Characterisation of the relaxant response to raloxifene in porcine coronary arteries

The present study characterises the vasorelaxant response to raloxifene in isolated rings of porcine coronary artery. Tissues precontracted either with KCl (30 mM) or prostaglandin F2α (PGF2α; 3 μM) were concentration-dependently relaxed by raloxifene (0.1–10 μM). Relaxation was not inhibited by the estrogen receptor antagonist 7α-[9-[(4,4,5,5,5-pentafluoropentyl)sulfinyl]nonyl]-estra-1,3,5(10)-triene-3,17β-diol (ICI 182,780; 1 μM). Preincubation with raloxifene (1–3 μM) caused an inhibition of the KCl or PGF2α-induced contraction. The effects of raloxifene were independent of the endothelium. The relaxant response to raloxifene was slow in the onset and could not be reversed after repeated washings. Raloxifene did not affect Ca2+ release from intracellular stores since it failed to inhibit a transient contraction induced by caffeine (10 mM). Raloxifene-induced relaxation was not influenced by the intracellular calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid tetrakis(acetoxymethyl ester) (BAPTA-AM; 10–20 μM). Calcium-induced contractions in Ca2+-free high K+ (60 mM) depolarising medium were concentration-dependently inhibited by raloxifene (0.3–3 μM). If arterial rings were incubated with the L-type Ca2+ channel activator (S)-(–)-1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)phenyl]-3-pyridine carboxylic acid methyl ester ((S)-(–)-Bay K 8644; 0.1 μM), cumulative concentration–response curves to Ca2+ were shifted to the left. Raloxifene (0.3–3 μM) inhibited the effect of (S)-(–)-Bay K 8644 in a concentration-dependent manner. 4-(4-Fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole (SB 203580; 10 μM), an inhibitor of p38 mitogen-activated protein kinase (MAPK), diminished raloxifene-induced relaxation in endothelium-denuded arterial rings. Western blot analysis demonstrated that raloxifene stimulated p38 MAPK. It is concluded that raloxifene has an inhibitory effect on voltage-gated and receptor-operated L-type Ca2+ channels in porcine coronary arteries, thus inducing vascular relaxation independent of the endothelium. p38 MAPK is, at least in part, involved in the relaxant response to raloxifene.