimulated the activity of the receptor might help increase the amount of sperm in semen in some men with fertility problems, she says.