IDENTIFICATION AND CHARACTERIZATION OF A HIGH-AFFINITY PERIPHERAL-TYPE BENZODIAZEPINE RECEPTOR IN RABBIT URINARY-BLADDER

The present study used radioligand binding and in vitro contractility experiments to identify and characterize a peripheral-type benzodiazepine receptor PBR in rabbit urinary bladder. [H-3]PK11195 bound to bladder membranes with high-affinity and density (K-d = 5.2 nM., B-max = 268 fmol./mg. protein), indicating the presence of a PBR. [H-3]flunitrazepam bound with high-affinity and density (K-d = 1.2 nM., B-max = 48 fmol./mg. protein). The rank order potency of various benzodiazepines and isoquinoline carboxamides in displacing the binding of [H-3]PK11195 was Ro5-4864 > diazepam = flunitrazepam >> Ro15-1788 = clonazepam. Ro5-4864 and PK11195 inhibited nerve-evoked contractions in a concentration-dependent manner (IC50 = 42 mu M. and 56 mu M., respectively). Carbachol- and KCl-induced contractions were also inhibited by Ro5-4864 and PK11195. KCl-induced contractions were inhibited to a greater extent than carbachol-induced or field-stimulated contractions with all the drugs tested. Both Ro5-4864 and PK11195 significantly increased the ED(50) for calcium-induced contractions following a cholinergic stimulus compared with control. These data demonstrate the presence of a PBR in urinary bladder capable of altering contractility in vitro through modulation of calcium activity.