Metabolic response of CD34⁺ cells to interleukin-8 in the presence of PF4. The metabolic response of CD34⁺ cells to PF4 and IL-8 was assessed by measurement of extracellular proton excretion (acidification rate) using a Cytosensor microphysiometer. After establishing a stable baseline, interleukin-8 (100 ng/mL) was infused over 20 minutes starting at time = 0 (closed circles). A brisk signaling response (increase in extracellular acidification over baseline) is evident. Squares: starting at t = — 16 minutes, PF4 (10 μg/mL) was infused through t = 20. At t = 0, the ongoing PF4 infusion was supplemented with IL-8 (100 ng/mL). Triangles: identical to squares except PF4 concentration is 100 ng/mL.