Researchers have developed a new technique to study blood cell behavior in small blood vessels, highlighting its efficacy for determining how blood cells regulate blood pressure, and ways in which the process may be altered by diet and disease. The study was conducted by researchers at Harvard University, including William Ristenpart, who is now a professor in the departments of food science and technology and chemical engineering and material science at UC Davis.
Red blood cells release the chemical ATP in response to shear stress, or pressure, that they encounter passing through narrow blood vessels. The ATP signals the blood vessels to expand and relieve the shear stress. It is believed that this process helps regulate blood pressure and that mechanical stretching of the red blood cell’s shape triggers ATP release.
Previous studies only provided before and after snapshots of ATP release in response to shear stress, said study co-author Ristenpart, who was a former postdoctoral fellow at Harvard.
“It was completely unknown before how long or how quickly blood cells respond to changes in shear stress,” he said.
To address this limitation, Ristenpart and colleagues designed a microscopic fluid transport system in which human blood cells flow through a transparent rubber channel that narrows and then widens again to mimic a constricted blood vessel.
“These high-resolution microfluidic tools are helpful for studies that explore and model how cells in the body experience stress to regulate blood flow to the surrounding tissue,” said Scott Simon, a professor of biomedical engineering who was not involved in the study.
By changing the width or length of the constricted channel in their unique system, the researchers could precisely control the strength or duration of the applied shear stress.
To follow the timing and amount of ATP released by blood cells under shear stress, researchers used a high-speed detector to measure light produced by the reaction between ATP and the enzyme luciferase.
From this data, the researchers determined that there is a considerable delay between when the cells first encounter increased shear stress and when they release ATP. Higher shear stress shortens the delay and increases the amount of ATP release.
Contrary to expectations, cell stretching itself does not trigger ATP release if the experienced shear stress lasts less than three milliseconds.
The researchers speculate that this minimum threshold represents the time for the cell’s outer membrane support network to respond to stretching and deformation. The longer delay that follows is required for specific proteins in the network to combine and form pumps in the membrane that transport ATP out of the cell.
These results suggest that there are two distinct processes involved in ATP release from blood cells, but more work needs to be done to corroborate the model and identify which proteins are responsible for ATP transport, Ristenpart said.
Although the system is artificial, there are ways to improve it and it is useful for isolating the effects of various factors that can trigger or modify ATP release, he said.
As part of the Foods for Health Initiative at UC Davis, Ristenpart will use the technique to examine how red blood cells respond to chemical byproducts called metabolites that are formed by the breakdown of food.
“We are very excited by [Ristenpart’s] work and its potential for interesting breakthroughs in the future,” said Bruce German, a professor of food science and technology who was not involved in the study.
Ristenpart has applied the approach to show that a metabolite of alcohol called FAEE, which binds to red blood cells, impeding their ability to deform under shear stress.
Instead of modifying shear stress as a stimulus, we can determine how the presence of different metabolites affects blood cell response by measuring the changes in ATP release, Ristenpart said.
Diseases such as atherosclerosis, pulmonary hypertension, cystic fibrosis and cancer are all believed to involve some defect in the ability of red blood cells to control ATP release. Other factors besides shear stress such as blood pH, oxygen and nitric oxide levels also affect ATP release and, by extension, the ability of red blood cells to regulate blood pressure.
“We can now determine the time scales associated with these processes, and that starts to give us a deeper insight into what’s going on fundamentally at the biochemical level,” Ristenpart said.
ELAINE HSIA can be reached at firstname.lastname@example.org