Coupling of thromboxane A2 receptor isoforms to Gα13: effects on ligand binding and signalling

AbstractPrevious subtyping of thromboxane A2 (TXA2) receptors in platelets and vascular smooth muscle cells was based on pharmacological criteria. Two distinct carboxy-terminal splice variants for TXA2 receptors exist and they couple to several different G protein α subunits including Gα13, but it has not been established whether either or both isoforms interact with and signal through it. We sought to determine: (1) which TXA2 receptor isoforms exist in vascular smooth muscle, (2) if Gα13 is present in vascular smooth muscle and (3) if Gα13 interacts with either or both of the two TXA2 receptor isoforms as determined by changes in ligand binding properties and generation of intracellular signals. Both TXA2 receptor isoforms and Gα13 were found in vascular smooth muscle cells. Both the alpha and beta isoforms of the TXA2 receptors were transiently transfected with or without Gα13 into COS-7 (radioligand binding assays) or CHO cells (agonist induced Na+/H+ exchange). Co-expression of each receptor isoform with Gα13 significantly (P<0.05) increased the affinity of each receptor for the two agonists, I-BOP and ONO11113, and decreased the affinity of the receptor for the antagonists, SQ29,548 and L657,925. I-BOP stimulated Na+/H+ exchange in vascular smooth muscle cells. Co-expression of Gα13 with each TXA2 receptor isoform in CHO cells resulted in a significant (P