A fast transient outward current in cultured cells from human pulmonary artery smooth muscle

A voltage-dependent transient outward current (I(to)) was observed in cells cultured from human pulmonary artery smooth muscle when K+, but not Cs+, was the dominant cation in the pipette solution. In 30% of cells investigated using the Cs+ pipette solution, a tetrodotoxin-sensitive inward current (I(in)) dependent on extracellular Na+ was evoked from depolarizations positive to -30 mV.Iin showed voltage-dependent inactivation with a membrane potential at 50% of the evoked current (V50%) of -75.53 +/- 0.81 mV and slope factor potential (Vs) of -10.73 +/- 0.01 mV. In the presence of 1 microM tetrodotoxin, I(to) was rapidly evoked by depolarizations positive to -40 mV and decayed with a single exponential time course (tau = 9.9 +/- 1.1 ms, pulse potential = +50 mV). I(to) also showed voltage-dependent inactivation with a V50% of -70.9 +/- 2.63 mV and Vs of -7.7 +/- 0.03 mV. I(to) was inhibited concentration dependently by the K+ channel blocker, 4-aminopyridine (4-AP), with a concentration of 4-AP at which I(to) was reduced to 50% of control of 36.5 +/- 2.8 microM. These cells possess voltage-dependent currents characteristic of the K(+)-selective fast transient outward and Na(+)-selective inward currents of smooth muscle cells.