Tissue endothelin-1 levels in perfused rat heart following stimulation with agonists and in ischaemia and reperfusion.

The purpose of this study was to compare coronary and interstitial endothelin-1 (ET-1) levels in perfused rat hearts under several experimental conditions, because the cardiac tissue concentration of ET-1 is not clear. Hearts were perfused in an upside-down position with a colloid-free buffer at a constant flow rate of 9 ml/min/g heart wet weight, and immunoreactive ET-1 was determined in timed collections of coronary effluent and interstitial transudate produced by the ventricles and appearing on their surface. Basal ET-1 release into effluent was 0.26 +/- 0.007 pg/min/g, and 0.005 +/- 0.0012 pg/min/g in transudate. Basal ET-1 concentration was 0.11 +/- 0.005 pg/ml (transudate) and 0.03 +/- 0.002 pg/ml (effluent), indicating four-fold higher transudate than effluent levels (P < 0.05). Following perfusion of hearts with collagenase to remove endothelial cells, ET-1 release into effluent was reduced to one-third and completely abolished in transudate, indicating that the peptide originated from the vascular endothelium. Perfusion of hearts with angiotensin II (0.1 mumol/l) or thrombin (5 U/ml) increased coronary perfusion pressure and ET-1 secretion, but little affected the transudate/effluent ET-1 concentration ratio (5.5 and 3.2, respectively). When coronary flow was reduced to ischaemic level (1 ml/min/g over several hours), ET-1 secretion rates into effluent were decreased by 55-65%, but increased three- to four-fold on reperfusion at normal flow (P < 0.05). The ET-1 concentrations in both fluids were still always below 1 pg/ml. No change in coronary perfusion pressure compared to time-matched normoxic controls was observed. In the presence of the ET-1 converting enzyme inhibitor, phosphoramidon (1.7 mumol/l), ischaemia-induced increases of ET-1 secretion were attenuated, and this was accompanied by a time-dependent rise in coronary perfusion pressure up to 60% (P < 0.05). These are the first measurements of endogenous cardiac tissue ET-1 levels; they do not support a vasoconstrictor (pro-ischaemic) action of endogenous ET-1 in rat hearts following ischaemia/reperfusion, but rather point to a possible vasodilator role of the peptide under these conditions.