The direct-acting cholinergic agonists are similar to ACh and react directly with receptor sites to cause the same reaction as if ACh had stimulated the receptor sites. These drugs usually stimulate muscarinic receptors within the parasympathetic system. They are used as systemic agents to increase bladder tone, urinary excretion, and gastrointestinal (GI) secretions and as ophthalmic agents to induce miosis to relieve the increased intraocular pressure of glaucoma. Systemic absorption usually does not occur when these drugs are used ophthalmically.
Direct-acting cholinergic agonists include bethanechol (Duvoid, Urecholine), carbachol (Miostat), cevimeline (Evoxac), and pilocarpine (Salagen). These agents are used infrequently today because of their widespread parasympathetic activity. More specific and less toxic drugs are now available and preferred.
The direct-acting cholinergic agonists act at cholinergic receptors in the peripheral nervous system to mimic the effects of ACh and parasympathetic stimulation. These parasympathetic effects include slowed heart rate and decreased myocardial contractility, vasodilation, bronchoconstriction and increased bronchial mucus secretion, increased GI activity and secretions, increased bladder tone, relaxation of GI and bladder sphincters, and pupil constriction.
The agent bethanechol, which has an affinity for the cholinergic receptors in the urinary bladder, is available for use orally and subcutaneously to treat nonobstructive postoperative and postpartum urinary retention and to treat neurogenic bladder atony. It directly increases detrusor muscle tone and relaxes the sphincters to improve bladder emptying. Because this drug is not destroyed by acetylcholinesterase, the effects on the receptor site are longer lasting than with stimulation by ACh.
Carbachol is available as an ophthalmic agent. It is used to induce miosis, or pupil constriction; to relieve the increased intraocular pressure of glaucoma; and to allow surgeons to perform certain surgical procedures.
Cevimeline and pilocarpine, which bind to muscarinic receptors throughout the system, are used to increase secretions in the mouth and GI tract and relieve the symptoms of dry mouth that are in seen in Sjögren’s syndrome. They are approved for use in adults and are given three times a day, often with meals.
These drugs are used sparingly because of the potential undesirable systemic effects of parasympathetic stimulation. For example, they are contraindicated with hypersensitivity to any component of the drug to avoid hypersensitivity reaction and in the presence of any condition that would be exacerbated by parasympathetic effects, such as bradycardia, hypotension, vasomotor instability, and coronary artery disease, which could be made worse by the cardiac- and cardiovascular-suppressing effects of the parasympathetic system. Peptic ulcer, intestinal obstruction, or recent GI surgery could be negatively affected by the GI-stimulating effects of the parasympathetic nervous system.
muscarinic – a. 毒蕈碱的
bladder tone – 膀胱张力
miosis – n. 缩瞳，瞳孔缩小
bethanechol – n. 氯贝胆碱 Duvoid – n. 【奥】杜沃特（氯贝胆碱商品名） Urecholine – n. 乌拉胆碱（氯贝胆碱商品名）
carbachol – n. 卡巴胆碱 Miostat – n. 迈斯特（卡巴胆碱眼内注射液）
cevimeline – n. 西维美林 Evoxac – n. 【奥】依沃塞克（西维美林商品名）
pilocarpine – n. 匹鲁卡品，毛果芸香碱 Salagen – n. 【奥】赛洛根（匹鲁卡品商品名）
vasodilation – n. 血管扩张
bronchoconstriction – n. 支气管收缩
nonobstructive – a. 非阻塞性的
postpartum – a. 产生的
urinary retention – 尿潴留
atony – n. 张力缺乏，弛缓
bladder atony – 膀胱乏力，膀胱张力缺乏
detrusor muscle – 逼尿肌
Sjögren’s syndrome -- 干燥综合症
sparingly – ad. 很少地，保守地
vasomotor – a. 司血管收缩的，血管舒缩的
vasomotor instability – 血管舒缩不稳定 注：带【奥】标志者为奥医临时用名，只为方便学员复习记忆用，不得作为临床治疗依据。
★Video★
Cholinergic Agonists
★Tests★
1. This drug is usually indicated for postoperative and postpartum urinary retention as well as bladder atony.
A. Carbachol
B. Bethanechol
C. Tacrine
D. Pyridostigmine
2. A patient on neostigmine complained of frequent diarrhea and salivation. Which is the best nursing action for this situation?
A. Educate client about drug therapy to alleviate anxiety as these are expected drug effects.
B. Withhold the dose of the drug and refer to physician.
C. Arrange for increasing the dose of the drug as it is not effective.
D. Document patient response and endorse accordingly for bedside care.
答案Answers 1. B. Bethanechol Rationale: This drug has specific affinity for receptors in the urinary bladder. 2. B. Withhold the dose of the drug and refer to physician. Rationale: Discontinue drug if excessive salivation, diarrhea, emesis, or frequent urination becomes a problem to decrease the risk of severe adverse reactions. The nurse should refer this to the physician.
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