Differential effects of ß-methylphenylethylamine and octopamine on contractile parameters of the rat gastrointestinal tract.

This study tested the effects of ß-methylphenylethylamine (ß-MPEA) and octopamine on contractile parameters of the gastrointestinal tract in rats. We hypothesized that some of their effects result from interactions with trace amine (TA)-associated receptors or serotoninergic 5-hydroxytryptamine (5-HT) receptors. ß-MPEA-induced contractions in rat gastric fundus strips under resting tonus conditions, but induced relaxation in preparations that were previously contracted with carbachol. Octopamine relaxed gastric fundus strips maintained at resting tonus or contracted with carbachol. The contractile effect of ß-MPEA was reduced by cyproheptadine and methiothepin, antagonists of excitatory 5-HT receptors. The relaxing effect of ß-MPEA on gastric fundus was insensitive to pretreatment with N-(3-ethoxyphenyl)-4-(1-pyrrolidinyl)-3-(trifluoromethyl)benzamide (EPPTB) and tropisetron, antagonists of TA1 and 5-HT4 receptors, respectively. Both EPPTB and tropisetron inhibited the relaxant effects of octopamine on carbachol-contracted preparations. Contrarily, EPPTB did not reduce the relaxant effects of RO5263397 (TA1 agonist) or zacopride (5-HT4 agonist). Octopamine, but not ß-MPEA, delayed the gastrointestinal transit of a liquid test meal in awaken rats. In isolated preparations of the small intestine under resting conditions, ß-MPEA did not alter the basal tonus, but octopamine relaxed it. Intestinal preparations previously contracted with carbachol relaxed after the addition of octopamine and decreased the magnitude of their spontaneous rhythmic contractions in a tropisetron-dependent manner. Thus, ß-MPEA and octopamine exerted pharmacological actions on the rat gastrointestinal tract. The excitatory effects of ß-MPEA involved 5-HT receptors. Octopamine inhibited the rat gut contractility through the likely involvement of 5-HT4 and TA receptors. Overall, octopamine effectively inhibited rat gastrointestinal transit.

Neryl butyrate induces contractile effects on isolated preparations of rat aorta.

Neryl butyrate is a constituent of volatile oils obtained from aromatic plants. Aliphatic organic compound analogues chemically close to neryl butyrate possess vasodilator properties in rat aorta. To evaluate whether neryl butyrate has relaxing properties, this study tested its effects on isolated rat aorta. Unlike the analogues, neryl butyrate did not show relaxant profile in aortic rings precontracted with phenylephrine, but induced a contraction when it stimulated aortic rings under resting tonus. The contractile effect augmented in endothelium-denuded aortic rings. Treatment of endothelium-intact preparations with the nitric oxide synthase inhibitor L-NAME or the guanylyl cyclase inhibitor ODQ also augmented the contractile effect of neryl butyrate. Such phenomenon was absent in the presence of the cyclooxygenase inhibitor indomethacin. Contractile responses decreased in the presence of verapamil, a L-type Ca2+ channel blocker, or when Ca2+ was removed from the extracellular solution. Antagonists of α-adrenergic receptors (prazosin and yohimbine), but not the thromboxane-prostanoid receptor seratrodast, reversed the contraction induced by neryl butyrate. The α1A selective antagonist RS-17053 antagonized the neryl butyrate-induced contraction. The contraction caused by neryl butyrate was decreased by inhibiting the phospholipase C or the rho-associated kinase with U-73122 or Y-27632, respectively. Injected intravenously to awake rats, neryl butyrate induced arterial hypotension and bradycardia. Decreased frequency was also present in isolated right atrium preparations. In conclusion, the contractile effects of neryl butyrate were inhibited by α-adrenergic antagonists, indicating the involvement of α-adrenoceptors in the mechanism of action. In vivo, neryl butyrate caused hypotension, suggesting that other systemic influence than vasoconstriction may occur.

Dual excitatory and smooth muscle-relaxant effect of ß-phenylethylamine on gastric fundus strips in rats.

ß-Phenylethylamine (ß-PEA) is a trace amine with chemical proximity to biogenic amines and amphetamines. It is an endogenous agonist of trace amine-associated receptors (TAARs) that acts as a neuromodulator of classic neurotransmitters in the central nervous system. At high concentrations, ß-PEA contracts smooth muscle, and a role for TAARs in these responses has been postulated. The high dietary intake of trace amines has been associated with such symptoms as hypertension and migraine, especially after the intake of foods containing such compounds. In gastrointestinal tissues, TAAR expression was reported, although the effect of ß-PEA on gastric contractile behaviour is unknown. Here, isolated strips that were obtained from the rat gastric fundus were stimulated with high micromolar concentrations of ß-PEA. Under resting tonus, ß-PEA induced contractions. In contrast, when the strips were previously contracted with KCl, a relaxant response to ß-PEA was observed. The contractile effect of ß-PEA was inhibited by 5-hydroxytryptamine (5-HT) receptor antagonists (i.e., cyproheptadine and ketanserin) but not by the TAAR1 antagonist EPPTB. In gastric fundus strips that were previously contracted with 80 mmol/L KCl, the relaxant effect of ß-PEA intensified in the presence of 5-HT receptor antagonists, which was inhibited by EPPTB and the adenylyl cyclase inhibitor MDL-12,330A. The guanylyl cyclase inhibitor ODQ did not alter the relaxant effects of ß-PEA. In conclusion, ß-PEA exerted dual contractile and relaxant effects on rat gastric fundus. The contractile effect appeared to involve the recruitment of 5-HT receptors, and the relaxant effect of ß-PEA on KCl-elicited contractions likely involved TAAR1 .

