Characterization of peripheral benzodiazepine receptors in purified large mammal pancreatic islets.

In this work, we evaluated the biochemical properties of peripheral benzodiazepine receptors (PBRs) in the porcine endocrine pancreas and their role in insulin release. Binding of [3H]1-(2-chlorophenyl-N-methyl-1-methyl-propyl)-3-isoquinolinecarboxa mide ([3H]PK-11195), a specific ligand of PBRs, to islet membranes was saturable and Scatchard's analysis of saturation curve demonstrated the presence of a single population of binding sites, with a dissociation constant (Kd) value of 4.75 +/- 0.70 nM and a maximum amount of specifically bound ligand (Bmax) of 4505 +/- 502 fmol/mg of proteins. The pharmacological profile of PBRs was determined as the ability of PK-11195 and several benzodiazepine compounds to displace [3H]PK-11195 from these binding sites. The rank order of potency yielded the following affinity results: PK-11195 > 7-chloro-1,3-dihydro-1-methyl-5-(p-chlorophenyl)-2H-1,4-benzodiazepine-2 -on (Ro 5-4864) > diazepam > or = flunitrazepam >> flumazenil. Secretion studies demonstrated that PK-11195 (1 and 10 microM) and Ro 5-4864 (10 and 50 microM) significantly potentiated insulin secretion from freshly isolated porcine islets at 3.3 mM glucose. This potentiating effect was not observed at 16.7 mM glucose concentration nor by the addition of clonazepam. These results show the presence of PBRs in purified porcine pancreatic islets and suggest an implication of PBRs in the mechanisms of insulin release.