Pulsatile secretion of gonadotrophins, ovarian steroids and ovarian oxytocin during prostaglandin-induced regression of the corpus luteum in the cow

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Abstract
A luteolytic dose (500 .mu.g) of cloprostenol was given on day 12 of the estrous cycle to 5 heifers. Blood samples were collected simultaneously from the caudal vena cava and jugular vein at 5-20-min intervals from -6 to 0 (control period), 0 to 12 and 24 to 36 h after PG injection. Pulses of LH [luteinizing hormone] were secreted concomitantly with pulses of FSH during all sampling periods. During the control period separate FSH pulses were detected resulting in a shorter (P < 0.01) interpulse interval for FSH than LH (93 vs. 248 min). LH and FSH pulse frequencies increased (P < 0.01) beginning 1-3 h after PG to interpulse intervals of 59 and 63 min, respectively, and continued to be maintained 24-36 h after PG. Concomitantly there was a 2-3-fold increase (P < 0.01) in basal concentrations and pulse amplitude for LH (but not FSH). FSH basal concentrations and pulse amplitudes decreased (P < 0.05) in 3 heifers 24-36 h after PG. Pulsatile secretion of estradiol was observed at frequencies similar to LH during the periods 4-12 h (3 heifers) and 24-36 h (2 heifers) after PG, respectively, resulting in higher (P < 0.05) mean estradiol concentrations. Progesterone concentrations in the vena cava increased (P < 0.01) 5-10 min after PG, but decreased (P < 0.01) 67% by 20 min after PG. This decrease was followed by a rise (P < 0.05) beginning 2-3 h after PG and lasting for an average of 3.3 h. After a steady dcline, basal concentrations of 1.0 ng/ml were reached 24-36 h after PG. Basal oxytocin concentrations in the vena cava and jugular vein (8.2 and 4.3 pg/ml) increased (P < 0.01) to reach maximum concentrations (2029 and 701 pg/ml) 5-10 min after PG and then decreased over a 3-5 h period and were lowest (4.3 and 3.2 pg/ml) 24-36 h after PG. Maximum prolactin concentrations were higher and appeared 5-10 min earlier in the jugular vein compared to the vena cava. Progesterone and oxytocin secretion from the corpus luteum evidently is initially increased and then dramatically decreased by a luteolytic dose of PG. The reduction of progesterone concentrations below a certain threshold level in the presence of low estradiol concentrations probably eliminates the negative feedback effect on gonadotropin secretion, thereby allowing the frequency and amplitude of LH pulses, and to a lesser extent the frequency of FSH pulses, to icnrease. The increase in FSH and LH pulse frequencies probably stimulates, after a variable period of time, the development of a large follicle that secretes increasing concentrations of estradiol.