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  • Title: Sustained effects on synthetic ovarian steroids of rat myometrial contractility.
    Author: Parkington HC, Lipton A.
    Journal: J Endocrinol; 1976 Aug; 70(2):223-7. PubMed ID: 965882.
    Abstract:
    Ovariectomized rats were given a single dose of synthetic oestrogen or progestogen, or both combined. On selected days after treatment one rat from each treatment group was killed and myometrical contractility in response to spasmogens was measured isometrically in isolated tissue baths. Contractility persisted at a low level after ovariectomy without steroid replacement (controls) for the 60 days of the experiment. The powerful potentiating action of oestrogen reached a maximum in about 10 days and sustained a high level thereafter. Progestogen treatment did not influence contractility to a significant degree when compared with saline, except that the contractions were sometimes of higher frequency and more irregular in size. There was a less powerful potentiating action on contractility with combined oestrogen and progestogen treatment than with oestrogen alone. It reached a maximum in 4 days and declined more rapidly than with oestrogen-treated preparation but was still vigorous for up to 30 days. Virgin female Wistar rats were ovariectomized and 2 days later were given a single .2 ml dose of steroid im. Medroxyprogesterone acetate, 100 mg/kg of body weight or estradiol valerate 100 mcg were used. Group 1 (controls) received only solvents, Group 2 received progestogen in saline, Group 3 received the estrogen in oil, and Group 4 received both steroids. On selected days after treatment a rat from each group was killed, the uterus removed, and 2 myometrial strips were suspended in isolated organ baths on Statha G10B strain gauges for isometric recording. After 30 minutes, a dose of either 10 mcg/ml of acetylcholine or .25 mcg/ml of oxytocin was added to each bath. Tension was recorded for 10 minutes followed by a 15-minute wash. These doses were supramaximal. Responses began with a brief contraction and then became a series of fast powerful contrations which continued for 30-60 minutes. Each strip was later removed, prepared for sectioning, and length and cross-section were measured. Controls responded similarly to saline and to oil injections. There was a significnt initial increase in the response to both acetylcholine (p less than .05) and oxytocin (p less than .01). From Days 6 to 20 the responses did not change significantly. After Day 20 the response to acetylcholine declined significantly (p less than .01) but the response to oxytocin was unchanged for the 60 days of the test. With estrogen treatment, increase in response to both spasmogens was highly significant (p less than .001) in the first 18 days at which time it reached a plateau and remained the same for 60 days. With the progestogen treatment the mean response to acetylcholine increased significantly for the first 2 days (p less than .01) then maintained a plateau for the next 3 days equal to the control group and subsequently declined slowly (p less than .001). The contractile response to oxytocin remained constant for 60 days. With estrogen plus progestogen treatment there was a significant increase in contractility up to Day 5 (p less than .001). From Days 5 to 9 plateau levels were maintained with acetycholine and at a lower level with oxytocin. After Day 9 there was a significant decline in both spasmogens effect (p less than .001). With controls and all steroid treatments there was a highly significant difference in responses to acetylcholine and to oxytocin, with acetylcholine consistently giving stronger responses (p less than .001) than oxytocin. Results indicated that medroxyprogesterone acetate required the presence of estrogen for its myometrial action. There was also evidence that estrogen actions were modified by progestogen. It is concluded that single doses of estrogen and progestogen had depot actions of adequate intensity and duration for prolonged studies in myometrial contractility and that there was significant interaction between them when given together. Rat myometrium retained a significant response to spasmogens for at least 60 days after ovariectomy without steroid replacement, perhaps from adrenocortical steroid secretion.
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