THE EXISTENCE OF A NEW SUBTYPE OF α‐ADRENOCEPTOR ON THE RAT ANOCOCCYGEUS IS REVEALED BY SGD 101/75 AND PHENOXYBENZAMINE

Abstract

1 Noradrenaline and Sgd 101/75 (4(2-imidazoline-amino)-2-methylindazol-chlorhydrate) acted as full agonists in contracting the rat anococcygeus. 2 Very low concentrations of phenoxybenzamine (0.3 nm for 2–30 min) reduced preferentially the effects of Sgd 101/75. Preparations made insensitive to Sgd 101/75 by phenoxybenzamine still contracted to noradrenaline, this contraction occurring in the presence of high concentrations (400 μm) of Sgd 101/75. 3 It is concluded therefore that the α₁-adrenoceptor in this preparation is not a homogeneous entity and must be subdivided into at least two subtypes.