Relaxation of the Pelvic Ligaments of the Guinea Pig Induced by Progesterone. ∗

Abstract

Early in the search for the active principle of the corpus luteum, 2 methods of extraction were reported to yield substances differing somewhat in their physiological effects. The crude extracts of Corner and Allen 1 induced progestational changes in the test rabbit.

Hisaw and his collaborators 2 , 3 described an alcohol-insoluble fraction separable from corpus luteum extracts which possessed the specific capacity of causing relaxation of the guinea pig pubis—a phenomenon which normally occurs toward the end of pregnancy. This substance was given the name “relaxin” by these investigators, who considered it to be separate and distinct from the progestational fraction which they obtained in an ether-soluble form.

A hormonal control of the pelvic changes in the guinea pig, based upon an ‘estrin-relaxin’ action, has been described in detail by Hisaw, 4 , 5 but it would appear that the need for postulating a hormone specific for the production of pelvic relaxation should not be considered imperative until the purified hormone of the corpus luteum, progesterone, 6 , 7 has been demonstrated incapable of evoking this response. With the advent of the crystalline substance the possibility of putting it to test appears, singularly enough, to have escaped attention. Accordingly, experiments were devised to determine its adequacy in fulfilling, in addition to its well demonstrated activities, the one of causing relaxation of the pelvic ligaments in the virgin female guinea pig.

Adult females, weighing 350 to 634 g, were oöphorectomized under ether anesthesia and beginning 3 to 4 days later were injected with varying dosages of estrogens,‡ followed by a course of progesterone injections, either alone or together with continued estrogen administration. Dosages and details of administration are listed in Table I, which demonstrates that in all cases in which progesterone was given, in total quantities from 0.8 to 2.2 mg, marked relaxation occurred, easily detectable on palpation.