A COMPARISON OF PLASMA VASOPRESSIN AND OXYTOCIN CONCENTRATIONS DURING THE ESTROUS-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 (mid-luteal 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<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<0.001). Analysis of the frequent samples showed a significant (P<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. The concentrations of vasopressin measured are unlikely to be sufficient to influence the activity of the reproductive tract in vivo.