Acetylcholine gastrointestinal effects

The most prominent pharmacologic effects of cholinesterase inhibitors are on the cardiovascular and gastrointestinal systems, the eye, and the skeletal muscle neuromuscular junction (as described in the Case Study). Because the primary action is to amplify the actions of endogenous acetylcholine, the effects are similar (but not always identical) to the effects of the direct-acting cholinomimetic agonists. 

It is less powerful and slower in action than neostigmine but it has a longer duration of action. It has relatively mild gastrointestinal effect but an antimuscarinic drug may still be required for the stomach cramps. A total daily dose of 450 mg should not be exceeded in order to avoid acetylcholine receptor downregulation. Immunosupressant therapy is usually added if the dose of pyridostigmine exceeds 360 mg daily. It is only available as a tablet, unlike neostigmine which comes as a tablet and injection. 

Effect on smooth muscles Acetylcholine causes increase in tone, amplitude of contractions, peristalsis and secretory activity of the gastrointestinal tract. It causes contraction of smooth muscles of gall bladder and relaxation of sphincters of gastrointestinal and biliary tract. 

Opioids induce an inhibitory effect on gastrointestinal motility and fluid secretion (Kromer, 1990). The effect is peripherally and centrally mediated. The peripheral component is related to p- and K-receptors in intestinal organs, which are densely equipped with opioid receptors. They are located at parasympathic ganglia and inhibit the release of acetylcholine, which stimulates the contraction of smooth muscles. Inhibition of the intestinal fluid secretion is mediated via inhibition of adenylate cyclase. The intestinal effects of opioids extend to all parts of the gut and results in inhibition of stomach emptying and inhibition of secretion and motility of duodenum, jejunum, colon and rectum. 

Artemisia monosperma The relaxant effects of 7-0-methyleriodictyol, a flavone isolated from the aerial parts of A. monosperma, on smooth muscle contraction has been studied. The inhibition of contraction induced by known therapeutic agents such as acetylcholine and oxytocin are consistent with the use of this plant in the treatment of certain gastrointestinal disorders. 

Choline esters are poorly absorbed and poorly distributed into the central nervous system because they are hydrophilic. Although all are hydrolyzed in the gastrointestinal tract (and less active by the oral route), they differ markedly in their susceptibility to hydrolysis by cholinesterase in the body. Acetylcholine is very rapidly hydrolyzed (see Chapter 6 Introduction to Autonomic Pharmacology) large amounts must be infused intravenously to achieve concentrations high enough to produce detectable effects. A large intravenous bolus injection has a brief effect, typically... 

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