A persistent scarcity of oxygen in body tissues - a widespread problem in patients with heart or lung disease - can create a defect of red blood cells that further exacerbates the condition by constricting blood vessels in the lung, Howard Hughes Medical Institute researchers at Duke University Medical Center have found.
What's more, the team demonstrated through studies in people and animals that inhalation of a 'souped up' form of nitric oxide, which targets red blood cells, reverses the blood abnormality to restore normal lung pressure.
The team's findings appear in the online Early Edition of Proceedings of the National Academy of Sciences (October 3-7, 2005). The work was supported by the National Heart, Lung, and Blood Institute and the National Science Foundation. Stamler is a paid consultant for Nitrox LLC, a biotechnology company developing NO-based drugs for disorders of the heart, lung and blood.
The potentially fatal lung condition, pulmonary hypertension, is characterized by high blood pressure in the lungs. The disorder is a common complication of chronic diseases such as emphysema, arthritis, sickle cell disease and heart failure. However, pulmonary hypertension can also arise in otherwise healthy people for unknown reasons. Symptoms include shortness of breath under minimal exertion, fatigue, chest pain, dizzy spells and fainting.
"Many people suffer pulmonary hypertension as a complicating factor of other chronic disease," said study senior author Jonathan Stamler, M.D. "In such cases, the lung condition is often predictive of poorer outcomes. For others, pulmonary hypertension is the primary disease."
"We have now established a molecular defect of the red blood cells as an important contributing cause of hypertension in the lung," added Timothy McMahon, lead author of the study. Physicians had previously considered an abnormality within the lung itself as the primary source of the condition, he explained. Physicians had not considered red blood cells as a cause of lung disease.
"We have found that when red blood cells are exposed to abnormally low oxygen for long periods, they become depleted of an essential substance that they normally release to relax blood vessels in the lung," McMahon continued. "But not only do blood cells, which of course perfuse the lung, cause lung problems, we've also found that inhalation of a new drug designed to correct the blood defect can reverse this condition."
Stamler's group reported in 1996 that hemoglobin in red blood cells acts as a finely tuned biosensor, adjusting blood flow to provide exactly the optimum amount of oxygen to tissues and organs. The blood cell adjusts blood flow by changing shape and releasing a nitric oxide-like molecule called s-nitrosothiol (SNO), which the cell carries through the bloodstream along with oxygen.
When oxygen levels are high, hemoglobin scavenges excess oxygen and NO, constricting blood vessels and reducing blood flow. When oxygen levels drop, the NO is released to relax blood vessels and improve blood flow. The Duke team now finds that with prolonged oxygen shortage, or hypoxia, blood cells become depleted of SNOs, therefore losing their ability to relax blood vessels.
More recent evidence from the Duke group has indicated that other types of SNOs might offer new therapeutic approaches to diseases of the heart, lung and blood. For example, the researchers found that SNOs played a critical role in septic shock, a common cause of death in intensive care units. They later showed that the compounds are lacking in the blood of patients with sickle cell disease and also play a part in preventing asthma. The latest findings extend the role of SNOs in red blood cells to include pulmonary hypertension.
A new chemical therapy, which replenished SNO levels in the blood of patients, restored the red blood cells' ability to dilate vessels, lowered pressures, and improved the transfer of oxygen to tissues. Similarly, in the lab, exposure of red blood cells to sustained hypoxia led to a deficiency of the SNO vessel relaxant, according to the researchers. The SNO-deficient blood cells failed to relax blood vessels of the lungs in laboratory studies and constricted pulmonary blood vessels in pigs, they reported. Restoration of SNO levels in the animals likewise lowered pressures in the lungs.
The researchers demonstrated that under conditions of prolonged oxygen deficiency, which is very common in sick patients, red cells become deficient for SNO, thereby losing their capacity to relax blood vessels and boost blood flow, Stamler said. Pressure came down in the lungs of animals given red blood cells replete in SNOs, whereas transfusion of red blood cells deficient in SNO raised pressures, the team reported.
To examine the relevance of the findings to human disease, the researchers compared the level of SNO in the blood of patients with pulmonary hypertension to that of healthy people. Normal individuals had five times more SNO in their blood than did those with elevated lung pressure. In fact, those with the lung condition almost completely lacked hemoglobin with bound SNO, a finding consistent with the effects of hypoxia observed in the lab, Stamler said. That SNO-deficiency led to impaired blood vessel dilation by the red cells, they showed.
The researchers reasoned that if deficiency of SNO in red blood cells causes the lung condition, then restoring SNO levels should reverse the disease. Ten patients treated with an inhaled SNO-generating gas exhibited an increase in SNO in the bloodstream, found the researchers. After therapy, patients' red blood cells again relaxed blood vessels in a manner comparable to that of normal red cells. In addition, the pressures came down in the lungs of the patients.
"We have followed this process all the way from characterizing the molecular defect of red blood cells through the translation of this basic scientific finding into a promising new therapy," Stamler said. A larger clinical trial effort now underway will further examine the therapy's potential to relieve pulmonary hypertension, he said.
Collaborators on the study include Timothy McMahon, Gregory Ahearn, Martin Moya, Andrew Gow, Yuh-Chin Huang, Raphael Nudelman, Yun Yan, Abigail Krichman, Thomas Bashore, Robert Califf, Claude Piantadosi and Victor Tapson, all of Duke. Benjamin Luchsinger and David Singel of Montana State University also contributed to the research.