Distribution of binding sites for thromboxane A(2) in the mouse kidney

Thromboxane A(2) (TxA(2)) is a potent vasoconstrictor eicosanoid that has been implicated in the pathogenesis of both human and experimental renal diseases. The biological actions of TxA(2) in the kidney are mediated through specific cell-surface receptors. In this report, we characterize the distribution of thromboxane receptors (TxR) within the normal mouse kidney by receptor autoradiography. With the iodinated TxR agonist [I-125]BOP, TxA(2) binding sites were detected throughout the kidney. Competitive inhibition curves of whole kidney binding demonstrated a half-maximal inhibitory concentration of 6.5 nM. When Scatchard analysis was performed on anatomically discrete regions, the [I-125]BOP binding in the medulla was best fit by a one-site model, with a dissociation constant (K-d) of 8.2 +/- 2.2 nM. In contrast, [I-125]BOP binding in the cortex was better described by a two-site model, with estimated K-d Of 262 +/- 16 pM for a higher affinity site and 16.9 +/- 1.3 nM for a lower affinity site. These sites do not appear to represent receptor isoforms that arise from alternative splicing of mRNA. The lower affinity binding sites may represent a novel TxR or an alternative affinity state for the previously characterized high-affinity binding site.