This study demonstrates that aequorin, a luminescent natural dye, is useful for vascular cell intracellular concentration determination. A new single-photon counting technique was developed to resolve the effects of fluid flow shear stress on in human aortic smooth muscle cells (HASMCs). Confluent HASMCs were grown on petri dishes loaded with aequorin. Then the dishes were placed in a luminometer chamber after the physiological level of shear stress was applied to the HASMC surfaces. The chamber was housed inside a highly sensitive photomultiplier tube. It detected ultraweak photon emission in response to the transient. In the presence of extracellular , a shear stress of , applied for to the top surface of the HASMC monolayer, elicited a sharp increase in .