Figure 2. Effect of P_2x1 receptor desensitisation on thrombin-evoked non-capacitative Sr²⁺ entry in human platelets.

Fura-2-loaded human platelets were resuspended in HBS containing 1 mm CaCl₂ (A) or in Ca²⁺-free HBS (100 μm EGTA added; B) and then α,β-methylene ATP (α,β-MeATP; 10 μm) was added. A second addition of 10 μmα,β-methylene ATP in the presence of 1 mm external Ca²⁺ demonstrates desensitisation of the P_{2 x1} receptor (n = 4; A). Addition of 1 mm CaCl₂ after 10 μmα,β-methylene ATP and a second addition of 10 μmα,β-methylene ATP were without effect, indicating that desensitisation occurs independently of Ca²⁺ entry (n = 4; B). C and D, platelets were treated in Ca²⁺-free HBS (100 μm EGTA added) with α,β-methylene ATP (10 μm; D) or the vehicle (HBS; C) prior to stimulation with TG (1 μm) and IONO (50 nM); 3.5 min later SrCl₂ (300 μm) was added followed by thrombin (10 U ml⁻¹; n = 4).