In an ideal world, a cholinergic agonist would only produce its intended effect, but in reality, other parasympathetic effects often occur. For example, while aiming for benefits on bladder emptying, bethanechol use can produce gastrointestinal effects like diarrhea and cramps or pulmonary consequences like bronchial tube spasm. Acetylcholine receptors come in two main varieties: muscarinic receptors and nicotinic receptors.
These differ in bodily location and function. Different cholinergic agonists activate different types of ACh receptors in order to achieve a particular effect. The pharmacology of cholinergic agonists differs from other cholinomimetics drugs that promote the action of acetylcholine. Cholinergic agonists are direct-acting agents, stimulating the parasympathetic system themselves. In comparison, acetylcholinesterase inhibitors are another type of cholinergic drug class, one that is indirect-acting.
Acetylcholinesterase inhibitors block acetylcholinesterases, enzymes that break down ACh. The enzyme blockers result in more of the neurotransmitter acetylcholine being available and consequently more acetylcholine parasympathomimetic parasympathetic promoting activity.
Salagen and Evoxac stimulate muscarinic ACh receptors in the salivary glands with the primary goal of increasing saliva secretion.
While these two drugs act on muscarinic receptors stimulating salivary glands, bethanechol does so at the bladder in order to provide relief for those experiencing urinary retention. Miostat is a muscarinic receptor that is injected into the eye, resulting in miosis pupil dilation which is helpful during eye surgery.
Pilocarpine eye drops have a utility similar to Miostat but also have an indication for glaucoma treatment. Nicoderm CQ nicotine patches and Chantix varenicline act on nicotinic receptors. Therapeutically, they are used to reduce the craving for nicotine and increase the chance of successful smoking cessation. Most cholinergic agonists have not been studied adequately in pregnancy and breastfeeding, so alternatives are typically recommended.
However, Nicoderm CQ and related nicotine replacement products have enough benefit in helping to quit smoking in pregnancy and breastfeeding to justify the risk for most women. In addition, pilocarpine eye drops can also be used in these situations, and Miostat can be used during breastfeeding if needed.
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Cholinergic Agonist By: Aditya Arya 2. The cholinergic drugs: act on receptors that are activated by acetylcholine. The adrenergic drugs: act on receptors that are stimulated by norepinephrine or epinephrine. Sites of actions of cholinergic agonists in the autonomic and somatic nervous systems 4. Neurotransmission at cholinergic neurons Synthesis and release of acetylcholine from the cholinergic neuron.
Choline acetyltransferase catalyzes the reaction of choline with acetyl coenzyme A CoA to form acetylcholine an ester in the cytosol. Acetyl CoA is derived from the mitochondria and is produced by the Krebs cycle and fatty acid oxidation. Acetylcholine 7. Neural systems where ACh is a major neurotransmitter 8.
ACh is broken down in the synaptic cleft by the enzyme acetylcholinesterase AChE 9. Types of cholinergic receptors Locations of muscarinic receptors: These receptors have been found on ganglia of the peripheral nervous system and on the autonomic effector organs, such as the heart, smooth muscle, brain, and exocrine glands.
Although all five subtypes have been found on neurons, M1 receptors are also found on gastric parietal cells, M2 receptors on cardiac cells and smooth muscle, and M3 receptors on the bladder, exocrine glands, and smooth muscle.
When M1 and M3 receptors are activated, the receptor undergoes a conformational change and interacts with a G protein, designated Gq, which in turn activates phospholipase C. This acation can then interact to stimulate or inhibit enzymes, or cause hyperpolarization, secretion, or contraction. Nicotinic receptors. Those at the neuromuscular junction are sometimes designated NM and the others NN.
For example, ganglionic receptors are selectively blocked by hexamethonium, whereas neuromuscular junction receptors are specifically blocked by tubocurarine. Cholinergic Agonists Naturally occurring alkaloids, such as pilocarpine constitue the second group. Acetylcholine: is a quaternary ammonium compound that cannot penetrate membranes.
Its actions include Decrease in heart rate and cardiac output: The actions of acetylcholine on the heart mimic the effects of vagal stimulation. For example, acetylcholine, if injected intravenously, produces a brief decrease in cardiac rate negative chronotropy and stroke volume as a result of a reduction in the rate of firing at the sinoatrial SA node.
Decrease in blood pressure: Injection of acetylcholine causes vasodilation and lowering of blood pressure by an indirect mechanism of action. Acetylcholine activates M3 receptors found on endothelial cells lining the smooth muscles of blood vessels This results in the production of nitric oxide from arginine Nitric oxide then diffuses to vascular smooth muscle cells to stimulate protein kinase G production, leading to hyperpolarization and smooth muscle relaxation In the gastrointestinal tract, acetylcholine increases salivary secretion and stimulates intestinal secretions and motility.
Bronchiolar secretions are also enhanced. In the genitourinary tract, the tone of the detrusor urinae muscle is increased, causing expulsion of urine. In the eye, acetylcholine is involved in stimulating ciliary muscle contraction for near vision and in the constriction of the pupillae sphincter muscle, causing miosis marked constriction of the pupil.
Bethanechol: is structurally related to acetylcholine, in which the acetate is replaced by carbamate and the choline is methylated. It has a duration of action of about 1 hour. You are here Home » Pharmacology » Basic Pharamacology. Book navigation Immunology. Cardiovascular Medicine. Respiratory Medicine. Renal Medicine. Gastrointestinal Medicine. Hematology and Oncology.
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