The Failure of Progesterone to Affect Myometrial Activity in the Guinea-Pig

Abstract

SUMMARY A study was made of the effects of both systemic and intrauterine progesterone and medroxyprogesterone acetate (MPA) administration on the duration of pregnancy in the guinea-pig. In no case was pregnancy prolonged beyond normal term. When progesterone and MPA were administered to non-pregnant guinea-pigs (in doses up to 50 times that required to block myometrial activity in the rabbit) there was no effect on the amplitude and frequency of intrauterine pressure cycles recorded in vivo by means of intrauterine balloons. The response of the uterus to a standard dose (20 m-u.) of oxytocin was unchanged by progesterone. The administration of progesterone by an intrauterine, instead of a systemic, route did not alter the result. It is concluded that progesterone is not a myometrial blocking agent in the guinea-pig.

Quantitative Changes in Myometrial Activity in the Guinea-Pig During Pregnancy

“…However, the significant increase in the amplitude of the pressure cycles is contrary to the well-documented findings of a depressant effect of the corpus luteum on myometrial activity in the rabbit (Knaus, 1926;Allen & Corner, 1929;Schofield, 1957), human female (Knaus, 1934;Csapo & Pinto-Dantas, 1966;Coutinho, 1968) and cow (Evans & Miller, 1936;Porter, unpublished work) in vivo, and sow (Keye, 1923) and cow (Cupps & Asdell, 1944) in vitro. Although this apparent species difference cannot be explained as yet, it is consistent with earlier findings (Porter, 1969(Porter, , 1970 that exogenous progesterone had no inhibitory action upon myometrial activity in the non-pregnant guinea-pig.…”

“…Zarrow, Anderson & Callantine (1963) and Schofield (1964) reported that parturition in the guinea-pig could not be delayed by the systemic administration of progesterone. Not only was this confirmed recently (Porter, 1969(Porter, , 1970, but it was also found that even the intrauterine administration of large doses of progesterone failed to prevent delivery occurring at the normal time. Furthermore, in the non-pregnant animal, neither systemic nor intrauterine progesterone treatment had an effect upon the mechanical activity of the uterus in vivo ; a finding in contrast to the well documented effects of progesterone on myometrial activity in the rabbit (Csapo & Takeda, 1965;Porter, 1968).…”

“…The recording technique was similar to that described earlier (Porter, 1970). Recording balloons of 1-5-ml capacity were inserted into the gravid horns, whereas the balloons which were inserted into the sterile horns varied from 1 -5-ml to 50-ml capacity depending on the stage of pregnancy.…”

“…This correction is essential as was emphasized by Anderson, Turnbull & Murray (1967) since the intrauterine pressure produced by a given tension in the myometrium is inversely proportional to the radius of the uterine horn. By correcting the amplitude data on the basis of an arbitrarily selected uterine radius (0-5 cm), it was possible to render the intrauterine pressures directly proportional to (Porter, 1970) Analysis of the recordings from the gravid horns showed that there was significantly less 'total activity' than was found in non-pregnant animals ( there was also a slight increase in the mean amplitude of pressure cycles recorded after each oxytocin treatment, these changes were statistically signifi¬ cant only in the sterile horn during the last third of pregnancy.…”

Up-regulation of oxytocin receptors in non-pregnant rat myometrium by isoproterenol: effects of steroids

“…The idea of progesterone withdrawal as a prerequisite for parturition is, however, not valid in all species. In guinea pig (Porter 1970) and human pregnancy (Yannone et al 1968, Tulchinsky et al 1972) progesterone levels do not decline before the onset of labor and in human pregnancy (Lofgren & Backstrom 1997) high progesterone concentrations during parturition appeared to be related to effective labor. Interestingly, Vallet et al (1990) observed an increase in ewe uterine OTR following 12 days progesterone treatment and in accordance with the present results, Ruzycky & Crankshaw (1988) observed no inhibitory effect of progesterone on oxytocin induced uterine contractility in estrogen dominated rats.…”