Systemic effects and impact on organs
Upon administration into the body, the effects of muscarinic receptor stimulation predominate, reproducing the pattern of potent parasympathetic nervous system activation.
Key changes by organ system:
- Cardiovascular system: manifests as a decreased heart rate (bradycardia), vasodilation, and a marked drop in blood pressure.
- Smooth muscle: undergoes a sharp increase in tone. This presents as enhanced gastrointestinal motility, bronchospasm, as well as increased tone of the urinary bladder, gallbladder, and uterus.
- Exocrine glands: a generalized increase in secretion is observed, primarily in the bronchial, digestive, and sweat glands.
The dosage paradox: hemodynamic effects
The effect of acetylcholine on the cardiovascular system is directly dependent on the dose administered. Its direct action on heart rate can be modified by reflex influences from the sympathetic nervous system.
- Low doses (20–50 mcg/min, IV): the primary effect is a decrease in blood pressure due to vasodilation. In response to this hypotension, a compensatory reaction occurs — reflex tachycardia.
- High doses: direct excitation of cardiac muscarinic receptors predominates. Bradycardia and depression of atrioventricular (AV) conduction develop. The direct cardiodepressant effect overrides any reflex mechanisms, even against the background of persistently low blood pressure.
Cryptic nicotine-like action
Acetylcholine is also capable of stimulating nicotinic receptors; however, this effect only manifests when muscarinic receptors are blocked (e.g., following the administration of the muscarinic antagonist atropine).
In such cases, opposite effects are observed: tachycardia, vasoconstriction, and an increase in blood pressure (pressor effect). The mechanism of this phenomenon includes:
- Excitation of sympathetic ganglia.
- Stimulation of chromaffin cells in the adrenal medulla, leading to a massive release of epinephrine.
- Stimulation of the carotid bodies.
When extremely high doses are administered, stimulation is replaced by sustained depolarization of the postsynaptic membrane. This leads to a complete blockade of excitation transmission in cholinergic synapses.
Pharmacokinetics and clinical limitations
The acetylcholine molecule is a quaternary ammonium compound. It penetrates the blood-brain barrier extremely poorly and has virtually no effect on the CNS. Upon entering synapses or the bloodstream, the neurotransmitter is very rapidly degraded by acetylcholinesterase and plasma cholinesterase, causing the effect to last only a few minutes.
The lack of selectivity and short duration of action severely restrict the medical use of lyophilized acetylcholine chloride. It is primarily used in experimental pharmacology. In clinical practice, it is rarely used:
- Therapy: subcutaneously or intramuscularly as a vasodilator for peripheral vascular spasms (intermittent claudication) and retinal artery spasms.
- Eye surgery: topically in combinations to achieve miosis during surgical interventions.
Synthetic analogue: Carbachol
For clinical needs, a synthetic analogue was developed — carbachol (carbacholine). In terms of its pharmacological spectrum, it is a complete analogue of acetylcholine.
Its primary advantage is resistance to degradation by acetylcholinesterase. Thanks to this, the drug's effect lasts from 1 to 1.5 hours. Carbachol is used as eye drops for the treatment of glaucoma and is also administered parenterally for urinary bladder atony.
Briefly: answers to topical questions
Where in the body are nicotinic receptors localized?
Nicotinic receptors are localized in various structures of the nervous system and muscle tissue. Two main types of receptors are distinguished based on their location:
- Nm receptors — found in skeletal muscle (neuromuscular junctions).
- Nn receptors — located in autonomic ganglia (sympathetic and parasympathetic), chromaffin cells of the adrenal medulla, carotid bodies (sinocarotid zone), and the CNS.
What symptoms occur in an acetylcholine overdose (cholinergic crisis)?
An acetylcholine overdose leads to a cholinergic crisis caused by excessive receptor stimulation and depolarization block. The following clinical manifestations occur:
- Symptoms of muscarinic stimulation — enhancement of parasympathetic effects (nausea, vomiting, diarrhea, hypersalivation, bradycardia, lowered blood pressure, bronchospasm).
- Symptoms of nicotinic stimulation — paradoxical worsening of muscle weakness (myasthenic symptoms) due to impaired neuromuscular transmission, as well as muscle twitching (fasciculations).
To recall the systemic effects of acetylcholine, think of parasympathetic function: "rest and digest." The pulse drops (bradycardia), blood pressure falls, digestion is active (motility, glandular secretion), and pupils constrict (miosis).
Class and exam questions
Why do low doses of acetylcholine cause tachycardia?
In low doses, the vasodilatory effect predominates, leading to a drop in blood pressure. In response to severe hypotension, the sympathetic nervous system triggers a compensatory reflex tachycardia.
How will the effects of acetylcholine manifest when administered alongside atropine?
Atropine blocks muscarinic receptors, so only the cryptic nicotine-like (nicotinic-cholinergic) action of acetylcholine will manifest. Tachycardia, vasoconstriction, and a sharp increase in blood pressure will occur due to the release of epinephrine.
Why does carbachol act longer than acetylcholine?
Carbachol has an altered chemical structure, preventing it from being recognized or degraded by the enzyme acetylcholinesterase. Its action lasts 1–1.5 hours, whereas acetylcholine is degraded within minutes.
What else to study on this topic
- Mechanisms of reflex regulation of heart rate in response to changes in vascular tone.
- The role of carotid bodies in mediating pressor effects of neurotransmitters.
- The phenomenon of sustained postsynaptic membrane depolarization and depolarization block.
- Topography of muscarinic and nicotinic receptors in the autonomic nervous system.
Master the entire topic in the «Pharmacology» course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Acetylcholine" and 5,304 other subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 116 video lessons on the subject
Other topics in the section «Drugs acting on cholinergic synapses»
Free video lessons: pharmacologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.