A comparison of the actions of noradrenaline and UK-14,304 in the longitudinal smooth muscle of the rat isolated portal vein — no evidence for a population of post-junctional alpha2-adrenoceptors

The pharmacological characteristics of post-junctional alpha-adrenoceptors mediating contractions of the longitudinal smooth muscle of the rat isolated portal vein have been examined. Responses to the noncumulative addition of either noradrenaline, or the selective alpha2-adrenoceptor agonist UK-14,304, were equally sensitive to a low concentration (0.005 μmol/l) of prazosin. Idazoxan (0.1 μmol/l–0.5 μmol/l), a selective alpha2-adrenoceptor antagonist, was less potent than prazosin against both agonists. The combination of 0.1 μmol/l idazoxan and 0.125 pmol/l prazosin failed to produce a greater inhibition of responses to UK-14,304 than 0.125 μmol/l prazosin alone. A study involving the effect of various antagonists against a single concentration producing a submaximal response to UK-14,304, provided evidence for a prazosin-resistant component of responses which was sensitive to phentolamine. This component could, therefore, be attributed to an alpha-adrenoceptor. However, this particular response could not be ascribed to stimulation of an alpha2-subtype since the selective alpha, -adrenoceptor antagonist, corynanthine, produced greater inhibition than the selective alpha2-adrenoceptor diastereoisomer rauwolscine.