GESTATION 



1015 



substance is relaxin or a contaminant of the 

 relaxin extract. 



Krantz, Bryant and Carr (1950) reported 

 than an aqueous extract of the corpus lu- 

 teum would produce an inhibition or de- 

 crease of uterine activity in the guinea pig 

 and rabbit previously primed with estrone. 

 This has been amply confirmed with both 

 in vivo and in vitro preparations involving 

 spontaneous contractions measured isomet- 

 rically in the guinea pig (Felton, Frieden 

 and Bryant, 1953; Wada and Yuhara, 1956; 

 JMiller, Kisley and Murray, 1957) , rat (Saw- 

 yer, Frieden and Martin, 1953; Wada and 

 Yuhara, 1956; Bloom, Paul and Wiqvist, 

 1958), and mouse (Kroc, Steinetz and 

 Beach, 1959). However, Miller, Kisley and 

 Murray (1957) failed to show any action of 

 relaxin on uterine motility in the rabbit 

 and the human being in vitro. Thus, the in- 

 formation on the rabbit is contradictory and 

 a similar situation exists with regard to the 

 human female for whom both positive and 

 negative results have been reported follow- 

 ing treatment with relaxin for threatened 

 abortion (McGaughey, Corey and Thorn- 

 ton, 1958; Stone, Sedlis and Zuckerman, 

 1958; Eichner, Herman, Kritzer, Platock 

 and Rubinstein, 1959). In briefly summar- 

 izing the action of relaxin on the uterine 

 myometrium it should be pointed out that 

 ( 1 ) relaxin inhibits uterine motility in an 

 estrogen-primed animal, (2) the action may 

 be species-limited, and (3) relaxin treat- 

 ment docs not interfere with the action of 

 pitocin. 



XIII. Parturition 



A. PROGESTERONE 



A number of theories have been suggested 

 to explain the hormonal control of parturi- 

 tion. The most popular is that parturition 

 is due to a decrease in the level of progester- 

 one which allows oxytocin to exert its ef- 

 fect on the uterus. Evidence has already 

 been presented indicating that pregnancy 

 can be maintained in the castrated rabbit 

 by an extract of corpora lutea, or progester- 

 one, and even prolonged in rats (Nelson, 

 Pfiffner and Haterius, 1930; Miklos, 1930), 

 mice (Mandelstamm and Tschaikowsky, 

 1931), and rabbits (Zarrow, 1947a). Snyder 



(1934) and Koff and Davis (1937) pro- 

 longed gestation in rabbits by inducing the 

 formation of new corpora lutea during the 

 last trimester of pregnancy. 



Knaus (1930) originally noted a marked 

 antagonism between posterior pituitary ex- 

 tract and the corpus luteum hormone and 

 Koff and Davis (1937) reported that in pro- 

 longed gestation induced by progesterone, 

 posterior pituitary extract was ineffective 

 until two days after the last injection. 

 Csapo (1956a) performed a series of elegant 

 experiments and concluded that progester- 

 one blocks the uterine contractions, and 

 that premature labor could not be induced 

 with oxytocin before the 30th day of gesta- 

 tion in the rabbit except for a very small 

 percentage of animals. This has been con- 

 firmed by Fuchs and Fuchs (1958). 



Zarrow and Neher (1955) found the se- 

 rum gestagen levels in the pregnant rabbit 

 fell only after parturition was under way. 

 Hence the problem arose as to how parturi- 

 tion could begin while a high blood concen- 

 tration of gestagen was present. A partial 

 answer was obtained in experiments by 

 Csapo (1956b) and Schofield (1957) who 

 showed that the progesterone-dominated 

 uterus of the pregnant rabbit becomes es- 

 trogen-dominated and responsive to oxy- 

 tocin 24 hours before parturition. Hence the 

 concentration of progesterone in the serum 

 is meaningless by itself and it could be 

 theorized that the significant point is the 

 ratio of estrogen to progesterone. Csapo 

 (1956a), however, offered an alternative so- 

 lution. He observed a local effect of pla- 

 cental progesterone on the myometrium so 

 that the myometrium closest to the placenta 

 is under a greater progesterone-dominance 

 than that portion of the myometrium lying 

 more distant. Hence the local level of pro- 

 gesterone would be the significant factor in 

 the onset of parturition and not the systemic 

 level. 



B. OXYTOCIN 



It is now generally believed that parturi- 

 tion is the result of the action of the pos- 

 terior pituitary hormone on the myome- 

 trium of the uterus sensitized by estrogen. 

 The development of this hypothesis followed 

 from the well known fact that oxytocin pro- 



