Evidence for more than one type of post-junctional alpha-adrenoceptor.
The concept of two types of alpha-adrenoceptor, alpha 1 located on smooth muscle and mediating contraction and alpha 2 located on nerve terminals and mediating inhibition of transmitter release, has broken down. In vivo it has been shown that post-junctional receptors, with characteristics closely related to those of the alpha 2-adrenoceptors at nerve terminals, can mediate pressor responses and are, "post-junctional alpha 2-adrenoceptors". Several differences among agonists in vitro have superficial similarities to the in vivo alpha 1/alpha 2 system but do not correspond precisely and seem to point to a subdivision of post-junctional alpha 1-adrenoceptors. A preliminary hypothesis is: in vivo alpha 1 is rapid in onset, short-lived, utilises internal Ca2+, prefers alkalosis and responds to short-term stimuli such as short bursts of nerve impulses or bolus injections of catecholamines; alpha 2 is slower in onset, longer-lived, utilises external Ca2+, prefers acidosis and responds to more prolonged stimuli such as circulating catecholamines; in vitro these categories of response occur but antagonists fail to define an alpha 1/alpha 2 split, suggesting that some critical factor is missing in vitro. The implications of these trends in alpha-adrenoceptor classification are discussed in relation to current pharmacological and biochemical methods for receptor typing, to the possible physiological actions and roles of such receptors and to structure/activity relationships among agonists and antagonists.
D000818:Animals; D055598:Chemical Phenomena; D002621:Chemistry; D006801:Humans; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects; D009411:Nerve Endings / Q000187:drug effects; D009638:Norepinephrine / Q000494:pharmacology; D051381:Rats; D011941:Receptors, Adrenergic / Q000502:physiology; D011942:Receptors, Adrenergic, alpha / Q000145:classification / Q000187:drug effects / Q000502:physiology; D013329:Structure-Activity Relationship; D009435:Synaptic Transmission / Q000187:drug effects
D011941:Receptors, Adrenergic; D011942:Receptors, Adrenergic, alpha; D009638:Norepinephrine