Effect of dopamine on electrical activity of isolated stomach muscle in cats

Ultrasonographic assessment of the effect of metoclopramide, erythromycin, and exenatide on solid-phase gastric emptying in healthy cats

Background

Available data on the effect of gastrointestinal motility‐modifying drugs in cats are limited. Most recommendations for drug usage and dosage are based on collective clinical experience.

Objectives

To assess the effects of metoclopramide, erythromycin, and exenatide on gastric emptying (GE) and gastric motility in comparison to placebo. We hypothesized that metoclopramide and erythromycin would have prokinetic gastric effects, whereas exenatide would prolong GE times and decrease the motility index (MI) of antral contractions.

Animals

Eight healthy domestic shorthair cats.

Methods

Each cat had 4 separate ultrasonographic assessments. In a prospective, randomized, double‐blind, 4‐way crossover design, cats received placebo, metoclopramide, erythromycin, or exenatide for 2 days followed by a minimum 5‐day washout period. Ultrasonographic GE times and MI were compared to placebo.

Results

When compared to placebo, the rate of GE was significantly faster after administration of metoclopramide and erythromycin. Significant differences were found at all fractions of GE after administration of erythromycin and all but 1 fraction after metoclopramide when compared to placebo. The rate of GE in the first half of the GE curve was significantly slower after exenatide administration. The total area under the Ml curve was significantly larger after administration of metoclopramide and erythromycin than after placebo.

Conclusions and Clinical Importance

Metoclopramide and erythromycin shorten GE times and increase the MI of antral contractions, thus having a prokinetic effect in the stomach of healthy cats, whereas exenatide causes an initial delay in GE.

The Inhibitory Effect of Dopamine on Cat Gastric Smooth Muscle

The inhibitory effect of dopamine has been studied in longitudinal and circular muscle strips of the cat gastric fundus. When tone was raised by transmural electrical stimulation and by administration of methacholine, dopamine concentration-dependently relaxed the strips but the inhibitory effect of dopamine was clearly more pronounced on electrically-induced tone. The effect of dopamine was not influenced by the presence of cocaine or hydrocortisone. The relaxant effect of dopamine, when tone was raised by methacholine, was not influenced by α- and dopamine receptor antagonists but it was significantly reduced by propranolol and ICI 118551 (erythro-dl-1-(7-methylindan-4-yloxy)-3-isopropylaminobutan-2-ol). The inhibitory effect of dopamine on the electrically-induced tone was significantly reduced by phentolamine; domperidone tended to reduce the effect of the lower concentrations of dopamine. In the presence of propranolol, phentolamine and rauwolscine concentration-dependently antagonized the inhibitory effect of dopamine on electrically-induced tone, while prazosin was without influence. These results indicate that the inhibitory effect of dopamine in the cat gastric fundus is mainly due to interaction with postjunctional β-adrenoceptors on the smooth muscle cells and with prejunctional α-adrenoceptors on the intramural cholinergic neurons

Review article: gastroparesis

BACKGROUND

Gastroparesis is a chronic disorder caused by stomach pump failure and characterized by profound nausea, vomiting and epigastric pain. Most often, the cause is unapparent and of the known associations, diabetes is the most common. Diagnosis is usually made using an isotope-labelled test meal. Treatment is incremental and includes education, dietary support, prokinetic and antiemetic agents. There are novel approaches including gastric neurostimulation.

AIM

To review current concepts of gastric motor function, aetiology, investigation and treatment of gastroparesis.

METHODS

A systematic web-based review of the literature was undertaken using a lexicon of terms associated with gastroparesis.

RESULTS

There are few controlled studies of this condition. Little is known about causation or underlying nerve, muscle or pacemaker pathology. Idiopathic gastroparesis occurs most commonly in women and gastric emptying is often abnormal in diabetes. Isotopic gastric scintigraphy remains the gold standard investigation, but alternative tests are being developed. Treatment is multimodal and includes education, and nutritional support. There are no adequately powered controlled trials to support a particular drug regimen. In intractable gastroparesis, gastric neurostimulation appears to offer benefit.

