Epinephrine biologic effects

The adrenal medulla synthesizes two catecholamine hormones, adrenaline (epinephrine) and noradrenaline (norepinephrine) (Figure 1.8). The ultimate biosynthetic precursor of both is the amino acid tyrosine. Subsequent to their synthesis, these hormones are stored in intracellular vesicles, and are released via exocytosis upon stimulation of the producer cells by neurons of the sympathetic nervous system. The catecholamine hormones induce their characteristic biological effects by binding to one of two classes of receptors, the a- and )S-adrenergic receptors. These receptors respond differently (often oppositely) to the catecholamines. 

The biological effects of epinephrine and norepinephrine are mediated by nine different adrenoceptors (a1A,B,D, a2A,B,c. Pi, P2. P3)-To date, only the classification into a-i, a2, pi and p2 receptors has therapeutic relevance. 

It lias been proposed that the conformational flexibility of most open-chain nearohormoncs. such as acetylcholine, epinephrine. serotonin, histamine, and related physiologically active biomolecules, permits multiple biological effects to he produced by each molecule, by virtue of their ability to. 

In 1817, die Swede Johann Arfvedson discovered the element lithium. Its biological effect is on intracellular influx of sodium during the process of axonal depolarization, which interferes with the synthesis and reuptake of neurotransmitters. In the 1950s, it began to be used in the treatment of bipolar disorder, because it dampens neurotransmission. It enhances the reuptake of dopamine, norepinephrine, and 5-HT into neuronal vesicles, reducing their action. It also reduces release of norepinephrine from synaptic vesicles and inhibits production of cAMP. It decreases the neuronal activity excited by 5-HT, dopamine, and epinephrine. 

Adrenergic drugs are natural or synthetic compounds that either partially or completely replicate the effects of norepinephrine (noradrenaline), epinephrine (adrenaline), and dopamine, and which cause a biological response similar to the activation of the sympathetic nervous system. They are also referred to as sympathomimeties beeause they mimic the stimulation of the sympathetic nervous system. 

ADRENAL MEDULLA HORMONES. Adrenaline (epinephrine) and its immediate biological precursor noradrenaline (norepinephrine, levartei-nol) are the principal hormones of the adult adrenal medulla. See Fig.l. Some of the physiological effects produced by adrenaline arc contraction of the dilator muscle of the pupil of the eye (mydriasis), relaxation of the smooth muscle of the bronchi constriction of most small blood vessels dilation of some blood vessels, notably those in skeletal muscle increase in heart rate and force of ventricular conlraction relaxation of the smooth muscle of the intestinal tract and either contraction or relaxation, or both, of uterine smooth muscle. Electrical stimulation of appropriate sympathetic (adrenergic) nerves can produce all the aforementioned effects with exception of vasodilation in skeletal muscle. 

The phosphodlesterase i which cleaves cyclic AMP is inhibited by methyl xanthines and it is entirely possible that some or all of the biological activity observed for these inhibitors of cyclic AMP degradation may be due to this action. For example, theophylline has been shown to increase phosphorylase a activity in the isolated heart and potentiate the effects of epinephrine in this assays . 

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