(A) Platelets stimulate the IL-6 production of SMC in a cell number dependent fashion. Human vascular SMC were incubated in 24-well plates until they reached confluency. The cultures were washed (RPMI 1640) and medium (squares) or different numbers of isolated platelets (Pla; circles) added. After centrifugation of plates (500g, 10 minutes), medium (open symbols), or medium containing thrombin (1 U/mL; closed symbols) was added. Supernatants were harvested after 24 hours and centrifuged (1,500g, 5 minutes). IL-6 activity in supernatants was measured in 7TD1-assay. Similar results were obtained in 5 experiments. (B) The activation of platelets with thrombin is specific. In a separate experiment SMC were incubated in 24-well plates until they reached confluency, washed, and medium (▪) or medium containing hirudin (20 U/mL; ▨) was added. Subsequently platelets at a ratio of 1,000 platelets per SMC and finally, medium (None) or medium containing thrombin (Thrombin; 1 U/mL) was added. After the incubation (24 hours) supernatants were harvested and IL-6 was measured in 7TD1-assay. Two additional experiments showed similar results.