1014 



SPERM, OVA, AND PREGNANCY 



15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 

 Hours after mating 



Fig. 16.37. Change from a positive to a negative staircase as the hormone dominance of 

 the myometrium moves from the estrus to the progestational state after mating. X and 

 O indicate the two strains of rabbits used. (From B. M. Schofield, J. Physiol., 138, 1, 

 1957.) 



the uterine horn in a wave starting at the 

 tubal end. Both amplitude and rate are in- 

 creased. During pregnancy the uterus be- 

 comes relatively quiescent. In general this 

 pattern of myometrial activity has been re- 

 produced with both hormones, estradiol and 

 progesterone. 



Recently Schofield 11957), using the 

 Csapo technique, has studied, in vivo, myo- 

 metrial activity in the rabbit. In a series of 

 experiments she was able to show in several 

 strains of rabbits that, when mating occurs 

 during estrus, the uterine myometrium is 

 dominated by estrogen. Within 20 to 28 



STAIRCASE 

 Negative 



Transient 

 Positive 



26 27 28 29 30 31 

 Day of pregnancy 



32 



Fig. 16.38. Change from negative through 

 transient to positive staircase as the hormone 

 dominance reverses at the end of pregnane}', indi- 

 cating estrogen dominance. X and O indicate the 

 two strains of rabbits used. (From B. M. Schofield, 

 J. Physiol. 138, 1, 1957.) 



hours after mating, the positive staircase 

 effect passes through a transient effect to a 

 negative effect indicating the development 

 of progesterone dominance (Fig. 16.37j. 

 This condition remained in effect through- 

 out pregnancy until 24 hours before parturi- 

 tion when a reversion to estrogen domina- 

 tion was indicated by the positive staircase 

 response ( Fig. 16.38) . Thus the progesterone- 

 dominated uterus is maintained throughout 

 pregnancy and the uterus is nonreactive to 

 oxytocin. Csapo (1956a) and others have 

 shown that labor cannot be induced by 

 oxytocin in the rabbit before day 30 of ges- 

 tation, but 24 hours later, on removal of the 

 progesterone block, 96 per cent of the rab- 

 bits delivered following treatment with oxy- 

 tocin. He believes that the specific action of 

 progesterone involves a blocking of the ex- 

 citation-contraction coupling which is a 

 consequence of the disturbed ionic balance 

 in the myometrial cell. Thus a block is set 

 up to the propagation of the contraction 

 wave which can be removed only by a de- 

 crease in the level of progesterone. 



The role of the water-soluble extract, re- 

 laxin in myometrial activity, is still un- 

 certain. That an inhibition of estrogen-in- 

 duced uterine contractions is obtained in 

 certain species, such as the rat, mouse, and 

 guinea pig, with relaxin preparations is un- 

 (luestionable. However, we still have not 

 answered the questions as to w'hether this 

 hormone plays a role in uterine contractions 

 under normal physiologic conditions and 

 whether the uterine contraction-inhibiting 



