Study of the mechanisms involved in the vasorelaxation induced by (-)-epigallocatechin-3-gallate in rat aorta

Abstract

This study investigated several mechanisms involved in the vasorelaxant effects of (-)-epigallocatechin-3-gallate (EGCG). EGCG (1 microM-1 mM) concentration dependently relaxed, after a transient increase in tension, contractions induced by noradrenaline (NA, 1 microM), high extracellular KCl (60 mM), or phorbol 12-myristate 13-acetate (PMA, 1 microM) in intact rat aortic rings. In a Ca2+ -free solution, EGCG (1 microM-1 mM) relaxed 1 microM PMA-induced contractions, without previous transient contraction. However, EGCG (1 microM-1 mM) did not affect the 1 microM okadaic acid-induced contractions. Removal of endothelium and/or pretreatment with glibenclamide (10 microM), tetraethylammonium (2 mM) or charybdotoxin (100 nM) plus apamin (500 nM) did not modify the vasorelaxant effects of EGCG. In addition, EGCG noncompetitively antagonized the contractions induced by NA (in 1.5 mM Ca2+ -containing solution) and Ca2+ (in depolarizing Ca2+ -free high KCl 60 mM solution). In rat aortic smooth muscle cells (RASMC), EGCG (100 microM) reduced increases in cytosolic free Ca2+ concentration ([Ca2+]i) induced by angiotensin II (ANG II, 100 nM) and KCl (60 mM) in 1.5 mM CaCl2 -containing solution and by ANG II (100 nM) in the absence of extracellular Ca2+. In RASMC, EGCG (100 microM) did not modify basal generation of cAMP or cGMP, but significantly reversed the inhibitory effects of NA (1 microM) and high KCl (60 mM) on cAMP and cGMP production. EGCG inhibited the enzymatic activity of all the cyclic nucleotide PDE isoenzymes present in vascular tissue, being more effective on PDE2 (IC50 approximately 17) and on PDE1 (IC50 approximately 25). Our results suggest that the vasorelaxant effects of EGCG in rat aorta are mediated, at least in part, by an inhibition of PDE activity, and the subsequent increase in cyclic nucleotide levels in RASMC, which, in turn, can reduce agonist- or high KCl concentration-induced increases in [Ca2+]i.

Figure 1

Chemical structure and absolute stereochemistry of (−)-epigallocatechin-3-gallate.

Figure 2

Cumulative concentration–relaxation curves for EGCG (1 μM–1 mM) in intact and endothelium-denuded rat thoracic aortic rings precontracted with NA (1 μM, panel a), PMA (1 μM) in 1.5 mM Ca²⁺-containing solution (panel b) and in Ca²⁺-free medium (panel d) or high extracellular KCl concentration (60 mM, panel c). Each point represents the mean value±s.e.m. (indicated by vertical bars) from five experiments. Level of statistical significance: ^*P<0.05 or ^**P<0.01 with respect to the maximal tension (E_max=100%).

Figure 3

Cumulative concentration–response curves for NA (panel a) and for CaCl₂ (panel b) in endothelium-intact rat thoracic aortic rings in the absence (control) or in the presence of different concentrations of EGCG. Each point represents the mean value±s.e.m. (indicated by vertical bars) from 10 (control with NA) or five (control with CaCl₂ and EGCG-treated rings) experiments. Level of statistical significance: ^*P<0.05 or ^**P<0.01 with respect to control curve.

Figure 4

Effects of EGCG (100 μM) and IBMX (10 μM) on the decrease of cGMP (panel a) and cAMP (panel b) production induced by NA (1 μM) or high KCl (60 mM) in cultured RASMC. Each point represents the mean value±s.e.m. (indicated by vertical bars) from the number of separate experiments shown within parentheses (in each separate experiment, the corresponding sample was assayed in duplicate). Levels of statistical significance: ^#P<0.05 and ^**P<0.01 with respect to control values.

Figure 5

Inhibition of ANGII-induced increase in [Ca²⁺]_i after 30 min pretreatment of RASMC with EGCG. Data are mean±s.e.m. from 14 and 15 cells, respectively.