Adrenoceptors interact not only with norepinephrine but also with the adrenal medullary hormone epineph-rine and a number of chemically related drugs. However, the responses produced by the drugs in different autonomic structures differ quantitatively or qualitatively from one another.
On the basis of the observed selectivity of action among agonists and antagonists, it was proposed that two types of adrenoceptors exist. These were designated as a- and (3-adrenoceptors. Subsequently, it has become necessary to classify the adrenoceptors further into a0 a2-, pi-, and p2-receptor subtypes.Table 9.1 indicates present knowledge of the distribution of the subtypes of adrenoceptors in various tissues.
The aj-adrenoceptors are located at postjunctional (postsynaptic) sites on tissues innervated by adrenergic neurons. a2-Adrenoceptors having a presynaptic (i.e., neuronal) location are involved in the feedback inhibition of norepinephrine release from nerve terminals (discussed later). a2-Receptors also can occur postjunc-tionally. The ^-adrenoceptors are found chiefly in the heart and adipose tissue, while p2-adrenoceptors are located in a number of sites, including bronchial smooth muscle and skeletal muscle blood vessels, and are associated with smooth muscle relaxation.
Activation of a1-adrenoceptors in smooth muscle of blood vessels leads to vasoconstriction, while activation of p2-adrenoceptors in blood vessels of skeletal muscle produces vasodilation. Activation of ^-adrenoceptors on cardiac tissue produces an increase in the heart rate and contractile force.
Norepinephrine and epinephrine are potent a-adrenoceptor agonists, while isoproterenol, a synthetic
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