Over the last 25 years, it has been established that the red blood cell (RBC) is a major determinant in blood flow, which it can modulate through release of adenosine triphosphate (ATP). Although RBCs store intracellular ATP in mM concentrations, measurements indicate that the cells release nM concentrations when stimulated by deformation, hypoxia (lowered oxygen tension), or incubation with pharmacological stimuli such as hydroxyurea (HU), which is the only approved drug for treatment of sickle cell disease. Upon release, RBC-derived ATP can induce vessel dilation via activation of endothelial cell nitric oxide synthase (eNOS) to produce nitric oxide (NO). To probe the fate of increased ATP release from human RBCs incubated with the drug hydroxyurea, a traditional soft polymer platform was utilized to facilitate measurement of cell-to-cell communication between RBCs and a cultured endothelium. This device contained an array of micron-scale channels through which RBC samples were pumped. The sample flow was separated from a detection well by a porous polycarbonate membrane. Stimulated ATP released from the RBCs diffused across the membrane to the detection wells and was measured using the luciferin-luciferase chemiluminescence assay, integrated with a plate reader for detection. RBCs incubated with 100 uM of HU released on average 2.06 ± 0.37 times more ATP relative to the control sample. Through the use of various inhibitors, this increase in ATP release was subsequently demonstrated to depend on RBC deformability, RBC NOS activity, and the cystic fibrosis transmembrane conductance regulator protein (CFTR). The fate of the measured RBC-derived ATP was also investigated by probing ATP signalling to an adjacent cultured endothelium.ATP release from RBCs increases in response to hypoxia, or lowered oxygen tension; however, the dependence of RBC ATP release on oxygen tension has not been investigated. To enable measurement of RBC ATP release and oxygen tension in a flowing stream of RBCs, a 3D printed device was designed to accomodate commercial transwell inserts for ATP measurements, as well as threaded Clark-type electrodes for amperometric oxygen measurements. The device consisted of a channel 2 mm wide and 0.5 mm in height with two ports for analyte detection and one threaded port for an electrode. The Clark-type electrode was fabricated from gold and silver wires secured into a finger tight fitting. Oxygen standards and RBC samples were prepared using air and argon purged buffers. Using the 3D printed device, RBC ATP release and oxygen tension were measured simultaneously from prepared RBC samples. Relative to controls, RBC ATP release increased significantly in response to systematically lowered oxygen tension with a maximum increase of 2.38 ± 0.43 fold more ATP when exposed to 5.35 ± 0.12 ppm oxygen. ATP release saturated, i.e., was not significantly different, at lower oxygen tensions. This increase in ATP release was inhibited by incubating RBCs with the cell stiffening agent, diamide. The dependence of hypoxic RBC ATP release on the conformation of heme in hemoglobin (Hb) is demonstrated by converting measured oxygen tensions to Hb saturation.The 3D printed platforms presented herein were also utilized as in vitro tools to model pharmacokinetic dosing profiles, specifically with applications for studying antibiotic resistance. The World Health Organization, Centers for Disease Control, and the White House have issued reports that outline strategies to combat antibiotic resistance. The Spence lab has developed a 3D printed device to mimic in vivo drug dosing profiles on an in vitro platform for applications in drug discovery. This 3D printed diffusion-based dynamic dosing device mimics the dosing capabilities of the hollow fiber chamber reactor (HFCR). The in vitro 3D printed device contains 6 ports to house commercial polyester transwell membrane inserts (0.4 micron) and in house fabricated 0.2 micron pore size inserts (polyester), which can be loaded with a sample of Escherichia coli. Chemically competent, kanamycin resistant E. coli were dosed with the DNA gyrase and topoisomerase IV inhibitor levofloxacin, which reached a maximum concentration (Cmax) of 21.0 ± 5.7 uM (0.4 micron pore size) and 68.0 ± 7.1 uM (0.2 micron pore size) of levofloxacin in approximately 1 hour. After dosing, the viability of the bacteria samples was measured using standard plating methods.
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