Relaxation of the Pelvic Ligaments of Castrate Hysterectomized Guinea Pigs Induced by Progesterone. ∗

Abstract

Haterius and Fugo 1 showed that in castrate female guinea pigs relaxation of the pelvic ligaments could be elicited by treatment with crystalline progesterone after preliminary conditioning with estrogens. These findings cast some doubt upon the existence of a separate relaxation hormone, or at least its indispensability in the process of pelvic relaxation.

More recently Hisaw et al. 2 confirmed the ability of crystalline progesterone to relax the pelvic ligaments in the spayed and estrogenized guinea pig. But they reported that in castrates which were hysterectomized the estrogen-progesterone treatment did not induce relaxation. The authors concluded therefore that under the conditions of their experiments athe uterus is indispensable for the formation of relaxin.

It would seem from this interesting report that the presence of the uterus in some unexplained way facilitates relaxation but is not a necessary factor in the development of this reaction. Similarly progesterone is not indispensable since relaxin alone produces relaxation in the estrogenized hysterectomized castrate. Surprisingly, however, the combination of two factors (uterus and progesterone) each shown to be nonessential, together give a positive reaction.

It was not the intention of this study to analyze the complicated nature of this reaction. Rather it was thought necessary to reinvestigate our earlier conclusion, namely, that progesterone alone was sufficient to produce relaxation when given in combination with estrogen.

Furthermore it occurred to us that perhaps the failure of Hisaw et al. to obtain relaxation in castrate hysterectomized guinea pigs might be due to a time factor since their experimental procedure consisted in the injection of 250 mg of estradiol on 4 consecutive days followed by a single injection of 5 to 10 mg of progesterone on the following day.

Consequently 12 virgin guinea pigs weighing approximately 200 g were castrated and hysterectomized under ether anesthesia.