EFFECTS OF CA2+ CHANNEL ANTAGONISTS AND BENZODIAZEPINE RECEPTOR LIGANDS IN NORMAL AND SKINNED RAT URINARY-BLADDER

The effects of Ca2+ channel antagonists and benzodiazepine receptor ligands against concentration-dependent contractions of rat urinary bladder induced by CaCl2 (0.1-50 mM, in K+-depolarized tissues), KCl (1-100 mM) and acetylcholine (0.1 mu M to 1 mM) were studied. Nifedipine (0.001-0.1 mu M), verapamil (0.01-1 mu M), diltiazem (0.01-1 mu M), cinnarizine (1-100 mu M), and trifluoperazine (1-100 mu M) each produced a concentration-related inhibition of the log concentration-effect curve for CaCl2. The rank order of potencies of these antagonists, measured as the IC50 against Ca2+ (25 mM)-induced contraction of depolarized bladder, was nifedipine (0.01 mu M) > diltiazem (0.36 mu M) = verapamil (0.41 mu M) greater than or equal to cinnarizine (2.57 mu M) > trifluoperazine (17.4 mu M). These antagonists depressed KCl-induced contractions with an effectiveness and potency similar to that displayed against CaCl2-induced contractions. Nifedipine, verapamil, and diltiazem but not cinnarizine and trifluoperazine had a preferential inhibitory effect on the contractions elicited by KCl when compared to those elicited by acetylcholine. Ro 5-4864, diazepam, midazolam and the non-benzodiazepine PK 11195, each at 1-100 mu M, depressed CaCl2- and KCl-induced contractions (IC50 values in the micromolar range). Benzodiazepines and PK 11195, all at 100 mu M, markedly depressed. acetylcholine-induced contractions. Flumazenil was scarcely effective. Cinnarizine (100 mu M) and trifluoperazine (100 mu M), but not the other Ca2+ channel antagonists and benzodiazepine receptor ligands tested, depressed Ca2+ (20 mu M)-evoked contractions of skinned bladder. It is concluded that the action of nifedipine, verapamil, and diltiazem is restricted to the plasmalemma whereas cinnarizine and trifluoperazine also act on the intracellular contractile apparatus. The inhibitory effects of micromolar concentrations of benzodiazepines appear to be related to an interaction with low-affinity sites functionally linked to L-type Ca2+ channels but intracellular actions are present at high concentrations.