OBJECTIVE: This study tested the hypothesis that the increase in uterine tachyphylaxis to β-adrenergic stimulation during pregnancy is associated with increased expression of the β-adrenergic receptor–inactivating protein kinases.

STUDY DESIGN: Messenger ribonucleic acid was isolated from snap-frozen myometrium collected from nonpregnant and pregnant rats ranging from 10 to 22 days of gestation. Autoradiographic analysis of β-adrenergic receptor–inactivating protein kinase messenger ribonucleic acid expression was performed after hybridization with specific complementary deoxyribonucleic acid probes for types 1 and 2 β-adrenergic receptor–inactivating protein kinases. Probe-specific hybridization was normalized for ribosomal ribonucleic acid detected with methylene blue. Protein expression was detected by Western analysis with use of specific polyclonal antibodies.

RESULTS: Myometrial β-adrenergic receptor–inactivating protein kinase type 2 messenger ribonucleic acid and protein levels increased during the course of pregnancy and in postpartum day 1. In contrast, type 1 levels remained unchanged during the same period. Estrogen treatment resulted in a modest 20% decrease in messenger ribonucleic acid levels of both subtypes. This effect was reversed with progesterone treatment.

CONCLUSION: These results suggest that the myometrium undergoes a functional remodeling late in pregnancy to a state promoting myometrial contractions. The increased myometrial expression of type 2 β-adrenergic receptor–inactivating kinase may explain the decreased effectiveness of β₂-adrenergic receptor–mediated contraction inhibition at the end of pregnancy and labor. (Am J Obstet Gynecol 1997;176:1077-83.)