1 Smooth muscle cells of the rat portal vein were dispersed by enzymatic treatment and recordings of whole-cell currents were made by the voltage-clamp technique. The effects of the potassium (K) channel openers, P1060 (0.3-10 mum) and aprikalim (3-30 mum) on these currents were investigated. Antagonism of these agents by glibenclamide and phentolamine was also studied. 2 When cells were clamped at - 10 mV, P1060 (I mum) and aprikalim (3 mum) each induced a slowly-developing K-current (I(KCO)), the noise of which gradually increased. The rate of onset Of I(KCO) was greater for P1060 than for aprikalim. Current-voltage plots showed that P1060 and aprikalim each caused an approximately 25 mV negative shift of the reversal potential at zero current. 3 P1060 (1 mum) and aprikalim (3 mum) each inhibited the slowly activating, slowly inactivating delayed rectifier current, I(TO). 4 Addition of MgATP (5mm) to the recording pipette inhibited the generation of I(KCO) by P1060 (I mum) and reduced the accompanying inhibition of I(TO). 5 Stationary fluctuation analysis of the current, noise associated with I(KCO) induced by P1060 (1 mum) or aprikalim (3 mum) at a holding potential of - 10 mV indicated that the unitary conductance of the underlying K-channels was 10.5 pS at 0 mV under the quasi-physiological conditions of the experiment. 6 In the absence of K-channel openers, neither phentolamine (30-1 00 muM) nor glibenclamide (1 mum) affected the magnitude of control non-inactivating currents. However, phentolamine (30-100 mum), but not glibenclamide (1 mum) inhibited the control delayed rectifier current, I(TO). 7 After induction of I(KCO) by P1060 (I mum) or aprikalim (3 mum), subsequent exposure to glibenclamide (I muM) or phentolamine (30 mum) inhibited this current. After aprikalim-induced reduction of I(TO) had developed, subsequent exposure to glibenclamide was able partially to reverse the inhibition of I(TO) whereas phentolamine was without effect. Pre-exposure to glibenclamide (1 mum) prevented both the generation of I(KCO) by aprikalim (3 mum) and the inhibitory effect of this agent on I(TO). 8 It is concluded that P1060 and aprikalim each induce the current I(KCO) by opening the same small conductance, ATP-sensitive K-channel (K(ATP)), an effect which can be inhibited by glibenclamide or phentolamine. The opening of K(ATP) by both P1060 and aprikalim probably involves competition between these agents and ATP for the ATP-control site associated with the channel. Inhibition of the delayed rectifier current, I(TO), by P1060 and aprikalim was glibenclamide-sensitive and may be caused by the induction of a state of run-down in the channel which underlies this current.
