Alternative splicing produces a divergent cytoplasmic tail in the human endothelial thromboxane A₂ receptor

Thromboxane A₂ (TxA₂) causes contraction of vascular smooth muscle and aggregation of platelets; paradoxically, it also induces formation of the vasodilator and antiaggregant prostacyclin by human endothelium. To determine if the molecular structure of the endothelial TxA₂ receptor differs from that of the previously characterized receptor from placenta, we isolated a putative TxA₂ receptor cDNA from a human endothelial library. The predicted amino acid sequence revealed a structure of 369 amino acids, in which a novel cytoplasmic tail replaced the carboxyl-terminal portion of the previously characterized TxA₂ receptor; this divergence in cytoplasmic domains resulted from the nonsplicing of a potential intron in the placenta TxA₂ receptor. Northern hybridization reveals that the expression of the TxA₂ receptor in endothelial RNA decreases 6-fold following stimulation with an endoperoxide analog. Polymerase chain reaction using oligonucleotide primers specific to each cytoplasmic domain revealed that only the novel receptor was expressed in endothelium, while both receptors were expressed in placenta. Overexpression of the endothelial TxA₂ receptor cDNA in Chinese hamster ovary cells conferred the ability to bind a known receptor antagonist and mobilize Ca²⁺ in response to TxA₂ mimetics. This finding of a new TxA₂ receptor in endothelium suggests that a family of these receptors may result from alternative splicing of the cytoplasmic (carboxyl) tail.