Effect of thrombin and thrombin receptor agonist peptide on [Ca²⁺]_i in Jurkat clones deficient in either the tyrosine kinase p56Lck or tyrosine phosphatase CD45. The cell lines used were the leukemic cell line Jurkat (clone JE6.1, ⊡) and two variants lacking p56Lck (JCaM1, ▴) or the CD45 molecule (J45.01, ⊠). Cells were loaded with indo-1 as described in Materials and Methods. Thrombin 100 nmol/L or thrombin receptor agonist peptide (2.5 μmol/L) was added at 0 time and fluorescence monitored as a function of time. (A and C) Time course of thrombin and thrombin receptor agonist peptide effects. (B and D) Dose-response curves for thrombin (2 to 100 nmol/L) and thrombin receptor agonist peptide (0.1 to 2 μmol/L) effects. Results are representative of three different experiments.