Endothelium-independent relaxation to raloxifene in porcine coronary artery

Although the vascular action of raloxifene has been studied in several vascular beds, the underlying mechanisms are still incompletely understood. The role of endothelium in raloxifene-induced vascular responses was controversial. The present study was designed to examine endothelium-independent effects of raloxifene in isolated porcine left circumflex coronary arteries. Arterial rings were suspended in organ baths and changes in isometric tension were measured. The large-conductance Ca²⁺-activated K⁺(BK_Ca) currents were recorded using a whole-cell patch-clamp technique. Treatment with raloxifene (1-10 μmol/l) reduced the contractions to 9,11-dideoxy-11α,9α-epoxy-methanoprostaglandin F_2α (U46619), serotonin (5-HT), endothelin-1 in normal Krebs solution and to CaCl₂ in a Ca²⁺-free, high K⁺-containing solution. In endothelin-1-contracted rings, raloxifene (0.3 to 50 μmol/l) caused relaxations which were comparable in rings with and without endothelium. The raloxifene-induced relaxation was reduced by putative K⁺ channel blockers, iberiotoxin and tetraethyl ammonium chloride (TEA⁺) in rings with and without endothelium, or by elevated extracellular K⁺ ions (30 mmol/l K⁺ and 60 mmol/l K⁺). 13-methyl-7-[9-(4,4,5,5,5-pentafluoropentylsulfinyl)nonyl]-7,8,9,11,12,13,14,15,16, 17-decahydro-6H-cyclopenta[a] phenanthrene-3,17-diol (ICI 182,780) did not affect raloxifene-induced relaxation. Raloxifene enhanced the outward BK_Ca currents, which were sensitive to inhibition by iberiotoxin. In summary, the present study shows that raloxifene acutely relaxes porcine coronary arteries via an endothelium-independent mechanism without involving the ICI 182,780-sensitive estrogen receptors. Raloxifene mainly acts on the vascular smooth muscle cells to induce vasorelaxation by the inhibition of Ca²⁺ channels and the activation of BK_Ca channels. The former mechanism appears to play a more significant role.