Desensitization of responses to HRgpA by prior treatment of platelets with PAR-1 and PAR-4 agonist peptides.

Calcium responses to 1 nM HRgpA and 100 μM ADP following treatment with: (A) 150 μM TRAP; (B) 500 μM TRAP-4; or (C) 500 μM TRAP-4 and 150 μM TRAP. The ADP treatment was used in each case to demonstrate that prior treatments had not significantly depleted calcium stores, so that essentially normal responses could be obtained with a noninteracting agonist. In panel C, the response to ADP prior to any additions to platelets is shown as a bold line for comparison. Pretreatment with HRgpA also desensitized responses to TRAP-4 and TRAP (D).