[The transitional phenomenon in contraction of uterine muscle].

Since the discovery of pregnancy reversal by Dale) and Cushny,2) it is widely believed that progesterone plays a cardinal role for the maintenance of pregnancy in mammals by keeping the estrogen-dominated uterus quiescent and hyposensi tive to any given stimulation till term. If so, what is the mode of action of pro gesterone on the uterine muscle? Taking advantage of findings and methods used in skeletal muscle physiology,a) Csapo4,5) recently has performed considerable investigations on the influence of ovarian hormones to the structure and the function of uterine muscle. Among these, the stair-case phenomenon6) revealed a close relation to the hormonal status. The stair-case phenomenon, which was first described by Bowditch7) using frog heart and developed by Hajdu and St. Gyorgyi8), indicates that the maximum tension depends on the frequency of successive stimuli. According to Csapo et al. 6.11) two types of stair-case were exhibited in rabbit uterine muscle; the positive and the negative stair-cases. The effects of ovarian hormones on the stair-case in myometrium are highly characteristic and while the estrogen-dominated uterus gives the positive stair-case, the negative stair-case appears in the progesterone-dominated uterus specifically. It is therefore worthwhile to analyse the particular phenomenon with a hope that it will offer any suggestion to the mode of action of progesterone on the uterine muscle. The present paper reports some findings on the negative stair-case with a discussion about the mechanism by which these findings are exerted.