Vol. 27, No. 2 Printed in Japan

Gastroenterologia Japonica Copyright 9 1992 by The Japanese SocieO~ of Gastroenterology

Effect of luminal administration of thyrotropin-releasing hormone or somatostatin on gastric pH and interaction of these peptides in rats Atsushi FURUSAWA, Kimitomo MORISE, Yoshiaki MAEDA 1, Kiyoshi UCHIDA, Hiroshi KANEKO, Kouirhi NAKADA, and Terunori MITSUMA 2 /First Department of Internal Medicine, Nagoya University School of Medicine, Nagoya, Japan; and eFourth Department of Internal Medicine, Aichi Medical University, AichL Japan

Summary: The effect of intraluminal administration of thyrotropin-releasing hormone (TRH) on gastric pH and release of luminal somatostatin, and a possible interrelationship between TRH and somatostatin in the rat stomach were studied. TRH was administered into the stomach via an intragastric tube at various doses (50 pg/kg-10/zg/kg) and gastric pH was measured after 15 min. The intraluminal administration of TRH significantly decreased gastric pH at doses over 1.0 ng/kg. Time-course studies at a dose of 100 ng/kg TRH exhibited a significant decrease in gastric pH at 15, 30 and 60 min. Furthermore, TRH administration caused a significant increase in immunoreactive-somatostatin (ir-somatostatin) concentrations in the gastric wall and a significant decrease in ir-somatostatin concentrations in the gastric juice. On the other hand, intraluminal administration of somatostatin caused a significant increase in ir-TRH concentrations in the gastric wall and a significant decrease in ir-TRH concentrations in the gastric juice, and significantly raised gastric pH at 5 rain. These findings suggest that luminal TRH may exert a regulatory effect on gastric acid secretion, and that TRH may have a possible interaction with somatostatin in the modulation of gastric acid secretion. Gastroenterol Jpn 1992;27:165-171. Key words:

gastric pH; luminal administration; rat stomach; somatostatin; thyrotropin-releasing hormone

Introduction Thyrotropin-releasing h o r m o n e (TRH) is a tripeptide that was originally isolated from the hypothalamus 1. TRH-like immunoreativity is widely distributed t h r o u g h o u t the central nervous system, gastrointestinal (GI) tract, and pancreas of a variety of m a m m a l i a n species 2'3. Recent studies in laboratory animals have demonstrated that various peptides including gastrin are released into the gastric l u m e n under various conditions 4. We have previously reported the presence of TRH in the gastric juice 5'6 and demonstrated that histamine stimulated TRH release into the gastric juice from the rat stomach wall5. T R H has been shown to be released into the gastric juice from

the rat stomach wall under water-immersion stress 6. However, the physiological significance of T R H in gastric juice has not yet been documented. Somatostatin is also present in the gastric wall7 and gastric juice 4, and exerts a restraint on acid secretion 8. It has been proposed that the mechanism of gastric acid suppression by somatostatin may be involved in a direct action of somatostatin on parietal cells and an inhibitory effect on gastrin release 9. Therefore, we designed experiments to determine whether intraluminal administration of T R H or somatostatin had any effect on gastric acid secretion, and w h e t h e r TRH and somatostatin had any interaction in the rat stomach.

Received April 25, 1991. Accepted October 18, 1991. Address for correspondence: Atsushi Furusawa, M.D., The First Department of lnternal Medicine, Nagoya University School of Medicine, 65 Tsurumai-cho, Showa-ku, Nagoya 466, Japan.

166

A. Furusawa et al.

Materials and Methods

1) Animals Male Sprague-Dawley rats (SLC INS. Shizuoka, Japan), weighing 200-250 g, were kept in temperature (23~ - and humidity (60%)-controlled rooms lighted from 9.00 to 21.00 h and fed laboratory chow and water ad libitum. The rats were deprived of food overnight before the experiment.

2) Neuropeptides Synthetic TRH and somatostatin were purchased from The Peptide Institute, Inc. (Osaka, Japan).

