Occurrence of alpha 1s-adrenoceptors in the mouse but not in the rabbit isolated anococcygeus preparations.

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 microM), 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-3 microM 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 (alpha 1 and alpha 1s-adrenoceptors), but only one (alpha 1-adrenoceptor) in preparations derived from the rabbit.