Mechanism of the Lower Esophageal Sphincter Relaxation ACTION OF PROSTAGLANDIN E1 AND THEOPHYLLINE RAJ K. GOYAL and SATISH RATrAN From the Gastroenterology Section, Department of Medicine, Baylor College of Medicine, Houston, Texas 77025 A B S T R A C T The intravenous injection of prosta- glandin E1 (PGE1) causes a dose-dependent relaxation of the lower esophageal sphincter (LES) in the intact, lightly anesthetized opossum. The action of PGE1 is not inhibited by the drugs that produce muscarinic or nico- tinic cholinergic antagonism or alpha and beta adrenergic antagonism in the doses that inhibited the action of re- spective agonists. Moreover, this action is not affected by exogenous gastrin pentapeptide. The action of PGE1 on the LES is mimicked by isoproterenol, theophylline ethylenediamine, and dibutyryl cyclic AMP. Both theophylline, a phosphodiesterase inhibitor, and isopro- terenol, an adenyl cyclase stimulator, added to the ac- tion of PGE1. On the other hand, adenyl cyclase in- hibitor nicotinic acid, as well as phosphodiesterase stim- ulator, imidazole inhibited its action. Further, both nico- tinic acid and imidazole inhibited the degree of LES re- laxation produced by esophageal distension. These stud- ies suggest that intracellular cyclic AMP may act as the "second messenger" in the regulation of the lower esophageal sphincter relaxation. INTRODUCTION The lower esophageal sphincter (LES)' relaxes in re- sponse to swallowing to allow the passage of an ingested bolus of food (1), but the mechanism of this relaxation is not well understood. We have found that prosta- glandin E1 (2) and theophylline are potent relaxants of the LES. Beta adrenergic stimulation has previously been reported to cause inhibition of the circular muscle Received for publication 14 July 1972 and in revised form 30 October 1972. 'Abbreviations and trivial namnes used in this paper: cAMP, cyclic AMP; hexamethonium, hexamethylenebis (trimethylammonium chloride); isoproterenol, di-fi- (3,4-di- hydroxyphenyl) -a-isopropylamino ethanol; LES, lower esophageal sphincter; phentolamine, 2- (N- (m-hydroxy- phenyl)-p-toluidinomethyl) imidazoline; propranolol, 1-(iso- propylamino) -3- (1-naphthyloxy) -2-propanol hydrochloride; PGE,, prostaglandin E1; theophylline, 1-3-dimethylxanthine. from the lower esophagus (3) including the sphincteric zone (4). Prostaglandins of E type, theophylline and isoproterenol, have been shown to enhance cyclic AMP in many tissues (5). We present here indirect evidence in an in vivo system which suggests that PGEi, the- ophylline, and isoproterenol may produce LES relaxa- tion by enhancing cyclic AMP in the lower esophageal sphincter. METHODS Studies were done in the opossum (Didelphis virginiana) because the lower part of the esophagus and the lower esophageal sphincter, like that of man, is composed of smooth muscle fibers (6). The animals weighed between 2 and 3.5 kg and were of either sex. The animals were anesthetized with intraperitoneal sodium pentobarbital, 50 mg/kg, and were strapped supine to the animal board. The LES pressure was continuously monitored with a water- filled polyvinyl catheter. An assembly of three polyvinyl catheters (ID = 0.86 mm and OD = 1.17 mm) glued to- gether with tetrahydrofuran, and each with side opening 1 cm apart, was connected to three pressure transducers. The catheters were constantly perfused with bubble-free water at a rate of 4.6 ml/h with a constant infusion pump (Harvard Apparatus Co., Inc., Millis, Mass.). The catheter assembly was introduced through the animal's mouth so that all the openings recorded intragastric pressures. The assembly was then gradually withdrawn so that the proxi- mal opening was in the body of the esophagus, the middle one recorded the highest pressure in the lower high pres- sure zone and the distal one measured intragastric pressures. In initial studies a catheter assembly with openings I cm apart was used in order to evaluate the possibility of the movement of the LES in relation to recording opening. Movement of the catheter assembly would shift the high pressure zone in either the proximal or the distal lead. LES relaxation produced by PGE, and other agents could not be related to such a shift in the high pressure zone. Test substances were injected through an intravenous cannula as a single bolus or by a slow continuous infusion. In some experiments, dibutyryl cyclic AMP and acetyl- choline were injected directly into the left gastric artery which supplies the lower esophagus through its esophageal branch (Fig. 1). 5-10 min of normal base-line pressure was recorded between injections. The LES pressures were measured at end inspiration. The peak change in the sphincter pressure produced after drug administration was The Journal of Clinical Investigation Volume 52 February 1973 337