The peripheral, high affinity benzodiazepine binding site is not coupled to the cardiac Ca2+ channel.

Ro 5-4864, the prototype ligand of the peripheral benzodiazepine binding site caused a decrease of the action potential duration in isolated guinea-pig cardiac myocytes. Voltage-clamp experiments showed that, at concentrations below 3 X 10(-6) M, Ro 5-4864 caused a parallel outward shift of the membrane current elicited by depolarization to + 10 mV from a holding potential of -50 mV. The peak inward Ca2+ current (ICa) and the inwardly rectifying K+ current were not affected. ICa was reduced by Ro 5-4864 at concentrations above 3 X 10(-6) M. At these concentrations, Ro 5-4864 also caused a negative inotropic effect in isolated guinea-pig papillary muscles, reduced K+ depolarization-induced contractures of the isolated rat aorta and inhibited [3H]nitrendipine binding to guinea-pig cardiac membranes. These data provide no evidence that the peripheral high affinity benzodiazepine binding site is coupled to the cardiac Ca2+ channel. The possibility cannot be excluded that a postulated micromolar affinity benzodiazepine receptor is associated with the Ca2+ channel.