Koichiro Takeda

Dual effect of trimebutine on contractility of the guinea pig ileum via the opioid receptors

Preparations of longitudinal muscle attached to myenteric plexus from guinea pig ileum were used to observe the effect of trimebutine on intestinal motility. Electrical stimulation at 0.2 Hz and 5 Hz produced contraction mediated by the release of acetylcholine in the preparations. The response to low-frequency stimulation (0.2 Hz) was inhibited by trimebutine (10(-8)-10(-5) mol/L), and the response to high-frequency stimulation (5 Hz) was enhanced by the drug at low concentrations (10(-8)-10(-7) mol/L) and inhibited by high concentrations (10(-6)-10(-5) mol/L). This enhancement was mimicked by [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin, and was antagonized by naloxone but not by MR2266. Enhancement by trimebutine was inhibited by yohimbine. Trimebutine (greater than or equal to 10(-8) mol/L) inhibited stimulation (5 Hz)-evoked release of norepinephrine, and the trimebutine effect was antagonized by naloxone but not by MR2266. Low concentrations of trimebutine inhibit norepinephrine release via the mu-opioid receptor and enhance intestinal motility by preventing the adrenergic inhibition of acetylcholine release. Inhibition by trimebutine was antagonized either by naloxone or MR2266. High concentrations of trimebutine may inhibit acetylcholine release via the mu- and kappa-opioid receptors, after which the intestinal motility is inhibited. Trimebutine at further high concentrations (greater than 10(-5) mol/L) contracted single smooth muscle cells from the circular muscle layers but not from the longitudinal muscle layers. The usual dose of trimebutine may exert dual effect on the intestinal motility indirectly through cholinergic and adrenergic neurons without direct effect on the smooth muscle.

Expression of the GABAB receptor in Xenopus oocytes and inhibition of the response by activation of protein kinase C

The functional GABAB receptor was expressed in Xenopus oocytes by injecting mRNA obtained from the cerebellum of the rat. Application of GABA in the presence of bicuculline induced a hyperpolarization under current‐clamp conditions and an outward current under voltage‐clamp conditions. Baclofen mimicked the effect of GABA in the presence of bicuculline, and the effect of baclofen was antagonized by phaclofen. The GABA‐induced outward current was slightly inhibited by treatment with GDP‐β‐S and was completely inhibited by treatment with GTP‐γ‐S. The activation of protein kinase C by 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA), but not 4α‐phorbol‐12,13‐didecanoate, suppressed the GABAB receptor‐mediated hyperpolarization, and the effect of TPA was antagonized by sphingosine. Thus, activation of protein kinase C inhibits the expressed GABAB receptor‐mediated response.

Study of 16,16-Dimethyl Prostaglandin E2 for Prevention of Stress Ulcer after Hepatectomy of Experimental Cirrhotic Liver and Its Influence on Hepatic Regeneration

The influence of 16,16-dimethyl prostaglandin E2 (16,16-dm PGE2; an agent used for the prevention of stress ulcer after hepatectomy of the cirrhotic liver) on liver regeneration after hepatectomy was studied in rats. The following results were obtained. Ulceration after the stress of 6 h of water immersion was markedly suppressed in rats treated with 30 r/kg of 16,16-dmPGE2 as compared with the untreated controls. In animals that received hepatectomy alone, the gastric pH and gastric mucosal blood flow showed significant reduction from the preoperative levels. In animals that received hepatectomy plus 16,16-dmPGE2 treatment the postoperative reduction in the gastric pH and gastric mucosal blood flow was suppressed, suggesting the effectiveness of 16,16-dmPGE2 treatment in the prevention of stress ulcer after hepatectomy of the cirrhotic liver. The 3H-thymidine uptake percentage and thymidine activity 24 h after hepatectomy and the DNA content 30 h after hepatectomy were significantly higher in animals treated with 16,16-dmPGE2 than in the untreated controls. In animals that were treated intraperitoneally with 50 mg/kg of indomethacin 6 h before hepatectomy the mitotic index 30 h after hepatectomy was markedly lower than that in untreated controls. This indomethacin-induced reduction in the mitotic index tended to be normalized by treatment with 16,16-dmPGE2. These results suggest that 16,16-dmPGE2 treatment effectively prevents stress ulcer and favorably affects hepatic regeneration after hepatectomy of the cirrhotic liver.

