Distribution of binding sites for thromboxane a₂ in the mouse kidney

Thromboxane A₂ (TxA₂) is a potent vasoconstrictor eicosanoid that has been implicated in the pathogenesis of both human and experimental renal diseases. The biological actions of TxA₂ 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 [¹²⁵I]BOP, TxA₂ 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 [¹²⁵I]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, [¹²⁶I]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.