Investigation into different types of post- and presynaptic α-adrenoceptors at cardiovascular sites in rats

An investigation of presynaptic α‐adrenoceptor subtypes in the pithed rat heart

“…The existence of presynaptic ml-receptors in the pithed rat heart has been suggested previously (Kobinger & Pichler, 1980). The present results confirm and extend those findings, but whereas Kobinger and Pichler concluded that pre-and postsynaptic a,-receptors (likewise a2) differed from each other, the present data are explicable without the necessity of a further subdivision of x-adrenoceptors.…”

“…It is now known that a2-adrenoceptors are not restricted to nerve terminals but are also present on vascular smooth muscle cells (Drew & Whiting, 1979;Docherty, MacDonald & McGrath, 1979; Kwa & van Zwieten, 1979), and this and other observations have led to a more general subclassification independent of receptor location (Starke & Langer, 1979). However, it has generally been assumed that presynaptic a-adrenoceptors are exclusively, or at least predominantly, of the M2-type, although one study suggests that al-receptors may also be present (Kobinger & Pichler, 1980). The object of the present investigation was to examine presynaptic a-adrenoceptors in the pithed rat heart, and to demonstrate that, as well as a2-receptors, a I-receptors are present.…”

No evidence for differences between pre‐ and post‐junctional α2‐adrenoceptors

“…Absolute antagonist potencies would probably also be similar pre-and post-junctionally in the pithed rat if the concentration of antagonist at the receptors were measured given that a whole series of antagonists had similar relative potencies pre-and post-junctionally. Previous accounts ofdifferences between pre-and post-junctional Mc2-adrenoceptors were from experiments carried out in vivo (Kobinger & Pichler, 1980;De Jonge et al, 1981;Hicks, 1981); no such differences have been found in vitro (present results and Skirby, 1984).…”

“…In the periphery, it is now well established that two types of zx-adrenoceptor exist, the a,-and X2-subtypes, and that both subtypes can be present postjunctionally on vascular smooth muscle (Docherty & McGrath, 1980a;Constantine et al, 1980) or prejunctionally on adrenergic nerve terminals (Kobinger & Pichler, 1980;Docherty, 1984). However, some authors have suggested that two subtypes of a-adrenoceptor are insufficient to explain all findings, and in particular that prejunctional and postjunctional X2-adrenoceptors may differ (Kobinger & Pichler, 1980;Hicks, 1981), although De Jonge et al (1981) have suggested that these differences may be small.…”

“…However, some authors have suggested that two subtypes of a-adrenoceptor are insufficient to explain all findings, and in particular that prejunctional and postjunctional X2-adrenoceptors may differ (Kobinger & Pichler, 1980;Hicks, 1981), although De Jonge et al (1981) have suggested that these differences may be small.…”