Impairment of rat oesophageal muscle contractility associated with experimental non-erosive oesophageal mucosal damage.

NEW FINDINGS: What is the central question of this study? Is the responsiveness of isolated segments of the rat oesophagus to contractile or relaxant stimuli susceptible to acute luminal exposure of the oesophagus to an acid solution that contains pepsin and bile salt? What is the main finding and its importance? The study reveals that luminal acidity is an important factor that disrupts barrier function in the oesophagus to allow the diffusion of noxious agents, such as bile acid, that alter the contractile status of the oesophageal body, even in the absence of inflammation. ABSTRACT: We investigated whether the experimental simulation of duodenogastro-oesophageal reflux alters the contractile responsiveness of rat oesophageal strips. After 30 min of luminal exposure to a solution at acid pH that contained pepsin and taurodeoxycholic acid, isolated strips of the rat oesophagus and gastro-oesophageal junction were subjected to contractile or relaxing stimuli. Acid challenge decreased the responsiveness of oesophageal strips to contractile stimulation, especially in oesophageal preparations that were mounted following the circular orientation of the muscularis externa layer. The contractility of longitudinal preparations of the rat oesophagus appeared less susceptible to the deleterious effects of acid challenge. In contrast, the responsiveness of ring-like preparations from the gastro-oesophageal junction to contractile stimulation was unaltered by acid challenge. Taurodeoxycholic acid decreased the responsiveness of circular oesophageal preparations to KCl, an effect that was exacerbated by luminal acidity. On the contrary, although the relaxant ability of the rat oesophagus did not change, acid challenge increased the relaxant efficacy of sodium nitroprusside and isoprenaline in strips of the gastro-oesophageal junction. A significant decrease in transepithelial electrical resistance was seen when the oesophageal mucosa was challenged at pH 1 but not at pH 4. Treatment with alginate blunted the deleterious effects of acid challenge on transepithelial electrical resistance and the responsiveness of oesophageal preparations to KCl. The present findings support the notion that luminal acidity is an important factor that disrupts barrier function in the oesophagus to allow the diffusion of noxious agents, such as bile acid, that alter the contractile status of the oesophagus.

Extracellular acidosis selectively inhibits pharmacomechanical coupling induced by carbachol in strips of rat gastric fundus.

NEW FINDINGS: What is the central question of this study? Acute acidosis that results from short-term exercise is involved in delayed gastric emptying in rats and the lower responsiveness of gastric fundus strips to carbachol. Does extracellular acidosis decrease responsiveness to carbachol in tissues of sedentary rats? How? What is the main finding and its importance? Extracellular acidosis inhibits cholinergic signalling in the rat gastric fundus by selectively influencing the Gq/11 protein signalling pathway. Acute acidosis that results from short-term exercise delays gastric emptying in rats and decreases the responsiveness to carbachol in gastric fundus strips. The regulation of cytosolic Ca2+ concentrations appears to be a mechanism of action of acidosis. The present study investigated the way in which acidosis interferes with gastric smooth muscle contractions. Rat gastric fundus isolated strips at pH 6.0 presented a lower magnitude of carbachol-induced contractions compared with preparations at pH 7.4. This lower magnitude was absent in carbachol-stimulated duodenum and KCl-stimulated gastric fundus strips. In Ca2+ -free conditions, repeated contractions that were induced by carbachol progressively decreased, with no influence of extracellular pH. In fundus strips, CaCl2 -induced contractions were lower at pH 6.0 than at pH 7.4 but only when stimulated in the combined presence of carbachol and verapamil. In contrast, verapamil-sensitive contractions that were induced by CaCl2 in the presence of KCl did not change with pH acidification. In Ca2+ store-depleted preparations that were treated with thapsigargin, the contractions that were induced by extracellular Ca2+ restoration were smaller at pH 6.0 than at pH 7.4, but relaxation that was induced by SKF-96365 (an inhibitor of store-operated Ca2+ entry) was unaltered by extracellular acidification. At pH 6.0, the phospholipase C inhibitor U-73122 relaxed carbachol-induced contractions less than at pH 7.4, and this phenomenon was absent in tissue that was treated with the RhoA kinase blocker Y-27632. Thus, extracellular acidosis inhibited pharmacomechanical coupling in gastric fundus by selectively inhibiting the Gq/11 protein signalling pathway, whereas electromechanical coupling remained functionally preserved.

A simple laboratory exercise with rat isolated esophagus and stomach fundus to reveal functional differences between striated and smooth muscle cells.

This study describes an undergraduate student laboratory activity using isolated preparations from rat gastrointestinal tissues that possess contractile profiles typically exhibited by striated and smooth muscle cells. While students are introduced to an ex vivo methodology, they can compare differences in trace experiments, twitch aspects, phasic and tonic properties, force-frequency relationships, and pharmacological responsiveness of esophageal (striated) and fundic (smooth muscle) segments. Muscle strips were subjected to electrical field stimulation (EFS) applied by platinum electrodes immersed in the physiological solution. The contractile profile of EFS responses varied between these two types of gut preparations. Atropine and tubocurarine revealed differential inhibitory influences in esophagus or fundus tissues; caffeine and procaine produced similar effects, i.e., potentiation and blockade of the EFS-induced contractile response in these tissues, respectively. Experimental results obtained during the activity helped the improvement of student learning about basic concepts previously discussed in theoretical lectures. To measure student learning with this laboratory exercise, a questionnaire was applied before and after the activity, and the number of expected correct answers, concerning the mechanisms of contraction in striated and smooth muscle, could be clearly evidenced.