Direct-acting sympathomimetic agent

Pharmacology Epinephrine, a direct-acting sympathomimetic agent, acts on and receptors. Topical application, therefore, causes conjunctival decongestion (vasoconstriction), transient mydriasis (pupillary dilation), and reduction in intraocular pressure (lOP). It is believed lOP reduction primarily is caused by reduced aqueous production and increased aqueous outflow. The duration of decrease in lOP is 12 to 24 hours. 

The drug exhibits a direct-acting sympathomimetic agent having predominantly P-adrenergic activity plus a selective action on the P2-receptors (i.e., (P2-agonist). This is, therefore, recommended for the treatment of severe and acute forms of bronchospasm via IM, IV, sub-cutaneous routes. 

It is closely related structurally to salbutamol, the only difference being that the former has a basic-pyridine nucleus while the latter has a benzene ring (nucleus). It is also a direct-acting sympathomimetic agent having a most prominent P-adrenoreceptor stimulant action, and a selective aetion on P2-receptors. 

Tricyclic antidepressants potentiate the pressor effects of directly acting sympathomimetic amines, such as adrenaline (epinephrine) or noradrenaline (norepinephrine), to cause hypertension. Small amounts of these, such as may be present in local anaesthetic solutions, can be dangerous. Tricyclic antidepressants will inhibit the antihypertensive effects of the older anti hypertensive drugs, such as adrenergic neurone-blocking agents, e.g. guanethidine, a-methyl-DOPA, and clonidine. 

Catecholamine-derived tetrahydroisoquinolines can be stored within adrenergic neurons, " and when released can either activate or block a-adrenergic receptors. A number of 6,7-dihydroxytetrahydroisoquinolines related to dopamine were, therefore, studied for their inhibitory effects on the accumulation of dopamine by rat brain slices. 6,7-Dihydroxytetrahydroiso-quinoline and 5-( —)-salsolinol were found to be effective inhibitors of dopamine accumulation, while 7 -( + )-salsolinol was less potent. 6,7-Dihydroxytetra-hydroisoquinoline is both a directly and indirectly acting sympathomimetic agent. ... 

Bromocriptine (Parl el) [Antiparkinsonian Agent/Dopamine Receptor Agonist] Uses Parkin on Dz, hyperprolactinemia, acromegaly, pituitary tumors Action Direct-acting on the striatal dopamine receptors X prolactin secretion Dose Initial, 1.25 mg PO bid titrate to effect, w/ food Caution [B, ] Contra Severe ischemic heart Dz or PVD Disp Tabs, caps SE X BP, Raynaud phenomenon (vasospastic disorder resulting in discoloration of the fmgers/toes), dizziness, N, hallucinations Interactions T Effects W/ erythromycin, fluvoxamine, nefazodone, sympathomimetics, antihypertensives X effects W/ phenothiazines, antipsychotics EMS Monitor BP may cause intolerance to EtOH OD May cause NA, severe hypotension give IV fluids symptomatic and supportive... 

Sympathomimetics (directly acting) Combining MAOis with agents such as epinephrine or norepinephrine is relatively safe since MAOis cause them to accumulate in the nerve terminal (thus, they do not directly act on postsynaptic receptors). 

Actions of direct-acting catecholamines are sharply increased. Reduces effectiveness of mixed or indirect sympathomimetics. Causes severe hypertension with MAO inhibitors. Causes severe bradycardia, heart block or failure with digitalis, quinidine, or beta blockers. Potentiates actions of antihypertensive agents or CNS-depressants. 

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