Occurrence of α1s-adrenoceptors in the mouse but not in the rabbit isolated anococcygeus preparations

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Abstract
1 Both Sgd 101/75 (4(2-imidazoline-amino-2-methylindazol-chlorhydrate) and noradrenaline contracted anococcygeal preparations from the mouse and rabbit. 2 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. 3 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. 4 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). 5 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.