Actions of cromakalim on ionic currents recorded from single smooth muscle cells of the rat portal vein

The effects of cromakalim (BRL34915), a novel K channel opener, on ionic currents in dispersed smooth muscle cells of the rat portal vein were investigated. Application of cromakalim (>3 μM) generated an outward current, the reversal potential of which (-79 mV) was almost the same as the theoretical K equilibrium potential (-80 mV) under the experimental conditions used. When the Ca in the physiological salt solution (PSS) was replaced with Mn, the cromakalim-induced outward current was inhibited markedly. When 4 mM ethylene glycol bis(β-aminoethyl ether(N,N'-N,N'-tetraacetic acid (EGTA) was added to the pipette solution (bath solution, Ca-free PSS + Mn), cromakalim inhibited the outward current evoked by command pulses. Using a pipette solution containing high Cs with 4 mM EGTA and a bath PSS containing 2.5 mM Ca with 1 μM tetrodotoxin, the Ca inward current was isolated. Cromakalim (>3 μM) inhibited the Ca inward current in a voltage-dependent manner. Cromakalim (30 μM) inhibited the Ca inward current to 0.6 times the control. The decay of the inward Ca current comprised fast and slow components. Cromakalim inhibited the latter component and shifted the voltage-dependent inactivation curve of this current to the left (more hyperpolarized direction) in a parallel manner and delayed recovery from inactivation as estimated using a double pulse protocol. By using a pipette solution containing high Cs with 4 mM EGTA and Ca-free PSS containing 2.5 mM Mn in the bath, Na inward currents, blocked by tetrodotoxin (IC₅₀ = 10 nM), were evoked by depolarizing pulses. Cromakalim (up to 30 μM) had no effect on the Na inward current. Thus, the smooth muscle relaxant actions of cromakalim under physiological conditions may not be caused solely by the opening of K channels and membrane hyperpolarization but in part by the inhibition of Ca inward currents.