Final answer:
Progesterone and relaxin are the two agonists that cause myometrial relaxation, with progesterone preventing contractions early in pregnancy and relaxin increasing pelvic elasticity for childbirth.
Step-by-step explanation:
The two agonists that cause myometrial relaxation are progesterone and relaxin. Progesterone works by maintaining the uterine lining and preventing contractions early in pregnancy. As the pregnancy progresses, the placenta begins to take over the secretion of progesterone to continue this effect. On the other hand, relaxin is secreted by the corpus luteum and then by the placenta, which increases the elasticity of the symphysis pubis joint and pelvic ligaments, making room for the growing fetus and aiding in the dilation of the cervix during labor.
Oxytocin, in contrast, is a powerful uterine muscle stimulant that promotes labor and delivery. It functions in a positive feedback loop to enhance the strength of uterine contractions as labor nears. Prostaglandins, too, play a role in enhancing uterine contractile strength alongside oxytocin, but these substances are not involved in myometrial relaxation.