Epigallocatechin gallate elicits contractions of the isolated aorta of the aged spontaneously hypertensive rat

Abstract

The present study examined the effect of the green tea catechin epigallocatechin gallate (EGCG) on endothelium-dependent responses in the aorta of 36-week-old spontaneously hypertensive rats (SHR). Isometric tension was measured in isolated aortic rings. The release of prostanoid end products was determined using enzyme immunoassay kits and the intracellular reactive oxygen species (ROS) concentration using confocal microscopy. EGCG did not improve endothelium-dependent relaxations evoked by acetylcholine, except in the presence of indomethacin. EGCG did not inhibit endothelium-dependent contractions induced by acetylcholine or ATP. At 10(-6) M and higher concentrations, EGCG caused increases in tension in the SHR aorta. The EGCG-induced contractions were accompanied by an increased production of ROS. The amount of prostanoid end products was increased significantly by EGCG, indicating that their production followed the activation of cyclooxygenase (COX). These prostanoids in turn stimulated thromboxane-prostanoid (TP) receptors and caused contractions. EGCG induced significantly smaller contractions in aortae of normotensive Wistar-Kyoto rats (WKY), accompanied with a lower production of ROS and a lesser release of prostanoids. These observations suggest that EGCG-induced contractions occur more readily in blood vessels of hypertensive than normotensive animals. The present findings indicate that the increased oxidative stress in the ageing hypertensive animals contributes to the loss of the beneficial effects and the enhancement of the adverse effects of EGCG.