Influence of steroid hormone treatments or pregnancy on [3H]prazosin and [3H]rauwolscine binding to myometrial α-adrenoceptors in the ewe

Abstract

The adrenergic antagonists [3H]prazosin and [3H] rauwolscine were used to identify alpha 1- and alpha 2-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-17 beta or progestagen plus oestradiol-17 beta, 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 mumol/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 alpha 1- and alpha 2-adrenoceptors respectively. Saturation binding studies with [3H]prazosin showed that the number of alpha 1-adrenoceptors 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 (Kd) did not vary significantly between groups (Kd between 0.10 and 0.17 nmol/l). In contrast, saturation binding studies with [3H]rauwolscine revealed the presence of a high number of alpha 2-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.(ABSTRACT TRUNCATED AT 250 WORDS)