Effect of Benzodiazepine Ligands on Ca2+ Channel Currents in Xenopus Oocytes Injected with Rat Heart RNA

Voltage-dependent Ca2+ channel (VDCC) currents have been measured in Xenopus oocytes injected with heart RNA purified from 7 day old rats, using voltage clamp technique. The currents were evoked by depolarizing voltage steps, using Ba2+ as charge carrier. Electrophysiological analysis of the current revealed two components: a slow, L-type, dihydropyridine (DHP)-sensitive current and a transient, DHP-insensitive, current. The benzodiazepine (BZ) ligands diazepam and Ro5-4864 decreased the current with micromolar affinity, with potency order of Ro5-4864 greater than diazepam greater than clonazepam. The central antagonist Ro15-1788 did not interfere with the effect of these drugs, thus excluding the possible involvement of the "central type" receptor. The slow current that was increased about 3 fold in the presence of 0.5 microM Bay K 8644, was less potentiated when previously treated with Ro5-4864 or diazepam. Current-voltage relation of the peak inward current and the steady state activation curve showed a small shift towards negative potentials in the presence of 50 microM diazepam. It is concluded that the benzodiazepine ligands block DHP-sensitive voltage dependent Ca2+ channels with a very marginal effect on the transient, DHP-insensitive current. Also it is emphasized that Xenopus oocytes can serve as a useful model system to study the pharmacology of these important drugs on cardiac Ca2+ channels.