Alpha 2-adrenergic receptors in guinea pig myometrium in late pregnancy: evidence for a predominantly postjunctional location.

Uterine sympathetic nerves can exert an excitatory influence during pregnancy and parturition. Myometrial alpha 2-adrenergic receptor density falls sharply late in gestation, and diminished presynaptic neuromodulation could contribute to increased norepinephrine release reported at this time. Two studies were performed to investigate this potentially important mechanism. First, when alpha 2-adrenergic receptors were measured in well-innervated (tubal end and cervix), denervated (perifetal), and richly vascularized regions (placental) of late pregnant guinea pig uteri with the selective radioligand [3H]rauwolscine, high levels were detected in perifetal myometrium, particularly outside the site of placental implantation. Second, we report an unsuccessful attempt to demonstrate alpha 2-adrenergic receptor-mediated feedback suppression of electrically stimulated norepinephrine release from tubal and cervical regions of nonpregnant, midterm, and late-pregnant uteri. Together these results suggest that guinea pig myometrial alpha 2-adrenergic receptors are located mostly postjunctionally and are not associated with the vasculature. Thus an alpha 2-adrenergic feedback mechanism is unlikely to play an important role regulating uterine sympathetic neurotransmission.