α adrenoreceptors but not β adrenoreceptors increase in rabbit uterus with oestrogen

Abstract

STEROID hormones alter sympathetic function and modulate responses to catecholamines^1–3. The gonadal steroids, oestrogen and progesterone, can affect sympathetic response by altering catecholamine metabolism, and, in addition, they produce qualitative changes in the contractile response of smooth muscle to stimulation by adrenaline and noradrenaline^4–6. The response of human oviduct and uteri from several species changes from contraction to relaxation depending upon the concentrations of gonadal steroids. Contraction, which is mediated by α-adrenergic receptors, is observed in uteri from oestrogen-treated humans or rabbits, while relaxation, a β-adrenergic response, predominates during pregnancy or with progesterone treatment. Conversion between α and β response, depending on hormonal or other environmental factors, has prompted the hypothesis that α- and β-adrenergic receptors may be interconvertible^7–9. We used the radioligands ³H-dihydroergocryptine (DHE) and ¹²⁵I-iodohydroxybenzylpindolol ([¹²⁵I] IHYP) to quantitate α- and β-adrenergic receptors respectively in subcellular preparations of uteri from rabbits treated with oestrogen or oestrogen followed by progesterone. α-Adrenergic binding sites were three times greater in uteri from the oestrogen-treated animals in which α-adrenergic response was predominant than in uteri from animals treated with oestrogen followed by progesterone in which β-adrenergic response predominates. The number of β-adrenergic binding sites was unchanged by the different treatments and these sites were only 5% as numerous as ^α-adrenergic sites. These findings suggest that gonadal steroids may alter smooth muscle adrenergic response by alterations of adrenergic receptor number and do not support the hypothesis that α- and β-adrenergic receptors are simply interconvertible.