CONCLUSION

Despite a significant progress in the past decade, further controlled trials are required into the therapeutic options available for treating this intriguing condition.

Domperidone: review of pharmacology and clinical applications in gastroenterology

Domperidone is a dopamine-2 receptor antagonist. It acts as an antiemetic and a prokinetic agent through its effects on the chemoreceptor trigger zone and motor function of the stomach and small intestine. Unlike metoclopramide, it does not cause any adverse neurological symptoms as it has minimal penetration through the blood-brain barrier. It thus provides an excellent safety profile for long-term administration orally in the recommended doses. Domperidone is widely used in many countries and can now be officially prescribed to patients in the United States by an investigational new drug application for the treatment of gastroparesis and any condition causing chronic nausea and vomiting. In view of this additional clinical exposure of domperidone to a new generation of gastroenterologists and other specialists, the purpose of this timely review is to revisit the pharmacology, clinical application, and safety profile of this beneficial medication.

Inhibitory effect of dopamine on gastric motility in rats

BACKGROUND

There is disagreement with regard to the involvement of dopamine (DA) receptors in gastric motility. The mechanisms of the inhibitory effect of DA on rat gastric motility was investigated in association with acetylcholine (Ach) release in the present study.

METHODS

In vivo vagotomized, splanchnicectomized rats and control rats were used, and gastric movement was determined as the gastric motility index after DA administration. In vitro study of Ach release from the circular muscle strips of the gastric corpus was investigated after administration of domperidone, SCH23390, phentolamine, or propranolol.

RESULTS

In the in vivo study DA inhibited gastric motility in a dose-dependent manner. Vagotomy and splanchnicectomy had no effect on the inhibitory effect of DA. In vitro study DA inhibited [3H]-Ach release in a dose-dependent manner. The inhibitory effect of DA was antagonized by domperidone but not by phentolamine, propranolol, or SCH23390.

CONCLUSIONS

Inhibition of gastric motility by dopamine is independent of extrinsic innervation and seems to be mediated by DA2 receptors in the gastric wall.

Dopamine in gastrointestinal disease

Dopamine is an important enteric neuromodulator. Herein we review the data that support a role for dopaminergic involvement in experimental duodenal and gastric ulceration; gastric, pancreatic, and duodenal secretion; gastrointestinal motility; and gastric and intestinal submucosal blood flow regulation. There also is support for a role for dopamine and dopamimetic agents in the treatment of certain experimental gastrointestinal diseases because some highly selective dopamine agonists are gastroprotective when given either parenterally or centrally. Based upon these observations, we suggest that dopamine is a key element of the "brain-gut axis" and represents a potentially important target for pharmacotherapeutic exploitation.

The action of histamine on the isolated stomach muscle of the cat

The effect of histamine on contractile and electric activity was studied in the isolated stomach muscle strips of 138 cats. Histamine dose-dependently produced tonic and phasic contractions of the muscle preparations from the fundus and the corpus but only phasic contraction of the antral muscle preparations. The frequency of gastric slow waves (SWs) was also increased dose-dependently by histamine. The responses of muscle contractions and gastric SW frequency to histamine were completely blocked by pretreatment with pyrilamine (10(-6) M) and were significantly inhibited by atropine (10(-5) M) but not by cimetidine (10(-5) M), hexamethonium (10(-5) M), phentolamine (10(-5) M), or propranolol (10(-5) M). The inhibition by pyrilamine was competitive. Although atropine inhibited the effect of histamine significantly, it could not completely block the effect of histamine even at a high concentration (3 x 10(-5) M). It is concluded that histamine may participate in the regulation of gastric motility in the cat by acting on the H1 receptor to cause the release of acetylcholine and also other contractile substance(s).