Alpha-agonists constrict both arteries and veins; however, the vasoconstrictor effect is more pronounced in the arterial resistance vessels. Constriction of the resistance vessels small arteries and arterioles increases systemic vascular resistance , whereas constriction of the venous capacitance vessels increases venous pressure. These drugs include the naturally occurring catecholamines, dopamine, epinephrine and norepinephrine, as well as catecholamine analogs such as dobutamine.
Go to www. Methoxamine and phenylephrine are used as pressor agents in treating hypotension and shock. Alpha agonists are used intravenously to increase afterload in the setting of shock Neosynephrine or orthostatic hypotension midodrine.
Alpha agonists are also used intranasally as sinus decongestants pseudoephedrine, oxymetazoline. Most alpha agonists increase the risk of stroke, tachycardia and sudden cardiac death. Learning Objectives Lecture II 1. Understand the potential sites of action for sympathomimetics and the difference between a direct and indirect acting agonist. Understand the pharmacologic actions and therapeutic actions of drugs that act at the beta 1 and beta 2 -adrenergic receptors as well as the alpha 1 -adrenergic receptor.
Know the mechanism of action and effects of amphetamine and cocaine. Understand how the pressure of sympathomimetics alters the dental management of patients. Sympathomimetics: synthetic analogs of naturally occurring catecholamines that mimic the actions of the endogenous neurotransmitters.
These agents can be divided into direct and indirect acting sympathomimetics. Direct acting agonists or antagonists can act at postsynaptic receptors. Indirect acting agonists release neurotransmitters from presynaptic nerve terminals to produce a sympathomimetic effect.
Inhibition of the membrane uptake of catecholamines by drugs such as cocaine and tricyclic antidepressants produce a sympathomimetic effect. Inhibition of monoamine oxidase by drugs such as Tranylcypromine.
In congestive heart failure, the failing heart is not able to eject blood as efficiently as the normal heart. As a result there is a decrease in cardiac output which triggers a host of compensatory actions. These include fluid retention, vasoconstriction, an increase in peripheral vascular resistance, an increase in the levels of circulating catecholamines and tissue hypoxia. Dopamine and dobutamine activate the myocardial beta 1 receptor and thus increase the force of contraction of the failing heart.
This will result in an increase in cardiac output. These drugs are reserved for use in the acute management of heart failure. These agents have a higher affinity lower equilibrium dissociation constant for beta 2 receptors when compared to beta 1.
Therefore, they selectively activate beta 2 receptors when compared to beta 1. Uses 1. Airways dysfunction; bronchial asthma, chronic bronchitis, emphysema In airways dysfunction, beta 2 selective agonists relax airways thus decreasing airways resistance. Premature labor In premature labor, the beta 2 selective agonists relax uterine smooth muscle. Drugs that relax uterine smooth muscle are referred to as tocolytic agents.
Side effects related to dental practice 1. Xerostomia, with inhaler usage. These structural modifications of the parent catecholamine nucleus result in drugs that are orally active and have longer plasma half-lives.
However, these same modifications result in lower affinity for the receptor than do the endogenous agonists epinephrine or norepinephrine. There are two structural classes of alpha 1 agonists phenethylamines which are closely aligned in structure to epinephrine and the imidazolines, compounds structurally unrelated to epinephrine.
Levonordeferin is a phenyethylamine that has been used in dental practice in combination with local anesthetics. Hypotension-to increase blood pressure during a surgical procedure where a general anesthetic has induced hypotension 2.
Ophthalmic preparations-to induce mydrasis also in topical preparations for symptomatic release of eye irritation. Cough and cold preparations-Induces constriction of nasal mucosa decreases resistance to air flow.
Indirect Acting Sympathomimetics These agents require the presence of endogenous catecholamines to produce their effects. They have little activity if catecholamines are depleted. Cocaine: Blocks reuptake of monoamines into nerve endings. Cocaine also has local anesthetic activity. Amphetamine: Promotes the release of NE from nerve endings.
Shoukas AA, Sagawa K: Control of total systemic vascular capacity by the carotid sinus baroreceptor reflex. Influence on capacitive and resistive properties of the total systemic vascular bed of the dog. Pflugers Arch Eur J Physiol ; —6. Stokland O, Thorvaldson J, Ilebekk A, Kiil F: Factors contributing to blood pressure elevation during norepinephrine and phenylephrine infusions in dogs. J Pharmacol Exp Ther ; — J Cardiovasc Pharmacol ; — Hiley CR, Thomas GR: Effects of alpha-adrenoceptor agonists on cardiac output and its regional distribution in the pithed rat.
Br J Pharmacol ; — MacLean MR, Hiley CR: Effects of enalapril on changes in cardiac output and organ vascular resistances induced by alpha 1- and alpha 2-adrenoceptor agonists in pithed normotensive rats.
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