Concentrations of progesterone and oxytocin receptors in endometrium of postpartum cows expected to have a short or normal oestrous cycle.

Luteal lifespan is short after first postpartum ovulation in early-weaned beef cows unless cows are pretreated with a progestogen. Regression of the short-lived corpus luteum in the postpartum beef cow is due to a premature release of prostaglandin F2 alpha (PGF2 alpha) from the uterus. The premature release of PGF2 alpha may be mediated through lower concentrations of receptors for progesterone, higher concentrations of oxytocin receptors, or both, in the endometrium. Thirty-one beef cows were randomly assigned to four groups at parturition. Calves from cows assigned to the short cycle group (n = 6; control) and the short cycle/endometrium group (n = 10) were weaned at 30-32 days post partum. Cows in the normal cycle group (n = 5; control) and the normal cycle/endometrium group (n = 10) received norgestomet implants for 9 days beginning 21-23 days post partum and calves were weaned at implant insertion. Duration of oestrous cycle (x +/- SEM; P < 0.01) following first postpartum ovulation for the short cycle group was 11.5 +/- 1.9 days compared with 18.8 +/- 0.6 days for the normal cycle group. On day 5 following first postpartum ovulation, cows in the short cycle/endometrium and the normal cycle/endometrium groups were hysterectomized and endometrial tissue collected for measurement of progesterone and oxytocin receptors. Mean number of total progesterone receptors per cell was lower (P < 0.05) in the short cycle/endometrium group than in the normal cycle/endometrium group. Mean concentration of oxytocin receptors (fmol mg-1 protein) in the short cycle/endometrium group was higher (P < 0.05) than that in the normal cycle/endometrium group.(ABSTRACT TRUNCATED AT 250 WORDS)