Source of ovarian inhibin secretion during the oestrous cycle of the sheep

The source of inhibin secretion by the ovary in the sheep at different stages of the oestrous cycle was investigated by in-vivo cannulation of the ovarian veins. Twenty-four Scottish Blackface ewes were allocated to four groups of six ewes, i.e. those operated on during the luteal phase (day 10), and those operated on during the follicular phase 24-30, 36 and 60 h following an injection of 125 micrograms cloprostenol on day 10 of the luteal phase. Samples of jugular and timed ovarian venous blood were collected under anaesthesia before and after enucleation of the corpus luteum. Ovaries were then removed and follicles dissected out. Following injection of cloprostenol, luteal regression occurred as indicated by a fall in the secretion of progesterone. The concentration of inhibin in jugular venous plasma and its ovarian secretion rate were similar at all stages of the follicular phase and during the luteal phase. In contrast, the secretion rate of oestradiol rose from 2.86 +/- 0.73 pmol/min during the luteal phase to 8.70 +/- 2.24 pmol/min 24 h after injection of cloprostenol (P less than 0.05). Following enucleation of the corpus luteum the secretion rate of progesterone fell from 809 +/- 270 pmol/min to 86 +/- 30 pmol/min (P less than 0.001). There was also a smaller, artifactual fall in the secretion rate of oestradiol following enucleation of the corpus luteum, which was of similar size to a fall seen in the secretion rate of inhibin. This resulted in a significant (P less than 0.001) fall in the ratio of progesterone to inhibin, while the oestradiol to inhibin ratio remained unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)