Myometrial stretch and increases in wall tension have been implicated clinically in the initiation of labor and the etiology of preterm labor. We have taken a biomechanical and biochemical approach, using human tissues and rodent models to investigate the molecular mechanisms involved. Together with our collaborators, we have found, firstly, that the intracellular Ca requirement for activation of the contractile filaments increases during gestation. The decreased Ca sensitivity correlates with an increase in the expression of caldesmon, an actin binding protein and inhibitor of myosin activation, during pregnancy. In late pregnancy we observed an increase in ERK-mediated caldesmon phosphorylation, which appears to reverse this inhibitory effect during labor. Force generated by the myometrial contractile filaments is communicated across the plasmalemma and to the uterine wall through the focal adhesions. Using phosphotyrosine screening and mass spectrometry of stretched myometrial samples, we identified 3 stretch-activated focal adhesion proteins, FAK, p130Cas, and alpha actinin. FAK-Y397, which signals integrin engagement, is constitutively phosphorylated in term human myometrium whereas FAK-Y925, which signals downstream ERK activation, is phosphorylated during stretch. This Src mediated signaling appears to provide a tunable tension sensor in the myometrial cell. We have recently identified smooth muscle Archvillin (SmAV) as a new ERK scaffolding protein. A newly produced SmAV-specific antibody demonstrates gestation-specific increases in SmAV protein levels and stretch-specific increases in SmAV association with focal adhesion proteins. Thus, whereas increases in caldesmon levels suppress human myometrium contractility during early pregnancy, stretch-dependent focal adhesion signaling, and ERK activation contribute to subsequent myometrial activation. In parallel, biophysical measurements of smooth muscle compliance at both the cellular and tissue levels suggest that decreases in cellular compliance due to changing interactions of the actin cytoskeleton with the focal adhesions may also promote increases in uterine wall tension. Thus these results, taken together, suggest that focal adhesion proteins and their interaction with the cytoskeleton may present a new mode of regulation of uterine contractility.