Properties of the ATP-sensitive K+ current activated by levcromakalim in isolated pulmonary arterial myocytes

Tension and patch clamp recording techniques were used to investigate the relaxation of rabbit pulmonary artery and the properties of the K⁺ current activated by levcromakalim in isolated myocytes. Under whole-cell voltage clamp, holding at –60 mV in symmetrical 139 mm K⁺, levcromakalim (10 m) induced a noisy inward current of –116 ± 19 pA (n = 13) which developed over 1 to 2 min. This current could be blocked by either glibenclamide (10 m) or phencyclidine (5–50 M) and was unaffected when extracellular Ca²⁺ was removed. Both these drugs inhibited the levcromakalim-induced relaxation of muscle strips precontracted with 20 mm K⁺] _o . Application of voltage ramps in symmetrical 139 mm K⁺ confirmed that the levcromakalim-induced current was carried by K⁺ ions and was weakly voltage dependent over the potential range from –100 to +40 mV.The unitary current amplitude and density of the channels underlying the levcromakalim-activated whole-cell K⁺ current was estimated from the noise in the current record. We estimate that levcromakalim caused activation of around 300 channels per cell, with a single channel current of 1.1 pA, corresponding to a slope conductance of about 19 pS. Furthermore, cells dialyzed with an ATP-free pipette solution developed a large noisy inward current at –60 mV, which could subsequently be blocked by flash photolysis of caged ATP. Analysis of the noise associated with this current indicated that the single channel amplitude underlying the ATP-blocked current was 1.4 pA, a value similar to that estimated for the levcromakalim-induced current. We conclude that the conductance of this ATP-sensitive channel is likely to be small under physiological conditions and that it is present at low density.We thank SmithKline & Beecham for the gift of levcromakalim, ICI Pharmaceuticals for the gift of charybdotoxin and Prof. D. Colquhoun for the noise analysis programs. We also thank Mr. R. Davey for technical assistance with tension experiments. This work was supported by the British Heart Foundation and the Wellcome Trust. L.H.C. is a Wellcome Research Fellow and P.L. is an intermediate fellow of the BHF.