Reduction of infarct size by selective stimulation of prostaglandin EP(3)receptors in the reperfused ischemic pig heart.

We have previously identified prostaglandin EP(3)receptors in left ventricular myocardium. To assess the potential contribution of this receptor subtype to the anti-ischemic properties of E-type prostaglandins (i.e. PGE(1)), two groups of anesthetized open-chest minipigs were subjected to LAD occlusion (1 h) and reperfusion (3 h). In one group, the selective EP(3)receptor agonist M&B 28.767 (2 pmol/kgxmin) was infused into the LAD from 20 min before ischemia until the end of reperfusion. The other group received vehicle. M&B 28.767 did not alter the systemic hemodynamics, but significantly reduced infarct size (tetrazolium staining) and creatine kinase release by 53% and 48%, respectively. Ischemia-induced ventricular arrhythmias were mostly reduced. Further experiments analysed the effects of EP(3)receptor stimulation on normoxic myocardium. PGE(1), an unselective agonist to all EP receptor subtypes, as well as M&B 28.767 (2 pmol/kgxmin of each into the LAD) reduced the action potential duration (epicardial monophasic electrodes) and almost prevented the inotropic response to intravenous isoprenaline. This dual response is consistent with the EP(3)receptor coupling to an inhibitory G protein. This was confirmed in separate experiments with stable Chinese hamster ovary cell transfectants expressing the porcine EP(3)receptor, where M&B 28.767 inhibited the forskolin-induced increase in cAMP in a concentration-dependent manner. It is concluded that the protection of reperfused ischemic myocardium by E-type prostaglandins is mediated by EP(3)receptors, which seems to involve a combined activation of repolarizing membrane currents and an inhibition of deleterious effects caused by ischemia-induced catecholamine release.