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Figure 4 Effect of beta₃-adrenoceptor (ß₃-AR) agonists in human internal mammary arteries (IMA). (A) Typical records of relaxant effect of SR 58611A, a preferential ß₃-AR agonist, and acetylcholine (1 µmol/l) in human IMA rings. (B) Concentration-response curves to SR 58611A performed in the absence or in the presence of 10 µmol/l nadolol, an ß₁ and ß₂-AR antagonist, and 3 µmol/l L-748,337, a human ß₃-AR antagonist, in human IMA rings constricted with 1 µmol/l phenylephrine. The mean curves resulting from subtraction of the spontaneous relaxation of control vessels are shown. Results are expressed as percentage of relaxation from the contraction level induced by phenylephrine. Each point is the mean value of six experiments, and error bars represent SEM. *p < 0.05 and **p < 0.01 indicate significant differences from response to SR 58611A alone.