5-HT1-LIKE RECEPTOR-MEDIATED CONTRACTION IN THE HUMAN INTERNAL MAMMARY ARTERY

We wished to characterize the 5-hydroxytryptamine (5-HT) receptors med iating vasoconstriction in the human internal mammary artery (IMA). Se gments of the IMA obtained from patients undergoing coronary bypass su rgery were suspended in an organ bath and exposed to 5-HT and sumatrip tan (SUM), a 5-HT1-like receptor agonist, in the presence and absence of potassium chloride (KCI) and angiotensin II. 5-HT induced concentra tion-dependent contractions in all quiescent and precontracted prepara tions. SUM induced small contractions in 70% of quiescent IMA rings, w hereas it elicited marked and concentration-dependent contractions in all of the preparations given a moderate tone by a threshold concentra tion of KCI and angiotensin II. The efficacy of SUM was higher in prec ontracted 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) i nhibited responses to 5-HT, whereas it affected only the responses to the smaller concentrations of SUM. When methiothepin (30 nM) was appli ed 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 tha t of 5-HT in IMA.