Acid Secretagogue Action of Structurally Gamma-Aminobutyric Acid (GABA)-Related Compounds in Rats Katsuya YAMASAKI and Yoshiaki GOTO* Department of Pharmacology, Tokushima Bunri University, Yamashiro-cho, Tokushima 770, Japan Accepted October 13, 1989 Abstract-The properties of GABA related compounds on gastric function were studied in standardized perfused rat stomach preparations. Intravenous GABA (400 mg/kg) produced a rapid increase in acid secretion. Acid secretagogue actions of 3-aminobutyric acid and 2-aminobutyric acid were less potent than that of GABA. Intravenous injection of 5-amino-n-valeric acid (400 mg/kg) stimulated gastric acid secretion, but 6-amino-n-caproic acid was inactive. Isoguvacine (40 mg/kg, s.c.) stimulated acid output, whereas guvacine, an isomer of isoguvacine, did not. Systemically administered 3-hydroxy-GABA and 8-(p-chlorophenyl) GABA (PCPGABA) showed significant secretagogue actions. Acid responses to GABA-related compounds were significantly reduced by surgical truncal vagotomy and completely antagonized by atropine. The acid responses to PCPGABA and isoguvacine were partially augmented by yohimbine and propranolol. These results suggest that the secretagogue action of GABA is mimicked by structurally GABA-related compounds, which is mediated through cholinergic receptors, with slight implication of alpha-2 and beta-adrenoceptor mechanisms. It is well-established that the central (13, 14). GABAergic mechanisms to stimu nervous system participates in the regulation late acid secretion are, however, complicated of gastric acid secretion (1). The relationships because both types of GABAA and GABAB between neurotransmitters and gastric acid receptor agonists stimulate gastric acid secre secretion were widely studied with regard to tion responses. There is no evidence to show aspects such as inhibitory effects of norepi that GABA mechanisms in the regulation of nephrine (2), dopamine (3), calcitonin-gene gastric secretion is selectively activated by related peptide (CGRP) (4), nicotine (5), GABA or a GABA-moiety. This led us to ex bombesin (6), neurotensin (7) and somato plore the effect of structurally GABA-related statin (8) and secretagogue effects of nicotine compounds on gastric secretion. (9) and thyrotropin-releasing hormone (TRH) In the present study, we examined the ef (10). Gamma -amino-n-butyric acid (GABA) fect of various types of GABA-related com is well-established as an inhibitory transmitter pounds on gastric acid secretion by the route in the CNS. We previously reported that of intravenous administration in most cases GABA produced a dose-dependent increase because of its quick or rapid onset to cause in gastric acid secretion (11 ); and also a certain changes. We also compared the effect lipophilic derivative of GABA, 3-(p-chloro of GABA with those of other amino acids that phenyl)-GABA (PCPGABA), stimulated gas have chemical structures similar to GABA, tric acid secretion more potently at a small with special reference to structure-activity dose (2-8 mg/kg, s.c.) (12). The acid secre relationships, and also attempted to define the tagogue effects of GABA and GAB,^-A-mimetics autonomic neuronal mechanisms involved. have been also confirmed in dogs and humans Materials and Methods * To whoa, correspondence should be addressed . Measurement for gastric acid secretion: Male Wistar rats weighing 180-240 g were The cervical ligation of the esophagus was fasted in meta! cages with mesh bottoms for followed by resections of the cervical truncal 18-24 hr prior to the experiment but were vagi. Vagotomy was performed 30 min prior allowed free access to water ad libitum. Rats toa dministration of GABA or GABA-related were anesthetized with urethane (1.25 g/kg, compounds. i.p.). A cannula was inserted into the trachea Drugs: The drugs used were as follows: 4 followed by the ligation of the esophagus to amino-n-butyric acid (GABA), DL-2-amino avoid the reflex of the perfusate. A laparotomy n-butyric acid (2A-BA), DL-3-amino-n was performed, and the pylorus was ligated, butyric acid (3A-BA), 5-amino-n-valeric acid care being taken not to interfere with the (5A-VA), 6-amino-n-caproic acid (6A-CA), vascular supply and neural connections to the DL-4-amino -3-hydroxy-n-butyric acid stomach. A small incision on the forestomach (GABOB), atropine sulfate, yohimbine hy was made, and the gastrib lumen was gently drochloride, and DL-propranolol hydro rinsed with saline to remove the solid gastric chloride, which were all purchased from contents. A dual polyethylene cannula was Nacalai Tesque Ltd., Japan. Isoguvacine and introduced into the gastric lumen through the guvacine were from Funacoshi Pharm. Co., forestomach. The stomach was perfused with Japan; and DL-4-amino-3-(p-chloro saline, pH adjusted to 5.5 by HCI, using a phenyl)-butyric acid (PCPGABA) was a peristaltic