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(Circulation Research. 1997;81:765-773.)
© 1997 American Heart Association, Inc.

Articles

Expression, Function, and Regulation of E-Type Prostaglandin Receptors (EP₃) in the Nonischemic and Ischemic Pig Heart

Thomas Hohlfeld, Tom-Philipp Zucker, Jutta Meyer, , Karsten Schrör

From the Institut für Pharmakologie, Heinrich-Heine-Universität Düsseldorf (Germany).

Correspondence to Dr K. Schrör, Institut für Pharmakologie, Heinrich-Heine-Universität Düsseldorf, Moorenstr. 5, D-40225 Düsseldorf, Germany.

Abstract The action of prostacyclin, prostaglandin E₁ (PGE₁), and their mimetics on myocardial function includes changes in contractility, electrophysiological properties, and protection from injury caused by transient myocardial ischemia. This study was undertaken to investigate the basic properties of myocardial E-type prostaglandin (EP) receptors. Ligand binding studies using an enriched preparation of sarcolemmal membranes prepared from pig hearts revealed a single class of binding sites for [³H]PGE₁, with a K_d of 3.7 nmol/L and a B_max of 92 fmol/mg protein. Competition experiments indicated highest affinity for EPs, suggesting an EP receptor. In addition, the EP receptor subtype–selective agonists sulprostone (EP₁ and EP₃) and M&B 28.767 (EP₃) were active, suggesting the presence of an EP₃ receptor subtype. PGE₁ stimulated sarcolemmal GTPase and inhibited sarcolemmal adenylyl cyclase activity, indicating EP₃ receptor coupling to an inhibitory G protein (G_i). Additional in vivo experiments showed that intracoronary infusion of PGE₁ (1 nmol/min) decreased isoprenaline-stimulated left ventricular contractile activity without altering systemic vascular resistance. This inhibition of ß-adrenergic effects is compatible with the known myocardial anti-ischemic action of prostaglandins. Further experiments examined EP₃ receptor density and G-protein coupling in sarcolemma from ischemic and reperfused ischemic myocardium. In anesthetized open-chest minipigs, occlusion of the left anterior descending coronary artery for 60 minutes increased EP₃ receptor density by 50%, whereas receptor affinity was unchanged. This upregulation was prevented by pretreatment with colchicine (2 mg/kg IV), indicating microtubule-dependent receptor externalization. Northern hybridization showed comparable EP₃ receptor mRNA expression in control and ischemic myocardium. The increase of receptor protein was reversed during 60 minutes of reperfusion. G-protein coupling proved to be intact in ischemic and reperfused ischemic myocardial tissue, as shown by preserved GTP--S–induced decrease of [³H]PGE₁ binding. These data demonstrate for the first time that myocardial receptors for PGE₁ belong to the EP₃ subtype. The properties of this receptor include inhibition of adenylyl cyclase and upregulation during regional myocardial ischemia, suggesting an involvement in the anti-ischemic activity of E- and I-type prostaglandins.

Key Words: E-type prostaglandin receptor • prostaglandin E₁ • sarcolemma • adenylyl cyclase • myocardial ischemia