On the mechanism of papaverine inhibition of the voltage-dependent Ca++ current in isolated smooth muscle cells from the guinea pig trachea.

The effects of papaverine, a smooth muscle relaxant agent, on the voltage-dependent Ca++ current were examined in isolated smooth muscle cells from the guinea pig trachea. The tight-seal whole cell voltage clamp technique was used. Papaverine (1-100 microM) inhibited the Ba++ inward current (IBa) through the voltage-dependent L-type Ca++ channel in a concentration-dependent fashion. The inhibitory effect of papaverine on IBa appeared to have both tonic and use-dependent components. In addition to the reduction of the maximal conductance of IBa, papaverine (20 microM) shifted the quasi-steady-state inactivation curve of IBa to more negative membrane potentials by approximately 10 mV. These effects of papaverine on IBa were completely reversible. Although it has been suggested that papaverine inhibited phosphodiesterase to increase intracellular cyclic AMP, phosphodiesterase inhibitors (theophylline, 500 microM, and 3-isobutyl-1-methylxanthine, 500 microM), isoproterenol (2 microM) and dibutyryl cyclic AMP (1 mM) did not affect IBa. Thus, papaverine inhibits IBa in a way independent of intracellular cyclic AMP. Papaverine also had inhibitory effects on other membrane currents (i.e., the voltage-dependent transient outward K+ current and the Ca(++)-activated oscillatory K+ current), which may result in an enhancement of the excitability of the cells. These results suggest that inhibition of the voltage-dependent L-type Ca++ channel is involved in the papaverine-induced relaxation of the tracheal smooth muscle.