The initiation of labor is normally attributed to an endocrine cascade that leads to myometrial "activation" (increased expression of genes encoding "contraction-associated proteins" or CAPs, such as agonist receptors, gap junctions and ion channels) and the "stimulation" of the myometrium (increased production of uterotonic agonists). Our current evidence suggests that this model is incomplete and that mechanical signals from the growing fetus stimulate both the growth of the uterus and activation of the myometrium during labour. The long term objective of this project is therefore to define the contribution of mechanical stretch of the myometrium to the initiation of labor. Three specific aims will address this objective. I) A unilaterally pregnant rat model will be used to determine whether stretch induces CAP gene expression and to define the mechanisms by which this is achieved. With this model, the non-gravid horn can be experimentally distended or left undistended, to determine whether expression of genes critical to induction of labor are regulated by stretch and how this regulation interacts with systemic endocrine inputs. Regulation at the level of transcription and mRNA stability will be determined by nuclear run-on and mRNA half life assays, respectively. II) We will characterize the influence of stretch on myometrial growth. Cell proliferation and hypertrophy, and extracellular matrix accumulation, will be assessed in normal pregnant rats, then the unilaterally pregnant model will be used to assess the effects of stretch on this growth and its interaction with endocrine signals. III) We will investigate the role of focal adhesion- associated proteins in transducing stretch-induced myometrial growth and CAP gene expression. Expression and activation of proteins involved in the FAK and MAP kinase (ERK1/ERK2) pathways will be assessed using cultured myometrial cells that are stretched in custom-designed chambers and transfection with dominant negative FAK will be used to test whether MAP kinase activation is downstream from FAK. Finally, MAP kinase inhibition will be used to test whether this pathway drives expression of CAP genes that can be induced in cultured cells. This study may reveal the mechanism underlying the increased incidence of preterm birth, and associated morbidity and mortality, in pregnancies complicated by multiple fetuses or polyhydramnios, since such pregnancies are associated increased uterine wall tension.