Receptors for autonomic neurotransmitters

As discussed in the previous section, all the effects of the ANS in tissues and organs throughout the body, including smooth muscle contraction or relaxation alteration of myocardial activity and increased or decreased glandular secretion, are carried out by only three substances acetylcholine, norepinephrine, and epinephrine. Furthermore, each of these substances may stimulate activity in some tissues and inhibit activity in others. How can this 

The neurotransmitters of the ANS and the circulating catecholamines bind to specific receptors on the cell membranes of effector tissue. Each receptor is coupled to a G protein also embedded within the plasma membrane. Receptor stimulation causes activation of the G protein and formation of an intracellular chemical, the second messenger. (The neurotransmitter molecule, which cannot enter the cell, is the first messenger.) The function of intracellular second messenger molecules is to elicit tissue-specific biochemical events within the cell that alter the cell s activity. In this way, a given neurotransmitter may stimulate the same type of receptor on two different types of tissue and cause two different responses due to the presence of different biochemical pathways within each tissue. 

Furthermore, each class has at least two subtypes of receptors alr a2, Plr and P2- All of these receptors are linked to G proteins and second messenger systems that carry out the intracellular effects. 

Alpha receptors are the most abundant of the adrenergic receptors. Of the two subtypes, a,-receptors are more widely distributed on the effector tissues these receptors tend to be excitatory. For example, stimulation of aj-receptors causes contraction of vascular smooth muscle, resulting in vasoconstriction. 

Hypertension, or a chronic elevation in blood pressure, is a major risk factor for coronary artery disease congestive heart failure stroke kidney failure and retinopathy. An important cause of hypertension is excessive vascular smooth muscle tone or vasoconstriction. Prazosin, an aradrenergic receptor antagonist, is very effective in management of hypertension. Because oq-receptor stimulation causes vasoconstriction, drugs that block these receptors result in vasodilation and a decrease in blood pressure. 

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