Two components of potassium current activated by depolarization of single smooth muscle cells from the rabbit portal vein.

1. Using the patch-clamp technique at 20-23 degrees C membrane currents were recorded from single smooth muscle cells enzymatically isolated from the rabbit portal vein. Single-channel currents were observed in outside-out patches excised from these. 2. Outward current elicited upon depolarization from -70 mV was not activated as a result of Ca2+ influx. It could be divided into two components: an inactivating, 4-aminopyridine- and phencyclidine-sensitive low-noise current (IdK), and a non-inactivating, tetraethylammonium (TEA)- and charybdotoxin-sensitive high-noise current (IcK). 3. IdK activated with a threshold around -40 mV and was carried by K+. It was substantially inhibited by 4-aminopyridine (5 mM) or phencyclidine (0.1 mM) but was insensitive to TEA+ (4 mM), charybdotoxin (0.1 microM) or apamin (0.1 microM). Upon stepping to 0 mV it reached a maximum within about 0.2 s. The time course of its activation could be described by a fourth-order single exponential; the time constants of these exponentials changed e-fold every 56 mV. It inactivated in a time- and voltage-dependent manner with a fast and slow component, and was about 50% available at -30 mV. From single-channel recordings in isolated patches single channels underlying this current have a small unitary conductance (around 5 pS). 4. IcK did not inactivate significantly over 6 s. It activated with a less negative threshold than IdK, usually near 0 mV when the pipette solution contained 0.8 mM-EGTA with no added calcium. It was blocked by TEA (4 mM) or charybdotoxin (0.1 microM), but not by 4-aminopyridine (5 mM), phencyclidine (0.1 mM) or apamin (0.1 microM). Estimates of the single-channel conductance from the noise variance of the whole-cell current IcK indicated a value at +80 mV of 115 pS, very similar to that of the large-conductance Ca2(+)-activated K+ channels studied in these cells using single-channel recording. 5. The results suggest that outward current evoked by depolarization from the resting potential can be carried by 100 pS Ca2(+)-activated K+ channels and by small-conductance delayed-rectifier K+ channels. It is likely that opening of both types of channel contributes to the repolarization phase of the action potential in this smooth muscle.