The quantitatively major steroid hormones in ovarian and adrenal venous plasma of the female brush-tail possum were identified by gas chromatography–mass spectrometry. The ovarian vein plasma samples all contained oestradiol and its concentration was highest during the pro-oestrous phase of the reproductive cycle. During this phase the concentration of progesterone was below the limit of detection but at day 13 of the oestrous cycle and pregnancy, the concentration of progesterone exceeded that of oestradiol. Cortisol and corticosterone were the major steroid hormones found in all adrenal vein samples with cortisol predominant. Androgens with a 3-oxo structure, if present, were below the limits of detection in all plasma samples. Radioimmunoassays for the measurement of progesterone and oestradiol in peripheral plasma were used to follow changes in the concentrations of these steroids during the reproductive cycle.
Progesterone in serial blood samples was low at oestrus, rose gradually until day 7 and then increased more rapidly to reach a peak level of 21–29 nmol/l at around day 13. Any differences between the pregnant and non-pregnant cycles were minor. Oestradiol was only detected around oestrus when levels were variable (53·3±20·92 (s.e.m.) pmol/l; n = 4). The results indicate that the reproductive cycle of the brush-tail possum is characterized by a single peak of oestradiol at around pro-oestrus followed by gradually increasing levels of progesterone. Pregnancy appears to have no influence on the circulating concentrations of oestradiol or progesterone.
J. Endocr. (1985) 105, 53–62