Our red blood cells dutifully transport oxygen to our tissues – but could they play a more sophisticated role than we suspect? Rather than indiscriminately supplying oxygen, red blood cells may sense when our tissues need an oxygen boost and release substances to relax local blood vessels and increase blood flow to an oxygen-deprived area. We now know that, in our red blood cells at least, deoxygenated haemoglobin converts nitrite into nitric oxide. When nitric oxide escapes from the red blood cell, it diffuses into and relaxes the muscles around blood vessels, increasing local blood flow. If we can do this, perhaps fish can too, Frank Jensen and Claudio Agnisola reasoned(p. 3665).
To test whether fish red blood cells can convert nitrite into nitric oxide,Jensen and Agnisola selected the well-studied isolated trout heart setup as a model system, because it's easy to manipulate. But testing their idea wasn't straightforward; a sudden appearance of nitric oxide in a trout's circulation can have several explanations. `The endothelial cells of coronary blood vessels can also produce nitric oxide', Jensen explains. The pair needed to find a way to distinguish between nitric oxide produced by a trout's red blood cells and nitric oxide produced by blood vessel endothelial cells.
First, Jensen and Agnisola set out to show that the nitric oxide produced by endothelial cells can indeed increase coronary blood flow. They perfused a trout heart preparation with hypoxic saline and measured coronary blood flow and nitric oxide levels in the perfusate. Sure enough, they recorded high levels of nitric oxide and high coronary blood flow. But when they added an inhibitor that blocks nitric oxide synthesis in endothelial cells, nitric oxide levels plummeted and the trout's blood vessels constricted. The pair concluded that trout blood vessel endothelial cells produce nitric oxide, and that nitric oxide plays a role in blood flow regulation in the trout heart.
The next step was to show that trout red blood cells also produce nitric oxide. To do this, Jensen and Agnisola needed to replace the traditional saline perfusion with a red blood cell perfusion, which was not a trivial task. Eventually, they succeeded, and were rewarded with hearts that performed much better; the pair recorded improved oxygen consumption by the trout heart cells. They were now ready to test whether trout red blood cells convert nitrite into nitric oxide. Adding nitrite to the red blood cell suspension,they kept a close eye on nitric oxide levels in the perfusate. Sure enough,nitric oxide levels rose. Most importantly, when they added the inhibitor to stop nitric oxide production by the endothelial cells, nitric oxide levels were unaffected; the endothelial cells were not responsible for the elevated nitric oxide levels. Jensen and Agnisola conclude that the nitric oxide must have been produced elsewhere, most likely in the red blood cells.
But unfortunately, they didn't see any increased blood flow, so they can't prove that trout red blood cells play a role in local vasodilation of blood vessels. Jensen suggests a possible explanation: haemoglobin in the trout red blood cells may only have been sufficiently deoxygenated to convert nitrite into nitric oxide by the time the cells found themselves in the capillaries,after the blood had passed through the coronary arterioles. So, even though the red blood cells produced it, the nitric oxide never had a chance to exert its vasodilatory effects on the trout's arterioles.