Macroscopic K+ currents in single smooth muscle cells of the rabbit portal vein

1. Single smooth muscle cells isolated from rabbit portal vein were voltage clamped at room temperature using the whole-cell configuration of the patch-clamp technique. These cells exhibited a mean resting potential of -47.9 mV and a mean input resistance of 376 M omega. 2. Using small tip diameter micropipettes (to avoid dialysis of the cells), depolarizing voltage-clamp pulses from a holding potential of -50 mV elicited two distinct outward currents: a quasi-instantaneous background current and a time-dependent current that did not appear to inactivate (delayed rectifier). Upon return to the holding potential, an outward tail current decaying back to the holding current was observed. 3. The time course of development of the tail current as estimated from envelopes of tail current protocols followed the kinetics of activation of the delayed rectifier elicited during the preceding test pulse. The tail current reversed close to the equilibrium potential for K+ ions indicating that it is mainly carried by potassium ions. 4. Using large tip diameter micropipettes to internally dialyse the cells (EGTA = 0.1 mM; ATP = 5 mM), two additional outward currents having transient kinetics were revealed: a smooth transient outward current (Ito) and spontaneous transient outward currents (STOCs). Ito was found to be mainly selective for K+ ions and exhibited voltage-dependent inactivation with half-maximal availability near -40 mV. 5. Removal of calcium from the bathing solution significantly reduced the background current and abolished both Ito and STOCs. The delayed rectifier current appeared to be insensitive to this procedure. The two types of transient outward currents were never recorded when EGTA was elevated to 5 mM inside the micropipette whereas the background and delayed rectifier currents were not affected. These results suggested that Ito and the spontaneous transient outward currents are activated by internal calcium. 6. External application of TEA (0.5-20 mM) blocked all four outward currents. Calcium replacement by barium significantly reduced the background current and Ito, and had small effects on the delayed rectifier current. When potassium was replaced with caesium (130 mM) and TEA (20 mM) inside the pipette, none of the outward currents described was ever observed. In about 60% of the cells dialysed with this solution a small inward Ca2+ current was revealed. 7. External application of caffeine (5 mM) abolished STOCs in cells in which this activity was present under control conditions. In cells lacking this type of activity under control conditions caffeine induced and later abolished this type of current.(ABSTRACT TRUNCATED AT 400 WORDS)