Fig. 4. Concentration-effect curves for bupivacaine (  solid squares ) and levobupivacaine (  triangles ) with respect to ³[H]-norepinephrine (NE) overflow. Functions of best fit by least squares method are drawn through the data points and horizontal reference line gives level of ³[H]-norepinephrine release after tetrodotoxin (100 nm  ) application.  Error bars = SD. Six atria were tested in each group.