Human platelet aggregation and shape change are coupled to separate thromboxane A₂-prostaglandin H₂ receptors

The relationships of human platelet thromboxane A₂-prostaglandin H₂ (TxA₂/PGH₂) receptor occupation as assessed by equilibrium binding of the TxA₂/PGH₂ agonist [¹²⁵I]BOP to the functional responses of I-BOP-induced platelet shape change and aggregation were determined before and after specific, irreversible inactivation of platelet TxA₂/PGH₂ receptors with the photolyzable TxA₂/PGH₂ antagonist I-PTA-PON₃. I-BOP stimulated platelet shape change and aggregation with concentrations producing a half-maximal response of 173 ± 39 pM (n = 4) and 1.8 ± 0.4 nM (n = 6), respectively (means ± SE). Covalent inactivation of TxA₂/PGH₂ receptors with I-PTA-PON₃ caused rightward shifts of I-BOP shape change and aggregation dose-response curves and resulted in pharmacological dissociation constants (K(d)) of 134 pM and 1.95 nM, respectively. Isotherms of [¹²⁵I]BOP binding to intact platelets (n = 6) were shallow with Hill coefficients of -0.68 ± 0.03 and were best described by a two-site model with 222 ± 58 sites/platelet of high affinity (K(d) = 270 ± 60 pM) and 818 ± 90 sites/platelet of lower affinity (K(d) = 3.9 ± 1.2 nM). The relationship of I-BOP occupancy of high-affinity receptors to shape-change responses was linear (r = 0.97, P < 0.001), but the occupancy-response relationship for the lower affinity receptor and platelet aggregation was hyperbolic with half-maximal aggregation occurring after occupation of 25% of the receptors. After covalent inactivation of a variable proportion of the receptors with I-PTA-PON₃, the occupancy-response relationship for platelet aggregation resulted in a similar hyperbola indicating an excess of low-affinity receptors coupled to aggregation (spare receptors). These studies indicate that platelet shape change and aggregation are independently coupled to separate TxA₂/PGH₂ receptor subtypes.