Fig. 6. Effect of morphine on inward rectifier K+current (IK1). ( A ) IK1traces recorded using 300-ms voltage steps to between −120 and 0 mV from a holding potential of −40 mV in a representative ventricular myocyte during control, after the application of 0.1 μm morphine (6 min), and after washout of the drug (10 min). ( B ) ( a ) Current–voltage relation of IK1density determined with the same protocol as A in the absence ( open circles ) and presence of 0.1 μm morphine ( filled circles ). ( b ) Expanded plots for the outward components of IK1from B ( a ) to show the difference before and after the application of 0.1 μm morphine. ( C ) Mean values of relative IK1at −60 mV (n = 12) in the absence and presence of 0.1 μm morphine, and opioid receptor antagonists, i.e. , 5 μm nor-binaltorphimine (nor-BNI), 5 μm naltrindole (NTD), or 5 μm CTOP. The augmentation of IK1by morphine was not prevented by the pretreatment (5 min) with any opioid receptor antagonist. Data are expressed as mean ± sd. * P < 0.05 versus control.