Benzodiazepine receptors along the nephron: [3H]PK 11195 binding in rat tubules

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Abstract
Binding of [3H]PK 11195, an isoquinoline carboxamide derivative, was measured in microdissected tubule segments of rat nephron. High specific binding capacities (1.1–1.8 fmol·mm−1) were found in the thick ascending limb of the Henle's loop and in the collecting tubule, whereas specific binding could not be detected in the proximal tubule. In the medullary collecting tubule, the association and dissociation rate constants at 4°C were k 1 = 3.0 × 106 M−1·min−1 and k −1 = 0.021 min−1; the ratio k −1/k 1 = 7.0 nM was in agreement with the estimated equilibrium dissociation constant (K d = 2.4 nM). [3H]PK 11195 binding sites from medullary ascending limb and medullary collecting tubule revealed the following sequence of specificity: PK 11195 = Ro 5‐4864 ⪢ clonazepan, indicating that tubule binding sites might be the peripheral benzodiazepine receptors of the rat kidney.