DOWN-REGULATION OF MYOMETRIAL ATRIAL-NATRIURETIC-FACTOR RECEPTORS BY PROGESTERONE AND PREGNANCY AND UP-REGULATION BY ESTROGEN IN RATS

We previously reported that pregnancy and progesterone treatment abolish and ovariectomy reduces the tocolytic activity of atrial natriuretic factor (ANF) on rat uterus. The present study was carried out to determine whether the hormonal state of the animal influences myometrial ANF receptors. A single high-affinity binding site for I-125-labelled ANF(1-28) was detected in myometrial membrane preparations partially purified by discontinuous sucrose gradient centrifugation. The specifically bound ANF could be displaced completely by unlabelled ANF(1-28) (biologically active circulating form of ANF), partially (approximately 50%) by the clearance receptor (ANF-R2) ligand, ANF(4-23), and insignificantly by angiotensin II, vasopressin, carbachol and progesterone. Maximum binding capacities (fmol/mg protein +/- S.E.M.) of myometrial membrane preparations for ANF were as follows: vehicle-treated cyclic rats, 30.2 +/- 5.9 (n = 6); oestrogen-treated virgin rats, 55.8 +/- 7.1 (n = 8); progesterone-treated virgin rats, 21.9 +/- 2.4 (n = 6); 20-day pregnant rats, 18.9 +/- 4.3 (n = 4); ovariectomized rats, undetectable (n = 3); ovariectomized rats treated with oestrogen, 50.2 +/- 4.1 (n=3); virgin rats treated with progesterone antagonist RU 486, 64.5 +/- 5.2 (n = 3). Relative to cyclic rats, oestrogen and RU 486 caused a significant (P < 0.05) increase and progesterone and pregnancy caused a significant (P < 0.05) decrease in ANF myometrial receptors. The hormonal state did not change dissociation costants for ANF binding to myometrial preparations. ANF(4-23) displaced specifically bound ANF(1-28) by 50% in membrane preparations from oestrogen-treated and 100% in preparations from progesterone-treated rats suggesting that all binding sites in the latter group represented clearance receptors not involved in biological effects. The down-regulation of myometrial ANF receptors by progesterone and pregnancy and up-regulation by oestrogen correspond to reported changes in the effects of ANF on myometrial contractions.