Uterine Stimulants and Relaxants

The physiological processes involved in parturition (i.e., labor, delivery, and birth) require a complex interplay of hormonal action, neuronal activity, and uterine smooth muscle contraction. During the first two trimesters of pregnancy, the uterus remains in a relatively quiescent state, demonstrating little or no contraction of the myometrium.This inactivity is largely the result of the inhibitory action of high circulating levels of progesterone on the uterine musculature (see Chapter 63). During the final trimester, however, uterine smooth muscle becomes increasingly excitable, such that mild muscle contractions are seen (Braxton-Hicks contractions); these gradually increase in both strength and frequency, occasionally to the extent that they may even be thought to signal the onset of labor, a phenomenon termed false labor. Parturition requires in part the integration of processes that involve cervical canal dilation and uterine smooth muscle contractions that are strong enough to expel the fetus. Other physiological events must occur at the end of pregnancy to facilitate birth. The cervix begins to soften (cervical ripening) as a direct result of connective tissue dissociation; this process may involve the actions of the peptide hormone relaxin, which is produced both in the corpus luteum and in the placenta. Relaxin also aids in the dissociation of the connective tissue between the pelvic bones, a process that also aids in the facilitation of birth. At the true onset of labor, coordinated, rhythmic contractions of the uterus begin, and as labor progresses, the myometrial contractions increase in intensity and strength. These contractions force the fetus against the cervix, further dilating the cervix. Once the cervix has dilated sufficiently, the uterine contractions push the fetus through the birth canal. A variety of endocrine hormones play a role in initiating the changes in uterine contractility, especially during the final trimester. It is probable that the concentration of receptors responsive to the hypothalamic peptide hormone oxytocin (see Chapter 59) increases in the uterine musculature in response to the increasing levels of estrogen during pregnancy. Although circulating blood levels of oxytocin do not change markedly throughout pregnancy, it is likely that the augmented number of oxytocin receptors in the uterus makes the D R U G L I S T Uterine Stimulants and Relaxants