The costo‐uterine muscle of the rat contains a homogeneous population of β‐adrenoceptors

1 The effects of two selective β‐adrenoceptor antagonists on the inhibitory responses to some sympathomimetic amines of electrically‐stimulated preparations of costo‐uterine muscle, taken from virgin rats, have been examined quantitatively. 2 pA2 values for the antagonist, atenolol (β1‐selective) and ICI 118,551 (β2‐selective) were obtained using as agonists, fenoterol (β2‐selective agonist) and noradrenaline (α‐ and β‐adrenoceptor agonist, β1‐selective); and in addition, with ICI 118,551 only, isoprenaline (β‐agonist, non‐selective) and adrenaline (α‐ and β‐adrenoceptor agonist, β2‐selective). Catecholamine uptake mechanisms and α‐adrenoceptors were not blocked in any of these experiments. 3 Atenolol competitively antagonized the effects of fenoterol and noradrenaline to a similar extent, the pA2 values being 5.4 and 5.7, respectively. 4 ICI 118,551 competitively antagonized the effects of fenoterol, isoprenaline, adrenaline and noradrenaline to a similar extent; pA2 values ranged from 8.7 with noradrenaline to 9.1 with isoprenaline. 5 These results extend our previous observations which indicated that the adrenoceptors mediating inhibition of electrically‐evoked contractions of costo‐uterine muscle of the virgin rat are homogeneous and of the β2‐subtype. 6 The potency of the β1‐selective agonist RO 363 in producing inhibition of electrically‐evoked contractions of this tissue was also examined. RO 363 was 200 times less potent than isoprenaline but was a full agonist. This indicates that there is efficient coupling between β2‐adrenoceptor activation and tissue response in this non‐innervated preparation.