ACTIONS OF SOME AUTACOIDS AND PEPTIDES, INCLUDING RELAXIN, ON COSTO-UTERINE MUSCLE FROM RATS

SUMMARY 1. The actions of angiotensin II, bradykinin, oxytocin, arginine vasopressin, relaxin, serotonin and the prostaglandins E2 and F2α were examined on preparations of costo‐uterine muscle from stilboestrol‐treated rats. 2. All the agonists, except relaxin, when used in concentrations which contract the rat uterus, also produced contractions of costo‐uterine muscles. Concentration‐response curves were steep and maximal responses to the agonists were comparable. The negative log molar EC50 values were: serotonin, 6.5; angiotensin II, 8.8; bradykinin, 8.4; PGE2, 8.3; PGF2α, 7.1. The EC50 values (units/L) for oxytocin and vasopressin were 4.4 and 2.7 respectively. 3. Indomethacin (2.8 or 5 μmol/L) did not decrease the contractile effects of the peptides or serotonin. The effects of serotonin were reduced, but not reversed, by methysergide (0.94 μmol/L). 4. Porcine relaxin inhibited field stimulation‐induced contractions of costouterine muscle and uterine horns from immature rats pretreated with oestradiol cypionate and from stilboestrol‐treated mature rats. It was much less potent, and its effects were less clearly concentration‐related, on costo‐uterine muscle. 5. The inhibitory effects of relaxin on the uterus were unaffected by propranolol (1 μmol/L), confirming that on this tissue relaxin acts independently of the release of catecholamines. Progesterone (30 μmol/L) was also without effect on the action of relaxin on the uterus. 6. These results taken together indicate that the costo‐uterine muscle of the rat: (i) contracts in response to serotonin and the peptides angiotensin II, arginine vasopressin, bradykinin and oxytocin independently of the release of the contractile prostaglandins F2α and E2; and (ii) in contrast to the uterus, may lack a significant population of receptors for relaxin. These differences from the whole uterine horn of this species may reflect the absence of endometrial tissue, and/or of sensitivity of the smooth muscle cells of the costo‐uterine muscle to oestrogen.