In general, the responses to administered catechol-amines are similar to those seen after sympathetic nerve stimulation and depend on the type of adrenoceptor in the muscle.
Bronchial smooth muscle is relaxed by epinephrine and isoproterenol through their interaction with (2-adrenoceptors. Epinephrine and isoproterenol are potent bronchodilators, while norepinephrine has a relatively weak action in this regard (see Chapter 39).
Smooth muscle of the gastrointestinal tract is generally relaxed by catecholamines, but this may depend on the existing state of muscle tone. Usually motility of the gut is reduced by catecholamines while the gastrointestinal sphincters are contracted. Catecholamines appear to produce relaxation of the gut through an action on ^-adrenoceptors on ganglionic cells. Activation of these receptors reduces acetylcholine release from cholinergic neurons. Catecholamines also may produce gastrointestinal relaxation through an action on ( 2-adrenoceptors on smooth muscle cells. Contraction of the sphincters occurs through an action on aradreno-ceptors. These effects are quite transient in humans and therefore have no therapeutic value.
The radial (dilator) muscle of the iris contains a-adrenoceptors. Epinephrine and norepinephrine cause dilation of the pupil (mydriasis) by contracting the dilator muscle.
Uterine muscle contains both a- and (-adrenocep-tors, which mediate contraction and relaxation, respectively. The response of the human uterus to cate-cholamines is variable and depends on the endocrine balance of the individual at the time of amine administration (see Chapter 62). During the last stage of pregnancy and during parturition, epinephrine inhibits the uterine muscle, as does isoproterenol; norepinephrine contracts the uterus.
The detrusor muscle (which contains ( 2-adrenocep-tors) in the body of the urinary bladder is relaxed by epinephrine and isoproterenol. On the other hand, the trigone and sphincter (which contain a1-receptors) are contracted by norepinephrine and epinephrine; this action inhibits the voiding of urine.
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