MDC and SDF-1 stimulate platelet aggregation under low-shear conditions.

(A) The chemokine MDC is a stronger activator of platelet aggregation than SDF-1, and both chemokines are more effective in the presence of low levels of ADP or thrombin. Washed platelets stirred in a test tube were exposed to SDF-1 or MDC at 0.5 μg/mL in the presence or absence of ADP (0.05 μM) or thrombin (0.01 U/mL) for 10 seconds. (B) MDC and SDF-1 stimulation of platelet aggregation is labile, particularly for MDC. Additions were the same as in panel A, except that the chemokines were first mixed with the platelet suspensions, which were left at 37°C and were not stirred for 10 minutes before initiating low-shear conditions (stirring) and the addition of an agonist (ADP or thrombin). (C) Extracellular ADP is a major factor responsible for the ability of MDC and SDF-1 to stimulate platelet aggregation. Platelet suspensions were pre-incubated at 37°C with apyrase (5 U/mL ADPase activity) for 10 minutes under static conditions before an agonist, a chemokine, or both were added and low-shear stirring was initiated. Values are means ± SEM from 3 platelet preparations for panels A to C. The significance of the effects of MDC compared to SDF-1 (A) and of apyrase in each experimental situation (C) is *P < .05, **P < .01, ***P < .001.