Fig. 6. Effect of morphine on inward rectifier K⁺current (I^K1). (  A ) I^K1traces 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 I^K1density 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 I^K1from  B (  a ) to show the difference before and after the application of 0.1 μm morphine. (  C ) Mean values of relative I^K1at −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 I^K1by 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.