Expression of the GABAB Receptor in Xenopus Oocytes and Desensitization by Activation of Protein Kinase C

The γ-aminobutyric acid (GABA) receptors have been classified into two subtypes, termed GABAA and GABAB receptors, in the basis of their pharmacological properties (Bowery et al., 1984; 1989; Bormann, 1988). The GABAA receptor with its integrated Cl-channel is now well characterized. Studies revealed the amino acid sequence of the GABAA receptor (Schofield et al., 1987; Levitan et al., 1988). Stimulation of the GABAB receptor has been shown to induce two membrane effects, namely reduction in Ca2+ conductance and increase in K+ conductance (Bormann, 1988; Bowery, 1989), however, the GABAB receptor has not been purified. The expression of GABAB receptor in the Xenopus oocytes may provide information on molecular mechanisms of GABAB receptor-mediated responses.

Inhibitory Effect of Peptide YY on Gastric Acid Output in Rats

The administration of peptide YY (PYY: 0.8, 1.6 and 3.2 nmol/kg/h, i.v.) to fasting rats inhibited not only baclofen (2 mg/kg, s.c.)-stimulated gastric acid output and gastric mucosal blood flow, but also pentagastrin (8 micrograms/kg/h, i.v.)-stimulated gastric acid output. PYY (3.2 nmol/kg/h) reduced baclofen-induced acid output more than pentagastrin-induced acid output, i.e., by 61.8 +/- 11.5% compared to 35.3 +/- 8.2%. PYY inhibited acetylcholine (ACh) release from cholinergic nerve endings of gastric body evoked by electrical transmural stimulation (ETS: 1 msec, 10 V, 3 Hz, 30 sec) by 47.2 +/- 3.5%. The mechanism of the inhibitory effect of PYY on gastric acid output seems to involve decreased gastric mucosal blood flow and reduced ACh release from cholinergic nerves.

Dual effects of 5-hydroxytryptamine on the release of γ-aminobutyric acid from myenteric neurones of the guinea-pig ileum

The effects of 5‐hydroxytryptamine (5‐HT) on the release of γ‐aminobutyric acid (GABA) were examined in the longitudinal muscle‐myenteric plexus (LM‐MP) preparation of guinea‐pig ileum. 5‐HT increased the spontaneous release and inhibited the electrically‐evoked release of [3H]‐GABA. The 5‐HT‐evoked release was Ca2+‐dependent and tetrodotoxin‐sensitive, and was antagonized by (3α‐tropanyl)‐1H‐indole‐3‐carboxylic acid ester (ICS 205–930), but not by methysergide and ketanserin. The inhibitory effect of 5‐HT was antagonized by methysergide, but not by ketanserin and ICS 205–930. 8‐Hydroxy‐2‐(di‐n‐propylamino)tetralin mimicked the inhibitory effect of 5‐HT. Thus, 5‐HT may exert an excitatory effect on the enteric GABAergic neurone via the 5‐HT3 receptor and an inhibitory effect via the 5‐HT1A receptor.

The mechanism of acute gastric ulcer after induced hemorrhagic shock.

Changes in gastric mucosal blood flow were investigated for their relationship to gastric mucosal prostaglandin E2 (PGE2) and noradrenaline (NA) in rats with hemorrhagic shock. The results were as follows: 1) Gastric mucosal blood flow and NA decreased after hemorrhage. Gastric mucosal PGE2 initially increased after exsanguination and then markedly decreased. 2) Administration of NA before hemorrhage resulted in an increase of PGE2. However, the PGE2 value for animals receiving NA after hemorrhage was not different from that of non-NA-treated group. 3) Pre-treatment with PGE2 suppressed the reduction in both gastric mucosal blood flow and NA and the development of ulcer. These results suggest that the increase in gastric mucosal PGE2 in the early stage of shock might represent a phenomenon of adaptation by the adrenergic activation, and the decrease in PGE2 in the late stage might result from impaired synthesis of PGE2 due to persistent hypoxia and might be one of the possible factors in ulcer formation.

Experimental Study of Vagotomy for Prevention of Stress Ulcer after Hepatectomy of Cirrhotic Livers Its Influence on Hepatic Regeneration

We experimentally studied the influence of vagotomy on hepatic regeneration in rats after hepatectomy of cirrhotic livers. In animals that underwent hepatectomy plus vagotomy the reduction in gastric pH was suppressed, but gastric mucosal blood flow was less than that in control animals that received hepatectomy alone. The suppression of 3H-thymidine uptake percentage and thymidine kinase activity after hepatectomy was more marked in animals treated with hepatectomy plus vagotomy than in controls treated with hepatectomy alone. Hepatic DNA level tended to be lower in animals treated with hepatectomy plus vagotomy than in controls. In animals treated with hepatectomy plus vagotomy, the peak level of the mitotic index was lower and the hepatic regeneration rate was evidently suppressed. These results suggest that it is not appropriate to apply vagotomy, during hepatectomy of cirrhotic livers, for the prevention of postoperative stress ulcer because it causes a marked reduction in gastric mucosal blood flow and suppresses hepatic regeneration.