Adrenomimetic Drugs

=The adrenomimetic drugs mimic the effects of adrenergic sympathetic nerve stimulation on sympathetic effectors; these drugs are also referred to as sympathomimetic agents.

=The adrenergic transmitter norepinephrine and the adrenal medullary hormone epinephrine also are included under this broad heading.

= The adrenomimetic drugs are an important group of therapeutic agents that can be used to maintain blood pressure or to relieve a life-threatening attack of acute bronchial asthma.

=They are also present in many overthe-counter cold preparations because they constrict mucosal blood vessels and thus relieve nasal congestion.

Classification of Adrenomimetic drugs:-

⇨There are many drugs that are given bellow:-

⇾Albuterol 

⇾Amphetamine

⇾Dobutamine 

⇾Dopamine 

⇾Ephedrine 

⇾Epinephrine 

⇾Isoproterenol 

⇾Metaraminol 

⇾Methoxamine 

⇾Norepinephrine 

⇾Phenylephrine 

⇾Terbutaline 

CHEMISTRY:-

= The adrenomimetic drugs can be divided into two major groups on the basis of their chemical structure: the catecholamines and the noncatecholamines. The catecholamines include norepinephrine, epinephrine, and dopamine,all of which are naturally occurring,and several synthetic substances, the most important of which is isoproterenol (isopropyl norepinephrine).

=The L-isomers are the naturally occurring forms of epinephrine and norepinephrine and possess considerably greater pharmacological effects than do the D-isomers. Throughout most of the world, epinephrine and norepinephrine are known as adrenaline and noradrenaline, respectively. 

=Noncatecholamine adrenomimetic drugs differ from the basic catecholamine structure primarily by having substitutions on their benzene ring.

MECHANISM OF ACTION :-

=Many adrenomimetic drugs produce responses by interacting with the adrenoceptors on sympathetic effector cells. 

=The effect of a given adrenomimetic drug on a particular type of effector cell depends on the receptor selectivity of the drug,the response characteristics of the effector cells,and the predominant type of adrenoceptor found on the cells. 

 

=The interaction of compounds with these adrenoceptors initiates a chain of events in the vascular smooth muscle cells that leads to activation of the contractile process. Thus, norepinephrine and epinephrine, which have high affinities for α-adrenoceptors, cause the vascular muscle to contract and the blood vessels to constrict. Since bronchial smooth muscle contains β2-adrenoceptors,the response in this tissue elicited by the action of 2β-adrenoceptor agonists is relaxation of smooth muscle cells.

= Epinephrine and isoproterenol, which have high affinities for β2-adrenoceptors,cause relaxation of bronchial smooth muscle. Norepinephrine has a lower affinity for β2-adrenoceptors and has relatively weak bronchiolar relaxing properties.

=Adrenomimetic drugs can be divided into two major groups on the basis of their mechanism of action. Norepinephrine, epinephrine, and some closely related adrenomimetics produce responses in effector cells by directly stimulating α- or β-adrenoceptors and are referred to as directly acting adrenomimetic drugs.

=Many other adrenomimetic drugs, such as amphetamine,do not themselves interact with adrenoceptors,yet they produce sympathetic effects by releasing norepinephrine from neuronal storage sites (vesicles). The norepinephrine that is released by these compounds interacts with the receptors on the effector cells.

.=These adrenomimetics are called indirectly acting adrenomimetic drugs. The effects elicited by indirectly acting drugs resemble those produced by norepinephrine.

=An important characteristic of indirectly acting adrenomimetic drugs is that repeated injections or prolonged infusion can lead to tachyphylaxis (gradually diminished responses to repeated administration).

=This is a result of a gradually diminishing availability of releasable norepinephrine stores on repeated drug administration. The time frame of the tachyphylaxis will vary with individual agents. 

=The actions of many indirectly acting adrenomimetic drugs are reduced or abolished by the prior administration of either cocaine or tricyclic antidepressant drugs (e.g., imipramine).

= These compounds can block the adrenergic neuronal transport system and thereby prevent the indirectly acting drug from being taken up into the nerve and reaching the norepinephrine storage vesicles. Lipophilic drugs (e.g.,amphetamine),however, can enter nerves by diffusion and do not need membrane transport systems.