Desensitization of platelet thromboxane A₂/prostaglandin H₂ receptors by the mimetic U46619

Thromboxane A₂ (TXA₂) and prostaglandin H₂ (PGH₂) activate platelets through membrane receptors. This study sought to determine if changes occur in the TXA₂/PGH₂ receptor during its desensitization induced by exposure to its agonist 11α,9α-(epoxymethano)prosta-5Z,13E-dienoic acid (U46619). Washed human platelets were incubated for 30 min with U46619 in the presence of an antiaggregatory agent, iloprost. The platelets were washed and the aggregation response to U46619 was determined. The EC₅₀ increased from 372 ± 94 nM in the control group to 826 ± 143 nM for the U46619-pretreated group (n = 7). This desensitization was specific inasmuch as the aggregation responses to thrombin and the calcium ionophore A23187 were not affected by U46619 pretreatment. Desensitization was accompanied by a decrease in the number of binding sites for [³H]U46619 from 789 ± 189 in the control to 386 ± 120 sites per platelets in the U46619-treated group (n = 5) and a decrease in the number of binding sites for the TXA₂/PGH₂ receptor antagonist, [¹²⁵I]9,11-dimethylmethano-11,12-methano-16(3-iodo-4-hydroxyphenyl -13,14-dihydro-13-aza-15αβ-ω-tetranor-TXA₂ from 3988 ± 957 to 2443 ± 553 (n = 8). The K(d) for U46619 was 37 ± 10 nM in the control group and 23 ± 11 nM for the U46619-treated group (n = 5). The K(d) for l-9,11-dimethylmethano-11,12-methano-16(3-iodo-4-hydroxyphenyl)-13,14 dihydro-13-aza-15-αβ-ω-tetranor-TXA₂ changed from 58 ± 12 nM in the control to 44 ± 9 nM in the U46619-treated group (n = 8). The effects of U46619 on aggregation and binding were blocked by preincubating the platelets with L636499, a TXA₂/PGH₂ receptor antagonist. In conclusion, the TXA₂/PGH₂ receptor can be desensitized by U46619. This process does not require platelet activation and occurs via binding of U46619 to the TXA₂/PGH₂ receptor.