Alterations in the properties of the rat myometrium during gestation and post partum.

The relationships among progesterone and oestradiol contents in the sera, membrane activities, catecholamine actions and contents of cyclic nucleotides were investigated in the rat myometrium during gestation and post partum, and under conditions of hormone treatment. Progesterone contents declined and oestradiol contents increased in the sera on the 21st day of gestation. During the late stage of gestation, oestradiol related to regularity, and progesterone to an irregularity, in the spike generation of myometrial cells. In the longitudinal muscle, catecholamines dominantly activated the beta-adrenoceptors during gestation, but activation of the alpha-adrenoceptors was seen during and after delivery, while in the circular muscle, alpha-adrenoceptor dominancy altered to beta-adrenoceptors by application of catecholamines appeared simultaneously. At the late stage of gestation, oestradiol treatment rather than progesterone increased the cyclic AMP contents, in both muscle layers. In the light of the results, alteration in the myometrial properties during the last stage of gestation and delivery cannot be explained solely by progesterone withdrawal phenomena.