Scientists have discovered a human hormone that is chemically identical to ouabain, a heart drug derived from plants. The hormone may be a factor in causing high blood pressure.

Ouabain (pronounced WAH-bane) is closely related to digitalis, a drug made from the leaves of the foxglove plant that has long been used in patients with failing hearts to make the heart pump harder. The discovery that ouabain is naturally produced by the human body was made by researchers at the University of Maryland School of Medicine and the Upjohn Co. They isolated the substance from human plasma.

Human ouabain is probably made by the adrenal glands, which lie atop the kidneys. The scientists believe that people with high levels of the hormone may be at risk for developing high blood pressure because ouabain alters the ability of cells to get rid of sodium. The hormone's action may lead to a buildup of sodium and calcium inside the muscle cells in blood vessel walls. This, in turn, makes the muscle cells contract, narrowing the blood vessels and raising blood pressure. The research team is developing a blood test to measure ouabain levels in patients.

The findings were reported in Baltimore last week at the American Heart Association's annual conference on high blood pressure.