Stamler and McMahon (previously a student in Stamler's lab) found that stored blood rapidly loses the bulk of its NO within three hours of collection. Its ability to induce vasodilation likewise diminishes, they said.
"Everybody thinks if you give back oxygen-carrying red blood cells, you will get more oxygen to tissues," said Stamler. "No, that's not the case. The blood we give cannot open vessels, and thus cannot deliver oxygen, and we think that's because it's missing nitric oxide gas."
The researchers also found that stored red cells gradually lose their "membrane flexibility" -- a physical attribute of red cells that allows them to squeeze into narrow capillaries, and which may also be related to NO depletion.
Some five million patients receive blood transfusions annually in the United States, amounting to almost 14 million units of blood, according to a 2005 report.
Yet, except in cases of trauma, there are few guidelines on exactly who should receive blood transfusions, and little agreement over how much they should get. Existing clinical trial data, mostly in the form of retrospective analyses, suggest that, if anything, giving blood may harm patients more than it helps them.
"Overall, the [previous clinical] trials have shown that while bleeding is bad, and while decreases in blood count is bad, giving blood back is not good -- that's the paradox," explained Stamler. He said extensive data exists to support an association between blood transfusions and elevated risk of heart attack, organ damage, and death.
The new studies provide a molecular rationale for these observations. Not only can NO-depleted, stored red blood cells not open blood vessels, they may actually block capillaries in already oxygen-starved tissues, exacerbating tissue damage. Furthermore, donated blood cells may act as a "sink" to soak up NO, thereby causing
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