pump at a flow rate of 5 ml/min. generous gift from Ciba-Geigy, U.S.A. The Acid output was determined by titrating the chemical structures of these GABA-related perfusate with 0.02 N NaOH to pH 5.5 using compounds are shown in Fig. 1. The com an automatic titrator (Toa Electronic Co. Ltd, pounds dissolved in saline or distilled water Japan. TSB-10A) and continuously recorded were administered through the femoral vein or in terms of every 2 min response via a zero administered subcutaneously. Pretreatment suppression adaptor (Toa Electronic Co. Ltd, with atropine, yohimbine or propranolol was Japan, C96667T). Acid output was expressed performed 20 min before each secretagogue in terms of /i equivalents of hydrogen ions was administered. (freq.). After stabilization of basal acid secre Statistical analysis: Results are expressed tion for usually 30 min after the initiation of as the mean±S.E. Basal acid output was very the gastric lumen perfusion, each compound low and usually less than 2.0 aeq./10 min. was intravenously or subcutaneously given at For evaluation of drug effects, the net in a volume of 1 or 2 ml/kg. crease in gastric acid output was calculated Fig. 1. Chemical structures of GABA-related compounds used in this study. by subtracting the basal response from the little or no secretagogue actions. Intravenous acid response to each compound. Statistical injection of 5A-VA (400 mg/kg) and GABOB significances were determined by analysis of (200 mg/kg) markedly stimulated gastric variance followed by Dunnett's test for multi secretion, reaching maximal responses of group comparison of parametric data. 12.1±2.0 ueq./10 min (70 min after, N=6) and 7.2±1.5 ueq./10 min (60 min after, N=6), Results respectively. Isoguvacine also showed a Effect of GABA-related compounds on potent secretagogue effect at the dose of 40 gastric acid secretion: Typical responses of mg/kg, s.c. (maximal response: 15.7±3.2 GABA-related compounds on basal acid ueq./10 min at 90 min after the medication, secretion are shown in Fig. 2. GABA (400 N=6), while guvacine had no influence on mg/kg) given intravenously significantly in gastric acid secretion. Dose-response rela creased the gastric acid production. The tions of 5A-VA, GABOB and isoguvacine secretagogure effect was observed with 15 were also confirmed at the dose ranges of min latency and maintained for more than 120 100-400 mg/kg, i.v.; 50-200 mg/kg, i.v.; and min. The maximal response to GABA was 10-40 mg/kg, s.c., respectively. PC PGABA observed 60 min after the administration and was more than 100-fold potent than GABA was 8.4+1.2 ueq./10 min (N=7). The acid on a molecular basis. PCPGABA (4 mg/kg, secretagogue action of GABA was dose s.c.) exhibited secretagogue action and its dependent (100-400 mg/kg, i.v.). The effect peak response was obtained within 90 min of GABA in anesthetized rats was mimicked (25.6±4.3 ueq./10 min (N=10)). Beth by 5A-VA, GABOB, isoguvacine and PCP anechol (1 mg/kg) and histamine (10 mg/kg) GABA. The results obtained with structurally given subcutaneously caused potent acid related compounds are summarized in Fig. 3. increments, and the peak responses were 3A-BA, 2A-BA and 6A-CA, however, had 16.6±1.8 (30 min, N=10) and 17.7±2.9 (70 Fig. 2. Typical secretory responses to GABA, 5-aminovalleric acid (5A-VA), 4-amino-3-hydroxybutyric acid (GABOB) and isoguvacine in stornach perfused rats. Fig. 3. Secretagogue action of GABA-related compounds in rats . Each column represents the ne increase in gastric acid output for 60 min. GABA (400 mg/kg, i.v.), 3A-BA (400 mg/kg , i.v.), 2A-BA (400 mg/kg, i.v.), 5A-VA (400 mg/kg, i.v.), 6A-CA (400 mg/kg, i.v.), GABOB (200 mg/kg, i.v.), iso guvacine (40 mg/kg, s.c.), guvacine (40 mg/kg, s.c.), PCPGABA (4 mg/kg, s.c.), bethanechol (1 mg, kg, s.c.) and histamine (10 mg/kg , s.c.). Table 1. Influence of atropine and vagotomy on gastric acid response to GABA-related compounds in rats Atropine was subcutaneously pretreated 20 min before administration of GABA-related compounds . Each value represents the mean+S .E. of the net increase of acid output (,Ueq./60 min). 5A-VA: 5 aminovaleric acid, PCPGABA: fi-(p-chlorophenyl)-GABA . A significant difference was obtained from the secretagogue alone at the level of *: P 0.05 by Dunnett's test. The number of rats is indicated in parentheses. min, N=10) /eeq./10 min, respectively. compared to vagi-intact rats. When GABA Influence of cholinergic blockades on (400 mg/kg, i.v.), 5A-VA (400 mg/kg, i.v.), gastric acid response to GABA-related com isoguvacine (40 mg/kg, s.c.) and PCPGABA pounds: The results of the effectiveness of (4 mg/kg, s.c.) were given to vagotomized surgical and chemical vagotomy on the acid rats, the secretagogue actions of these com secretion in response to GABA-related com pounds were significantly attenuated, while pounds are shown in Table 1.