The adrenoceptors (adrenergic receptors) mediate the diverse effects of the neurotransmitters of the sympathetic nervous system, norepinephrine, and epinephrine, at virtually all sites throughout the body. During the last century, the adrenoceptors have been extensively studied by a variety of functional and molecular techniques, and have been progressively subdivided (Hieble et al. 1995a). Currently, nine adrenoceptors have been cloned, and they have been divided into three major categories, the a1-adrenoceptors, (^-adrenoceptors, and ^-adrenoceptors. Although there are pharmacological data that suggest the existence of additional adrenoceptor subtypes, it now seems likely that this results from multiple 'affinity states' of a particular adrenoceptor subtype, rather than from a new adrenoceptor protein yet to be cloned.
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