Agonist-induced calcium response in single human platelets assayed in a microfluidic device.

To facilitate drug discovery directed toward platelet-specific targets, we developed a platelet isolation and fluorophore-loading method that yields functionally responsive platelets in which we were able to detect agonist-induced calcium flux using a microfluidics-based screening platform. The platelet preparation protocol was designed to minimize preparation-induced platelet activation and to optimize signal strength. Measurement of platelet activation, as monitored by ratiometric determination of agonist-induced calcium flux in fluor-loaded human platelets, was optimized in a macrosample cuvette format in preparation for detection in a microfluidic chip-based assay. For the microfluidic device used in these studies, a cell density of 1 to 2 x 10(6) platelets per milliliter and a nominal flow rate of 5 to 10 nl per second provided optimal event resolution of 5 to 20 platelets traversing the detection volume per unit time. Platelets responded in a dose-dependent manner to adenosine diphosphate and protease-activating peptide (PAR) 1 thrombin receptor-activating peptide (TRAP). The work presented here constitutes proof-of-principle experiments demonstrating the enabling application of a microfluidic device to conduct high-throughput signaling studies and drug discovery screening against human platelet targets.