Tyrosine kinase inhibitors block CRP-induced aggregation and secretion of [³H]5-HT. (A) The effect of tyrosine kinase inhibitors on platelet aggregation induced by CRP (3 μg/mL). Platelets were resuspended in modified Tyrode's-HEPES buffer in the absence of EGTA and indomethacin and then preincubated with (i) vehicle, (ii) 3 μmol/L staurosporine (1 minute), or (iii) 100 μmol/L tyrphostin ST271 (5 minutes) before the addition of CRP (indicated by the arrowhead). The aggregation results are from one experiment that is representative of three others. (B) CRP-stimulated secretion of [³H]5-HT. (i) Platelets were stimulated with CRP (0.1 to 5 μg/mL) for 90 seconds and [³H]5-HT was analyzed as described in the Materials and Methods. (ii) Platelets were preincubated with 3 μmol/L staurosporine (stauro) or 100 μmol/L ST271 for 1 and 5 minutes, respectively, before the addition of CRP (1 μg/mL). (C) CRP-stimulated tyrosine phosphorylation is inhibited by staurosporine. The upper part shows whole cell tyrosine phosphorylation induced by CRP (3 μg/mL) in the presence of staurosporine (3 μmol/L) and the lower part shows phosphorylation of the tyrosine kinase syk under the same conditions. Results are the means ± SE of between two and four experiments.