Characteristics of beta-adrenergic receptors in longitudinal muscle membranes of rat uterus: changes in kinetic properties of the receptor during gestation.

Binding characteristics of beta-adrenergic receptors of longitudinal muscle membranes isolated from different stages of pregnant rat myometrium were investigated using [3H]dihydroalprenolol. Between Days 15 and 21 of gestation, the ratio of beta 1- and beta 2-adrenergic receptors of longitudinal membranes was constant. The membranes were found to be predominant in beta 2-adrenergic receptors. The concentration of longitudinal muscle beta-adrenergic receptors increased significantly during the last 7 days of gestation. Kinetic binding studies implied that the affinity of the membrane beta-adrenergic receptors decreased through a slight decrease in the association rate and a large increase in the dissociation rate with progression of pregnancy. A Scatchard plot indicated that longitudinal muscle in beta-adrenergic receptors on Days 15 and 18 constitute a single class of independent sites. By contrast, the dissociation kinetics, the convex downward curvature in a Scatchard plot and a Hill coefficient (h) of less than 1.00 of [3H] DHA binding to beta-receptors of muscle on Day 21 suggested the existence of negatively cooperative multiple binding sites for beta-adrenergic ligand. These results suggest that changes in the dynamics of uterus beta-adrenergic receptors play an important role in the onset of labor.