DURHAM, N.C. -- Investigators from Duke University Medical Center and the Howard Hughes Medical Institute (HHMI) have demonstrated that red blood cells play a crucial and active role in responding to the oxygen needs of tissues and that furthermore, the chemical nitric oxide is key to this process, leading the researchers to conclude that the chemical should be considered as the third major blood gas -- along with oxygen and carbon dioxide -- to be monitored in patients.
In their studies, the scientists tested the responses of the circulatory systems of human subject in specialized chambers where the scientists could control atmospheric pressure and gas concentrations. The scientists raised or lowered oxygen levels in the chambers and analyzed the response of the subjects' blood cells. From such analyses, the scientists demonstrated that nitric oxide in the blood cells is an active regulatory molecule that causes the oxygen-carrying hemoglobin molecules in red blood cells to undergo subtle shape changes in response to varying concentrations of oxygen in tissues. Nitric oxide works by relaxing or contracting blood vessels.
The researchers are publishing their findings in the July 2002 edition of Nature Medicine, which posted the study as an advance online publication today (June 3, 2002) at http://www.
The scientists said that their discoveries help explain why some treatments -- such as blood transfusions or drugs like erythropoietin that boost red blood cell production -- either don't work, or even lead to death. The findings could also explain why there is a direct relationship between high red blood cell counts and stroke, heart attack and hypertension, said the scientists. Additionally, the findings could offer new avenues of research in treating a whole range of those disorders, as well as sickle cell disease and pulmonary hypertension.
"One of the prevailing precepts of biology is that every cell regulates its major function," said Jonathan Stamler, M.D., HHMI investigator at Duke and senior member of the research team. "For red blood cells, their function is the delivery of oxygen to tissues, so the fact that it couldn't regulate blood flow, as was previously thought, seemed to me to make no sense. Now, we see the crucial role nitric oxide plays in the respiratory cycle, which is the basis of life for all mammals." According to the paper's first author, Duke University Medical Center pulmonologist Timothy McMahon, M.D., "The ability to monitor and manipulate nitric oxide along with oxygen and carbon dioxide should prove to be very useful in the diagnosis and treatment of many human diseases. Specifically, the knowledge that nitric oxide is intimately involved with red blood cells in blood flow regulation opens up huge new fields of research.
"As we develop further understanding of nitric oxide's role in oxygen delivery, these insights will be integrated into routine care of our patients," McMahon added.
The research effort was supported by the National Institutes of Health and the National Science Foundation.
The results of the teams' experiments provide insight into two long-running paradoxes faced by physicians: first, found the scientists, the oxygen content of blood often doesn't always correlate with the actual delivery of oxygen to the tissues. And secondly, the results provide a mechanism for well-known ability of blood vessels to respond to oxygen tension. The key finding, they said, is that individual red blood cells can respond quickly and locally to the oxygen needs of cells in the body's tiny capillaries, or microvasculature.
Hemoglobin, the molecule to which oxygen binds in red blood cells, occurs in two shapes, known as the R and the T shapes, that differ in their affinity for the three gases. Hemoglobin changes shapes in response to local oxygen concentrations, with nitric oxide playing the key role in either relaxing the capillaries to allow a more ready exchange of gas, or constricting the capillaries, found the scientists.
"Instead of the total oxygen saturation of blood in the circulatory system, the key determinant of the efficiency of oxygen delivery is the flow of blood in the microvasculature," Stamler said.
"Our data raises the possibility that the level of nitric oxide in the blood may provide physicians a keen insight into the state of a patient's microcirculation," Stamler continued. "The ability to monitor and manipulate levels of nitric oxide in red blood cells should be useful in assessing blood gases, in the diagnosing and treating disease of the heart, lung and blood, and in the rational development of therapeutics."
The findings help explain why many patients, especially those with heart disease, may not always benefit from blood transfusions and why some may actually be hurt. Recent data has even suggested increased deaths in a subset of these patients, Stamler said. Typically, heart patients with a hematocrit (concentration of red blood cells in a sample of blood) of less than 30 are automatically given transfusions in hopes of improving the ability of oxygen-starved tissues to be nourished, even though there is no generally accepted threshold for transfusion therapy, Stamler said.
"There is more to the story than just improving the amount of oxygen in the system -- if the mechanisms moderated by nitric oxide are not functioning properly, the oxygen can never leave the red blood cells and enter the tissue needing oxygen," Stamler said.
The Duke team has initiated small clinical trials to test their findings, particularly in disease states characterized by abnormal functioning of endothelial cells lining blood vessels.
"We will be trying to correlate the pathophysiology of these disorders with the activity of the red blood cells to get a better idea of how nitric oxide participates in these diseases," McMahon said. "We hope this will allow us to develop better ways of diagnosing and treating these diseases."
Currently, the technology to measure the levels of nitric oxide in the blood is complicated and time-consuming, the researchers say, and is not yet ready for clinical application. However, the researchers believe that technology used to measure nitric oxide in red blood cells can easily be adapted for future use in clinical settings and research.
Most of the sophisticated measurements made by the team took place in Duke's specialized hyperbaric chambers, in which researchers can change the pressures and concentrations of atmospheric gases and measure their effects on blood gas exchange.
"The hyperbaric chamber allowed us to manipulate oxygen levels of human volunteers," McMahon said. "By studying blood samples during hypoxia (too little oxygen) through hyperoxia (too much oxygen), we were able to measure the shape changes in hemoglobin and correlate them with the oxygen uptake and delivery by the red blood cells."
Duke pulmonary specialist Claude Piantadosi, M.D., directed the hyperbaric chamber studies.
Other members of the team were, from Duke, Richard Moon, M.D., Martha Carraway, M.D., Anne Stone, Bryant Stolp, M.D., Andrew Gow, Ph.D., John Pawloski, M.D., and Paula Watke. Ben Luschinger, Ph.D., and David Singel, Ph.D., Montana State University, were also members of the team.