3) Experimental design After light anesthesia with pentobarbital (20 mg/kg), a stainless steel cannula (2 m m external diameter) was inserted through the m o u t h and placed in the fundic region. Synthetic TRH and somatostatin freshly dissolved in 0.5 ml of 1 mM HC1 (pH 3.0) or 0.5 ml of vehicle (1 mM HC1, pH 3.0) were administered via the cannula. After the administration, gastric juice was obtained by the technique previously reported 5. Briefly, laparotomy was done by a midline incision and a new cannula was introduced through the pylorus via duodenotomy. The gastric juice was collected and centrifuged for 10 min at 2,000 rpm at 4~ The pellet was discarded and gastric pH was measured with a digital pH meter (CG 817, Schott Ger~ite, Hofheim, West Germany). The rats were sacrificed and their stomachs were carefully removed. The glandular stomach was excised from the remaining stomach. All specimens were stored at - 2 0 ~ until assayed 1~ a) TRH administration The rats were divided into two groups. Group 1 was composed of 72 rats. They were given vehicle or TRH (50pg/kg-10/zg/kg) and sacrificed 15 min after the administration. Group 2 was composed of 80 rats and given TRH (100 ng/kg) or vehicle. Eight rats in each subgroup were sacrificed at 0, 5, 15, 30 and 60 rain, respectively, after the administration. The rats in the 0 rain subgroup were sacrificed immediately after the administration. The vehicle-treated group in the

Vol. 27, No. 2

time-course study was also adopted as controls for the time-course experiment of somatostatin. b) Somatostatin administration Eighty eight rats were divided into two groups. Group 1, composed of 48 rats, was given somatostatin (0.1 ng/kg-10 ng/kg), and eight rats in each subgroup were sacrificed at 0 and 15 min, respectively. In group 2, 40 rats were given somatostatin (1.0 ng/kg) and 8 rats in each subgroup were sacrificed at 0, 5, 15, 30 and 60 min, respectively, after the administration.

4) Extraction and assay of TRH TRH was extracted from the gastric wall and gastric juice as previously described 5'6, and measured by a radioimmunoassay (RIA) 1~ I r - T R H concentrations in the gastric wall were expressed as pg/mg wet weight (pg/mg w.w.), and ir-TRH concentrations in the gastric juice were expressed as pg/ml. In this assay system, 2.0 pg/ml of TRH was consistently detected. The recovery rate of this extraction method was 70.3_+4.0% (mean_+ SD) (n=6).

5) Extraction and assay of somatostatin Somatostatin from the gastric wall and gastric juice were extracted by the m e t h o d previously described, and measured by a RIA u. T h e concentrations of somatostatin in the gastric wall were expressed as pg/mg wet weight (pg/mg w.w.) and pg/ml in the gastric juice. The antibody used in this experiment cross-reacted 0.4% with somatostatin-28 on a mass basis, but not other peptides. The recovery rate was 50_+ 5% (mean_+ SD) (n=6).

6) Stability of TRH and somatostatin in the gastric juice Stability of TRH and somatostatin in the gastric juice was examined by the following methods. Ten ng/ml of synthetic TRH or 100 n g / m l of synthetic somatostatin was added to the gastric juice which was adjusted to pH 2.0 or pH 7.0. The gastric juice was incubated for 15, 30 or 60 m i n at 37~ and TRH and somatostatin in gastric juice were measured by a RIA. The results were expressed as a percentage recovery of TRH or somatostatin added.

167

Luminal TRH and somatostatin

April 1992

(%) 100 1

4-

-L

_L ~-'--'I"----7/

.L "r 3 o-

60 H

40

p H 2.0

2.

o - - o p H 7.0

(.9

20

1

0

0'

ll5

30

2( ()

60 min

Stability of TRH

801

5

J-

i

9

)2

60

H

,10

0'

30

60

min

p H 2.0

o - < , p H 7,0

20 0

15

Fig. 3 Changes in gastric pH after intraluminal administration of TRH (100 ng/kg). F-I; Vehicle-treated group, Q; TRH administered group. Values are expressed as the mean+SE in each group of 8 rats. **P

Effect of luminal administration of thyrotropin-releasing hormone or somatostatin on gastric pH and interaction of these peptides in rats.

The effect of intraluminal administration of thyrotropin-releasing hormone (TRH) on gastric pH and release of luminal somatostatin, and a possible int...
525KB Sizes 0 Downloads 0 Views