CINCINNATI--Researchers at the University of Cincinnati (UC) AcademicHealth Center have found evidence of a hormone they say is responsiblefor certain types of high blood pressure (hypertension), and could alsocause preeclampsia, a potentially dangerous condition that occursduring pregnancy.
Work is now under way to locate and identify the hormone, which isbelieved to be produced in very small, but highly potent amounts, sothat its hypertension-causing action can be blocked.
Finding and neutralizing this "new player" in the mechanism ofhypertension, the UC scientists said, could provide a breakthrough inthe prevention of preeclampsia--which so far has been essentiallyuntreatable--and other hypertensive conditions.
One of the Cincinnati research team's areas of interest, preeclampsiais a leading cause of fetal complications, which include low birthweight, premature birth and stillbirth. It can lead to seizures, knownas eclampsia, the second leading cause of maternal death in the UnitedStates.
The researchers, Jerry Lingrel, PhD, chair of the University ofCincinnati's Department of Molecular Genetics, Biochemistry andMicrobiology, and Iva Dostanic-Larson, PhD, a postdoctoral fellow inthe department, report the findings of their three-year study in theOct. 17, 2005, online edition of the Proceedings of the NationalAcademy of Sciences (PNAS).
The work was funded by grants from the U.S. National Institutes of Health (NIH) and the Heart and Stroke Foundation of Ontario.
The focus of their research, said Dr. Lingrel, the principalinvestigator, is an area in the human cell known as the "sodium pump,"an enzyme (Na,K-ATPase) that has long been known to be involved in theregulation of blood pressure, excitability of muscle and nerve tissueand the uptake of a wide range of essential nutrients.
The sodium pump contains the target or "binding site" of a group ofdrugs called cardiac glycosides, such as digitalis, which are commonlyused to control congestive heart failure by increasing blood pressure.
Cardiac glycosides, used by physicians for centuries to treatcongestive heart failure, have largely been obtained from "external"sources like plants, and even frog skin.
The survival of this site in the cell over thousands of years ofevolution, however, has led scientists to believe that it must also bepresent for something other than externally derived, man-mademedications. They have been looking for a naturally occurring, internalor "endogenous" control agent.
The University of Cincinnati scientists say in their PNAS report thatthey have found "conclusive evidence" that the cardiac glycosidebinding site is also the receptor for an agent that occurs naturally inthe body. This finding in turn supports a long-held hypothesis thatthere must be a hormone in the body that regulates blood pressure byinteracting with the Na,K-ATPase binding site.
The next step, the Cincinnati researchers say, is to positivelyidentify the hormone, which they believe should be a relatively routinealthough painstaking procedure, so that its level can be manipulated tocontrol blood-pressure problems such as preeclampsia.
"For centuries physicians have controlled cardiac functionusing compounds like digitalis from the foxglove plant, which areclosely related to compounds found in the skin of a poisonous frog andthe bark of an African tree once used to make poison arrows," said Dr.Lingrel, "and they all worked on the sodium pump binding site."It turns out that almost all species, from fruit flies to humans, havea site that responds to these medications. The question is, did naturesomehow keep the site all through evolution just so that people whomanufacture drugs, or isolate them from plants, can use it as a target?
"No one would ever believe that," Dr. Lingrel said. "They'd say if it'sbeen so highly conserved, it has a real role in biology. And that'swhat we have shown.
"The site, with its ability to regulate blood pressure, is there torespond to something that occurs naturally in the human body. We're nowvery close to finding it."
Dr. Dostanic-Larson genetically engineered a mouse model specificallyfor this project. By identifying and replacing just two nucleotidesamong the 3.2 billion "building" blocks in mouse DNA, she was able toknock out the binding site's ability to function without altering theenzyme's other essential functions.
By altering the two nucleotides, the normally glycoside-sensitivetarget area in the sodium pump was made resistant to the glycoside drugouabain.
The researchers found that long-term, low-dose administration of thehuman hormone ACTH (adrenocorticotropic hormone) caused hypertension inwild-type control mice, but not in the ouabain-resistant animals.This, the researchers say, demonstrates for the first time that somesubstance produced naturally by the body must be a major regulator ofblood pressure.
"The point," Dr. Lingrel explained, "is that when the binding site isremoved, the compound produced by the body can't interact with it, andtherefore blood pressure is not regulated. In the logic ofbiochemistry, the compound interacting with the site has to be asteroid, and most probably a steroid hormone."
"The 'breakthrough,' if you like," said Dr. Lingrel, "is in saying thatthe binding site not only interacts with drugs, but there must be somesubstance the body makes that interacts with this site. That's thebottom line of this research.
"Once you know that blood pressure regulation occurs as a result ofinteraction between the binding site and a hormone--or one of severalhormones--you can neutralize the hormone, probably with a monoclonalantibody (an antibody engineered in a laboratory to react with aspecific target), and then the hypertension patient is going to be ingood shape."
A degree of hypertension is necessary in normal pregnancy, said Dr.Dostanic-Larson, so that enough blood gets to the baby. In some women,however, the hormone regulation goes wrong, increasing blood pressuretoo much and causing preeclampsia.
"This new player, when we identify it," Dr. Dostanic-Larson said, "ishighly likely to be overproduced in preeclampsia. The next step is tochase down the hormone. But in the meantime, we have a backup, becausewe know where the receptor is, and an alteration that changes thereaction between the hormone and the receptor will abolish this kind ofhypertension."
Preeclampsia typically starts after the 20th week of pregnancy, mostlyin first-time pregnancies, and is related to increased blood pressureand protein in the mother's urine, resulting from kidney problems. Itaffects the placenta, and can affect the mother's kidney, liver andbrain.
In a commentary on the research, Jack Kaplan, PhD, chair of theDepartment of Biochemistry and Molecular Genetics at the University ofIllinois at Chicago, said this "important work" provides a first cleardemonstration that some substance within the body binds to Na,K-ATPaseand is one of the regulators of blood pressure.
According to the National Heart, Lung and Blood Institute (NHLBI) ofthe NIH, there has been no proven way to prevent preeclampsia, and theonly "cure" is to deliver the baby. More than 146,320 cases ofpreeclampsia were diagnosed in the United States in1998, the NHLBI says.
Also on the research team were James Van Huysse, of the Heart InstituteHypertension Unit and the departments of medicine and biochemistry andmicrobiology and immunology at the University of Ottawa, Canada, andJohn Lorenz, of the Department of Molecular and Cellular Physiology atthe University of Cincinnati Academic Health Center.
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