Alpha-adrenergic receptors in human myometrium: changes during pregnancy

Myometrial biopsies from 80 women, delivered by cesarean section in 26-42 completed weeks of pregnancy, were analyzed using radiolabeled alpha 1-antagonist [3H]-prazosin and alpha 2-antagonist [3H]-rauwolscine. Statistical analysis was performed using Student's t test (two-tailed), where p < 0.05 was considered significant. The number of alpha 2-adrenergic receptors did not vary (mean value weeks 37-39 = 17.2 +/- 1.3 fmol/mg protein), while the dissociation constant (Kd) for alpha 2-receptors tended to increase with increasing gestational age, implying reduced affinity near term (p < 0.05, mean values 3.4 +/- 0.2 nM). alpha 1-Receptors, however, increased significantly (p < 0.01) in the pregnancy weeks 37-39 (mean value 29.2 +/- 2.9 fmol/mg prot) while Kd was unaltered. Increased number of alpha 1-receptors, along with a decreased affinity of alpha 2-receptors, could, in combination with other factors, play a role in the initiation of labor.