Abstract
1. The effects of fenoverine, an antispasmodic drug, have been studied on the Ca2+ channel currents of isolated cells from rat portal vein and pregnant myometrium by the patch-clamp technique (whole-cell configuration). 2. Fenoverine inhibited both fast and slow Ca2+ channel currents in a concentration-dependent manner. Half-inhibition of fast Ca2+ channel current (holding potential of -70 mV) and slow Ca2+ channel current (holding potential of -40 mV) in portal vein smooth muscle were obtained at concentrations of 7.5 and 1.9 microM, respectively. In myometrium, the fenoverine concentration which blocked 50% of the slow Ca2+ channel current (holding potential of -70 mV) was 2.3 microM. 3. Administration of fenoverine at rest reduced both Ca2+ channel currents. Currents activated repetitively, at a rate between 0.05 and 0.1 Hz, were inhibited equally which indicates an absence of use-dependent inhibition. 4. When cells held at depolarized membrane potentials at which fast or slow Ca2+ channel currents were strongly inactivated, the inhibitory effects of fenoverine were enhanced on both Ca2+ channel currents which indicates that the fenoverine-induced inhibition was voltage-dependent. The fenoverine concentrations which blocked the inactivated Ca2+ channels were 5-7 times lower than those which blocked the resting Ca2+ channels. 5. Our results show that fenoverine depresses inward currents through fast and slow Ca2+ channels. This effect may be explained by the preferential binding of fenoverine to resting Ca2+ channels. In addition, fenoverine has a higher affinity for inactivated Ca2+ channels than for resting channels.