What's often to blame? Lazy blood vessel response.
A study by Penn State College of Medicine researchers provides new data on blood vessel function and reveals the benefits of a measurement technique that gives more accurate data on the amount of norepinephrine (n r' ep-e-nef' rin) - a hormone vital for blood vessel response - in tissues adjacent to blood vessels. The study, led by Lawrence Sinoway, M.D., program director of the General Clinical Research Center (GCRC) at Penn State Milton S. Hershey Medical Center, and professor of medicine, Division of Cardiology, Penn State College of Medicine, was published July 26, 2002, in the online edition of American Journal of Physiology: Heart and Circulatory Physiology. Moving from a seated to standing position causes blood to pool in the legs. In most people, the body compensates for this drop in blood pressure by releasing norepinephrine, which triggers vasoconstriction, or blood vessel narrowing. Vasoconstriction revs up the pressure in the vessels and maintains adequate blood pressure. When a person stands and the vessels don't constrict, blood pressure drops and, temporarily, the brain doesn't receive enough oxygen. This causes dizziness and fainting.
Typically, norepinephrine levels are measured by taking blood samples.
"But because the norepinephrine sits in the tissue as well as in the blood plasma, when you measure it only in blood, you only account for a small part of it," Sinoway said. "What you really need to know is what's sitting in the muscle right next to the blood vessel because it's that norepinephrine that will signal vasoconstriction and maintain blood pressure when you stand up."
Conducted at the National Institutes of Health-funded GCRC, Sinoway used microdialysis to measure the muscle sympathetic nerve activity - or muscle activity prompted by
Contact: Valerie Gliem