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Oestrogen administration to spayed rats produces biochemical changes in the uterus. Wrenn, Wood & Bitman (1969a) found that oestrogen increased the glycogen and nucleic acid content and the weight of the uterus and Szego & Davis (1967) noted that oestrogen raised the level of cyclic AMP in the uterus. The presence of an IUD has been shown to cause similar changes (Wrenn et al., 1969b). There thus appears to be a similarity between the action of oestrogen and the effect of an IUD. The effect of an IUD on uterine cyclic AMP was investigated in rats during the oestrous cycle and early pregnancy. The activities of adenyl cyclase and phosphodiesterase were also studied. Batches of about ten female Sprague-Dawley rats about 10 weeks old and weighing from 170 to 200 g were used. The stage of the oestrous cycle was determined from vaginal smears. Nylon IUDs were made and inserted through the cervical os into one uterine horn at dioestrus as described by Wrenn et al. (1968). The rats were kept for 2 weeks and were observed to have regular cycles before their use in experiments. Rats at pro-oestrus were placed with males of proven fertility. The vaginal smears were examined daily and the day on which spermatozoa were found was designated Day 1 of pregnancy. Rats were anaesthetized with 30 mg pentobarbital/kg given intraperitoneally. The uteri were removed and immediately dropped into liquid air. Fat was removed from the frozen uteri. The IUD-bearing and contralateral control horns were weighed separately and ground in 10 vols of 6 % trichloroacetic acid in an all-glass homogenizer and the suspension was centrifuged. The supernatant was extracted four times with ten times its volume of peroxide-free ether. 50-µ1 vol. of the trichloroacetic acid-free supernatant was then transferred to a fresh test-tube, freeze-dried and mixed with 50 µ tris buffer supplied with the cyclic AMP kit (Code TRK 432, The Radiochemical Centre Ltd., Amer- sham, England). The assay for cyclic AMP was carried out according to the instructions given with the kit. A toluene/triton X-100 based scintillant (2 vols toluene containing 0-4% w/v PPO and 0-01 % w/v POPOP and 1 vol. triton X-100) was used and radioactivity was measured in a Packard Liquid Scin¬ tillation Counter. The recovery of cyclic AMP by this extraction process was

EFFECT OF IUD ON UTERINE CYCLIC AMP AND THE ACTIVITIES OF ADENYL CYCLASE AND PHOSPHODIESTERASE DURING THE OESTROUS CYCLE AND EARLY PREGNANCY IN RATS