The adrenergic antagonists [3H]prazosin and [3H] rauwolscine were used to identify α1- and α2-adrenoceptors respectively in the ovine myometrium. Ewes were allocated to four groups according to steroid hormone treatments or physiological status, namely ovariectomized ewes either as untreated controls, treated with oestradiol-17β or progestagen plus oestradiol-17β, and pregnant ewes at mid-gestation. Binding of both [3H]prazosin and [3H]rauwolscine to membrane preparations from the ovine myometrium was saturable, of high affinity and rapidly reversed by phentolamine (10 μmol/l). Based on the relative order of potency of selected adrenergic agonists and antagonists, the myometrial binding sites labelled by [3H]prazosin and [3H]rauwolscine were characterized as α1- and α2adrenoceptors respectively. Saturation binding studies with [3H]prazosin showed that the number of α1adrenoceptors was low (maximal binding capacity, Bmax, between 19 and 24 fmol/mg protein) and there were no noticeable differences between the animal groups. Moreover, the equilibrium dissociation constant (K
d) did not vary significantly between groups (K
d between 0·10 and 0·17 nmol/l). In contrast, saturation binding studies with [3H]rauwolscine revealed the presence of a high number of α2-adrenoceptors. Values of Bmax were far higher in the pregnant ewes (1096±241 fmol/mg protein; means ± s.d.) than in any of the non-pregnant ovariectomized ewes. For these latter groups, the highest Bmax values were found in the group treated with both progestagen and oestrogen (382±77 fmol/mg protein) compared with treatment with oestrogen alone (101±8 fmol/mg protein) or with controls (82±12 fmol/mg protein). The results of the present study, especially those obtained under a high-progesterone environment (e.g. pregnancy), strongly suggest a role of steroid hormones in the control of the number of α2-adrenoceptors in the ovine myometrium. In contrast, they did not yield any supporting data for a similar role on myometrial α1-adrenoceptors in the ewe.
Journal of Endocrinology (1990) 127, 471–479