Description:
Both Sgd 101/75 (4(2‐imidazoline‐amino‐2‐methylindazol‐chlorhydrate) and noradrenaline contracted anococcygeal preparations from the mouse and rabbit.The maximal response to and sensitivity (EC30 values) of each drug on the mouse was similar, whereas in the rabbit, Sgd 101/75 was a partial agonist relative to noradrenaline (intrinsic activity 0.50) and the EC30 value was 14.5 times higher than for noradrenaline.In both species, phenoxybenzamine (0.3 nm for 30 min followed by 20 washes over 30 min) reduced the effects of Sgd 101/75 preferentially. When noradrenaline was then added in the presence of a high concentration of Sgd 101/75 (400 μm), its maximum response and EC30 value were decreased in the rabbit, but not in the mouse.When a range of concentrations of phenoxybenzamine (30 pm‐3μm for 30 min followed by 20 washes over 30 min) was tested, the maximum response of the rabbit anococcygeus to noradrenaline declined in a concentration‐dependent manner. Similar experiments in the mouse resulted in two distinct sensitivities of the maximum response of noradrenaline to phenoxybenzamine (at 0.3 and 300 nm).The results indicate that noradrenaline acts on two receptors to contract the mouse anococcygeus (α1 and α1s‐adrenoceptors), but only one (α1‐adrenoceptor) in preparations derived from the rabbit.