Cholinergic agents direct-acting

Mechanism of Action A urinary antispasmodic agent that acts as a direct antagonist at muscarinic receptor sites in cholinergically innervated organs. Blockade of the receptors limits bladder contractions. Therapeutic Effect Reduces symptoms of bladder irritability and overactivity improves bladder capacity. 

The principal directly acting cholinergic agonists include methacholine, carbachol, and betanechol—agents which, unlike acetylcholine, are used clinically. 

Cholinesterase inhibitors have minimal effects by direct action on vascular smooth muscle because most vascular beds lack cholinergic innervation (coronary vasculature is an exception). At moderate doses, cholinesterase inhibitors cause an increase in systemic vascular resistance and blood pressure that is initiated at sympathetic ganglia in the case of quaternary nitrogen compounds and also at central sympathetic centers in the case of lipid-soluble agents. Atropine, acting in the central and peripheral nervous systems, can prevent the increase of blood pressure and the increased plasma norepinephrine. 

Because physostigmine acts on the enzyme cholinesterase— which is present at all cholinergic synapses—this drug increases acetylcholine effects at the nicotinic junctions as well as muscarinic ones. Bethanechol, on the other hand, is a direct-acting agent that is selective for muscarinic receptors and has no effect on nicotinic junctions such as the skeletal muscle end plate. The answer is (B). 

Pupil size and function can be affected by peripheral autonomic action and by centrally initiated impulses.The iris is an excellent indicator of autonomic activity because of the delicate balance between adrenergic and cholinergic innervation to the iris dilator and iris sphincter muscles, respectively. By acting directly on these muscles, both sympathetic and parasympathetic agents can influence pupil size and activity. 

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