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People May be Predisposed to High Blood Pressure

Lack of nephrons in kidneys may be to blame

WEDNESDAY, Jan. 8, 2003 (HealthDayNews) -- German researchers have found more support for the notion that an anatomical shortcoming may put people at greater risk for developing primary hypertension.

Specifically, the study found that people with fewer nephrons -- or filtering units -- in their kidneys were more susceptible to the condition.

Primary hypertension refers to high blood pressure for which the cause is unknown. Hypertension in general affects up to 50 million Americans, according to the Mayo Clinic. The vast majority of these people have primary hypertension.

"This is a provocative finding and it dovetails into theories that have been proposed over the last decade or more suggesting that the concept of how many nephrons we're endowed with translates into the expression of disease, in this case hypertension and/or other kidney disease," says Dr. David Roth, chief of nephrology at the University of Miami School of Medicine.

"This is a concept that's been proposed. It's been a theoretical one, mostly in rodents, without any firm evidence in human beings to suggest that it is a valid point or it isn't," Roth adds.

Dr. Sheldon Greenberg, program director of nephrology at Maimonides Medical Center in Brooklyn, N.Y., says, "Millions of people in the U.S. have hypertension for which there is no readily identifiable cause, and this would be one more hypothesis to explain it."

The new findings appear in tomorrow's issue of The New England Journal of Medicine.

For the study, the researchers actually counted the number of nephrons in the kidneys of 20 middle-aged white patients who had died in accidents. Ten of these patients had had documented hypertension and 10 (the controls) had not.

The process was a tedious, time-consuming one in which the investigators first divided each kidney into smaller pieces and counted the number of nephrons.

"The process of counting is quite complex and difficult to perform," says Dr. Kerstin Amann, senior author of the study and a professor of pathology at the University of Erlangen-N├╝rnberg in Erlangen, Germany.

To understand just how difficult and complex the process was, consider these numbers: Each kidney in a healthy patient had about 1.4 million nephrons. Those with hypertension had about 700,000 nephrons each, Amann says.

The difference in numbers is too great to ignore, experts say.

"That is a large difference," Roth says. "If it was only 50,000, you'd say that's nothing. But double is a significant finding even though the number of patients were small."

Says Amann: "It is well known that a low number of nephrons is accompanied by enlargement of the remaining nephrons. One can imagine that the workload of the kidney remains the same but must be dealt with by fewer nephrons, which then have to work much harder."

This condition is called hyperfiltration. Enlargement and hyperfiltration are risk factors for the development of renal diseases, which can result in higher blood pressure, Amann says.

Although the results seem startling, they are, at this point, just speculation. There's no way to know if the number of nephrons led to the development of hypertension in the 10 patients, Roth says.

There are other potential pitfalls to the study. For starters, all of the subjects were white, while hypertension is known to affect blacks in disproportionately high numbers. The study sample was also extremely small.

"It's always dangerous to make broad conclusions from extremely small numbers of study samples," Roth says. "This is also a group of Caucasians. We have to be careful that we don't extrapolate this information unjustifiably to other populations."

Nevertheless, the study raises some intriguing issues, experts say.

There is a possibility that conditions in the uterus might affect nephron development. Perhaps, improved nutrition for pregnant women and other strategies might affect the incidence of hypertension in the next generation, a point of view that is developed in an accompanying "perspective" article in the same issue of the journal.

"It's an interesting hypothesis that something done by your mother in pregnancy may predispose you later for hypertension," Greenberg says. "It's still hypothetical and probably not that clinically relevant yet."

The important thing is for people to get regular annual physicals, including having their blood pressure taken.

"There's still probably a large number of people who don't have their blood pressure checked for years and have slow organ damage and dysfunction from undiagnosed hypertension," Greenberg says. "That's probably the more important aspect for the public."

What To Do

For more on hypertension, visit the American Society of Hypertension or the National Institutes of Health.

SOURCES: David Roth, M.D., chief, nephrology, University of Miami School of Medicine, Miami; Sheldon Greenberg, M.D., program director, nephrology, Maimonides Medical Center, Brooklyn, N.Y.; Kerstin Amann, M.D., professor, pathology, University of Erlangen-Nürnberg, Erlangen, Germany; Jan. 9, 2003, The New England Journal of Medicine
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