Teams of scientists at Washington University School of Medicine in St. Louis have identified a mechanism that triggers increased blood flow to brain cells actively engaged in work. The findings appear in two papers in the Jan. 13 issue of the Proceedings of the National Academy of Sciences (PNAS) and are available online.
The researchers are hoping to apply the new insights to improve understanding of basic brain function and limit side effects of diabetes, but the new insights could have much wider ramifications.
"One can pick out any number of diseases where knowing how increased blood flow in the brain is activated will be very important and useful," says Marcus E. Raichle, M.D., professor of radiology, neurology and of anatomy and neurobiology. "Changes in blood circulation in the brain are linked, for example, to Alzheimer's disease as well as stroke."
Scientists have known since the late 1800s that when a muscle cell contracts repeatedly or a nerve cell increases its activity, the circulatory system responds by increasing blood flow to the activated cells. They assumed this happens so the blood can supply the cells with more sugar and oxygen as fuel for the increased workload.
Over the past decade, a growing body of evidence has suggested that this idea, logical as it seems, is incorrect. The new Washington University studies, one conducted in animals and the other in humans, give scientists a sense for how blood flow increases in the context of cellular exertion. Why blood flow increases still remains elusive, but knowing how the increase is triggered will provide vital aid to answering that question.
Raichle, who led the human study in PNAS, also directed a 1988 study that found increased brain activity increased blood flow much more than brain cells' consumption of oxygen. Later Washington University studies confirmed the surprising finding that the reason for increased blood flow wasn't to bring in additional sugar or oxygen.
Joseph Williamson, M.D., a retired Washington University pathologist, read Raichle's 1988 study and was intrigued. Williamson, lead investigator for the other Washington University study appearing in PNAS, specializes in the effects of diabetes. In addition to elevating sugar levels throughout the body, diabetes also increases blood flow and can cause damage to nerves, retinas and kidneys.
Williamson was struck by a similarity between working muscle cells and cells of diabetic patients in regions likely to be damaged by the disease: both experienced increases in the ratio of two forms of a key energy-producing compound, nicotinamide adenine dinucleotide (NAD).
"Because of its role as the major carrier of electrons and protons from fuels for energy metabolism, NAD is strategically positioned -- even uniquely positioned -- to coordinate blood flow with energy metabolism in the resting cell, in the working cell and in disease states like diabetes and hypoxia," Williamson says.
When in use as a carrier of electrons and protons, NAD is converted to NADH (NAD plus H, or one atom of hydrogen). Williamson thought the ratio between these two forms of the compound (NADH/NAD) might be modulating blood flow.
Two other compounds involved in energy production, pyruvate and lactate, can affects cells' ratio of NADH to NAD. Williamson thought it might be possible to use this connection to test his theory. With colleagues at Washington University, he demonstrated in a 2001 rat study that blood flow to working skeletal muscle increased even more than normal after lactate injections, which increase the NADH/NAD ratio. Injections of pyruvate, which decrease the ratio, had the opposite effect. They also found the same results in brain regions that process sensory information from rat whiskers.
In the new paper, the same effects were detected in the rat retina and visual region of the brain during optical stimulation. Williamson and his co-authors have also revealed that the increased NADH/NAD ratio activates a signaling pathway that promotes the creation of nitric oxide, a compound widely recognized for its ability to dilate blood vessels.
"This was the evidence that Marc Raichle's group needed to go ahead and look for this same effect in humans," Williamson says. "They needed some proof that the principle might be applicable to the visual region of the brain in humans, and they found that it is."
For the human research, seven subjects were studied using PET imaging scans. Participants were either asked to close their eyes during the scans or to fix their gaze on an unmoving central crosshair in an animated visual display.
Andrei Vlassenko, M.D., Ph.D., research associate in radiological sciences and an author of the human study, notes that without lactate injections, the blood flow increase to the visual cortex during the visual task was 19 percent. After lactate injections, it was 26 percent.
"That might not seem like a lot if you look strictly at the gain, but if you look at the gain as a percentage of original level of increase, that's fully one-third more," Vlassenko says.
Mark A. Mintun, M.D., professor of radiology and psychiatry, was lead author of the human study. He points out that the mechanism under study isn't the only way the brain controls blood flow. There are other mechanisms to respond to stress, hyperventilation, blood pressure alterations or other dramatic changes.
"What we're doing is isolating the response in blood flow when the brain function itself changes and using that response as a window for investigating brain metabolism," Mintun explains.
Mintun, Raichle and Vlassenko already have a follow-up study using pyruvate injections underway.
Williamson is using the results of the studies to build his case for a new understanding of how diabetes causes damage to tissues. He believes the same signaling pathways activated by increased NADH/NAD ratios may also trigger the production of chemically reactive compounds that damage cells.
Ido Y, Chang K, Williamson JR. NADH augments blood flow in physiologically activated retina and visual cortex. Proceedings of the National Academy of Sciences, early online edition, Dec. 29, 2003.
Mintun MA, Vlassenko AG, Rundle MM, Raichle ME. Increased lactate/pyruvate ratio augments blood flow in physiologically activated human brain. Proceedings of the National Academy of Sciences, early online edition, Dec. 29, 2003.
Funding from the Kilo Foundation and the National Institutes of Health.
The full-time and volunteer faculty of Washington University School of Medicine are the physicians and surgeons of Barnes-Jewish and St. Louis Children's hospitals. The School of Medicine is one of the leading medical research, teaching and patient-care institutions in the nation. Through its affiliations with Barnes-Jewish and St. Louis Children's hospitals, the School of Medicine is linked to BJC HealthCare.