alpha-Adrenoceptor agonists and antagonists are widely used perioperatively for internal mammary artery (IMA)-coronary artery bypass operations. To determine subtypes of alpha-adrenoceptors in the human IMA, we studied responses of isolated human IMA segments to alpha-adrenoceptor agonists, antagonists, and electrical stimulation in organ baths. The IMA ring segments (3 mm long) were set up at a physiologic and comparable condition according to their own length-tension curves. alpha 1-Agonist methoxamine (MO) induced 2.65 +/- 0.70 g force and alpha 1, alpha 2-agonist norepinephrine (NE) induced 4.07 +/- 0.70 g force. The contractions induced by both MO and NE were totally abolished by alpha 1-antagonist prazosin (0.1 microM) but not alpha 2-antagonist yohimbine. alpha 2-Agonist UK14304 induced only 0.39 +/- 0.17 g force, which was significantly less than that induced by MO or NE (p < 0.001). Contractions induced by electrical field stimulation (2, 10, 20 Hz) were decreased by alpha 1-antagonist prazosin 1 microM (p < 0.01) but potentiated by alpha 2-antagonist yohimbine. These results strongly suggest that in the human IMA the postjunctional alpha-adrenoceptors are predominantly of the alpha 1-subtype and therefore the alpha-adrenoceptor agonist-induced contraction and the sympathetic nerve stimulation-induced contraction is mediated mainly by activation of the alpha 1-adrenoceptors.