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.