'Peripheral-type' benzodiazepine receptors in the kidney: regulation of radioligand binding by anions and DIDS.

The high density of 'peripheral-type' benzodiazepine receptors (PBR) in the thick ascending loop of Henle (TAL) of the kidney prompted an evaluation of the effects of anions and anion channel blockers on these sites. [3H]Ro 5-4864 binding to rat kidney membranes was inhibited by halides in a concentration dependent fashion. Significant differences were observed in the efficacies (i.e. maximal effect achieved) of these halides to inhibit [3H]Ro 5-4864 binding. The effects of halides to inhibit this binding was manifest as a reduction in the apparent affinity of this radioligand with no effect on the maximum number of binding sites. The concentration necessary to achieve half maximal inhibition was around the physiologic range for Cl- in all extracellular body fluids. A strong correlation (r = 0.95; P less than 0.001) was obtained between the permeability of anions relative to Cl- and their efficacies to inhibit [3H]Ro 5-4864 binding. DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonic acid), an ion transport blocker, inhibited [3H]Ro 5-4864 binding to a maximum of 80%. The presence of Na2SO4 enhanced the potency of DIDS up to five-fold giving an IC50 of 43 +/- 6 microM. In contrast, the binding of [3H]PK 11195 was unaffected by DIDS and only slightly inhibited by I-. These data demonstrate that characteristics of radioligand binding to peripheral-type benzodiazepine receptors in the kidney are regulated by anions and anion transport blockers and suggest that the PBR may play an important role in anion transport. Additionally, these data suggest that the sites at which [3H]Ro 5-4864 and [3H]PK 11195 bind on the PBR may be overlapping, but not identical.