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Fifty-three Holstein heifers were used to study the effects of oxytocin administered alone and in combination with various gonadotropins on the weights and progesterone contents of corpora lutea. Pituitary gonadotropin levels were compared in untreated and oxytocin-treated heifers at three stages of the estrous cycle.
Oxytoein injections on Days 2 through 6 significantly reduced the weights and total progesterone contents but not the progesterone concentrations of glands removed on Day 7. The concurrent administration of either purified bovine luteinizing hormone (LH) or human chorionic gonadotropin (HCG) significantly increased these parameters from the oxytocin treatment levels to normal or supra-normal values. Neither purified bovine prolactin nor urea-incubated HCG overcame the inhibitory effects of oxytocin. Neither oxytocin treatment on Days 2 and 3 nor oxytocin together with HCG significantly altered luteal function when the glands were taken on Day 4.
It was concluded that LH is the luteotropic hormone and that the cow requires at least two periods of luteotropic stimulation for normal corpus luteum development: one at ovulation and another after Day 4 of the cycle.
Oxytocin depleted total pituitary gonadotropin levels by about half during estrus or on Day 7, but did not alter levels at Day 4.
Heifers' pituitary gonadotropin levels and the total progesterone in their corpora lutea were negatively correlated in control (-0.75 on Days 4 and 7 pooled) and positively correlated in oxytocin- (0.78 on Days 4 and 7 pooled) treated groups (P < 0.0001). On Day 7 the correlation coefficient in the oxytocin-treated group was 0.96. The positive correlations in the treated animals are interpreted to mean that pituitary gonadotropin levels reflect plasma levels, and that these levels are directly controlling (and limiting) progesterone synthesis in the corpus luteum. The negative correlations reflect pituitary storage of gonadotropin.