CHARACTERIZATION OF THE CLONED HEL CELL THROMBOXANE A(2) RECEPTOR - EVIDENCE THAT THE AFFINITY STATE CAN BE ALTERED BY G-ALPHA(13) AND G-ALPHA(Q)

Thromboxane A(2) (TXA(2)) induces activation of platelets and vascular smooth muscle contraction via cell surface receptors. A platelet type TXA(2) receptor from the megakaryocyte-like HEL cell was cloned with a deduced amino acid sequence identical to that previously reported fo r the human placental TXA(2) receptor. Transient expression of the HEL cell TXA(2) receptor cDNA and radioligand binding studies with the ag onist I-125- BOF showed a single class of binding sites with an affini ty comparable to a low affinity platelet TXA(2) receptor. Using a seri es of 13-azapinane TXA(2) analogs, which discriminate between TXA(2) r eceptor subtypes in platelets and vascular smooth muscle, we found tha t the cloned HEL cell TXA(2) receptor is characteristic of a platelet type TXA(2) receptor and that its binding characteristics are differen t from those of vascular smooth muscle cells. The affinity of the HEL cell TXA(2) receptor for I-125-BOP was significantly (P < .05) increas ed upon cotransfection with G alpha(13) alone, or with G alpha(q) alon e and with G alpha(13) and G alpha(12) together (n = 4-6), GTP gamma S significantly (P < .05) decreased the affinity of the receptor for I- 125-BOP in COS-7 cell membranes coexpressing HEL-TXR and G alpha(13) t o a value comparable to HEL-TXA(2) receptor alone. We conclude that I) the cloned HEL cell TXA(2) receptor has pharmacological characteristic s of a low affinity platelet type receptor and 2) that the affinity st ate of this receptor may be influenced by interaction with G alpha(13) and G alpha(q).