LUTEOLYTIC EFFECTS OF PROSTAGLANDINS IN RAT PREGNANCY, AND REVERSAL BY LUTEINIZING HORMONE

Abstract

ABSTRACT The influence of prostaglandin infusions on plasma progesterone (P) was determined in pregnant rats at various stages of gestation. Serial blood samples were collected from the right atrium through an indwelling catheter before, during and after prostaglandin (PG) administration. PGE2 and PGF 2α was given as intravenous infusions of 6 hours duration through the same catheter at rates varying from 0.33 to 5.5 μg/min (total dose: 125 to 2000 μg/rat). Control rats received an equal amount of 0.9 % saline solution. Prostaglandins suppressed plasma P levels at all stages of gestation, PGF2α being much more effective than PGE2. The effect of PGF2α was not dose-dependent above a threshold level of about 125 μg/rat. The decrease in plasma P was evident within 3 hours of the start of the infusion, with further decrease in the subsequent 3 hours of infusion. The degree of luteal suppression by PGF2α varied in the course of gestation, being greatest on days 10 and 18 of pregnancy and smallest on days 4 and 16. On days 10 and 18, plasma was reduced by 75 %, on the average, during the infusion of PGF2α with further decrease to 15 % of initial values within 24 hours. Pregnancy was terminated in these rats, but was maintained in most rats treated with PGF2α at other stages of gestation. Plasma P decreased, on the average, by 60 % during the infusion of PGF2α in all rats that remained pregnant, and progesterone concentrations remained at a level of 30 % of initial values for at least 2 days. Plasma P levels as low as 20 % of initial values were compatible with undisturbed gestation. Concomitant treatment with LH prevented the decrease in plasma P induced by PGF2α on days 4, 7 and 10, but had little effect on day 18. Prolactin, given alone or with oestradiol, was completely ineffective on day 10 but on day 4, prolactin had a marginal inhibitory effect on the PGF2α-induced fall in plasma progesterone.