5-HT1-like receptor-mediated contraction in the human internal mammary artery

We wished to characterize the 5-hydroxytryptamine (5-HT) receptors mediating vasoconstriction in the human internal mammary artery (IMA). Segments of the IMA obtained from patients undergoing coronary bypass surgery were suspended in an organ bath and exposed to 5-HT and sumatriptan (SUM), a 5-HT1-like receptor agonist, in the presence and absence of potassium chloride (KCI) and angiotensin II. 5-HT induced concentration-dependent contractions in all quiescent and precontracted preparations. SUM induced small contractions in 70% of quiescent IMA rings, whereas it elicited marked and concentration-dependent contractions in all of the preparations given a moderate tone by a threshold concentration of KCI and angiotensin II. The efficacy of SUM was higher in precontracted arteries. Concentration-effect curves (CEC) of 5-HT and SUM were not affected by the 5-HT3-receptor antagonist tropisetron (1 mu M). The nonselective antagonist, methiothepin (30 nM), shifted the CEC of SUM to the right. 5-HT2A-receptor antagonist, ketanserin (1 mu M) inhibited responses to 5-HT, whereas it affected only the responses to the smaller concentrations of SUM. When methiothepin (30 nM) was applied in the presence of ketanserin (1 mu M), a further inhibition in the responses to 5-HT was observed. These results suggest that 5-HT1-like receptors mediate the contractile action of SUM and contribute to that of 5-HT in IMA.