A comparison of plasma vasopressin and oxytocin concentrations during the oestrous cycle of the ewe.

Blood samples were collected twice daily (09.30 and 17.00 h) via indwelling jugular-vein cannulae from five ewes throughout an entire oestrous cycle. Additional frequent samples were taken at 30-min intervals for 8 h on each of days 3 (early luteal phase), 9 (midluteal phase), 12 (late luteal phase) and 0 (day of oestrus). Plasma concentrations of arginine vasopressin and oxytocin were measured in all samples by radioimmunoassay and progesterone was measured in the twice-daily samples only. Both oxytocin and progesterone showed the expected pattern of plasma concentrations, increasing during the early luteal phase, reaching a plateau and declining either preceding (oxytocin) or at (progesterone) luteolysis. Vasopressin concentrations showed a significant dependence on the day of cycle (P less than 0.05, analysis of variance) with concentrations lowest at oestrus and minor peaks on days 4 and 8-9. There was no correlation between the concentrations of vasopressin and progesterone. Vasopressin values were significantly higher in the morning than in the afternoon samples (1.3 cf. 0.9 pmol/l; P less than 0.001). Analysis of the frequent samples showed a significant (P less than 0.001) dependence on the time of day for vasopressin but not oxytocin. Values were high throughout the morning, declined to a trough at 15.00 h and rose again by 17.00 h. We conclude that there is a minor variation in the vasopressin concentration during the oestrous cycle, which is not related to the circulating progesterone concentration but could be regulated by oestradiol. We also provide evidence for a diurnal rhythm in the release of vasopressin into the plasma in the ewe.(ABSTRACT TRUNCATED AT 250 WORDS)