Pulsatile secretion of gonadotrophins, ovarian steroids and ovarian oxytocin during prostaglandin-induced regression of the corpus luteum in the cow
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Summary. A luteolytic dose (500 μg) of cloprostenol was given on Day 12 of the oestrous 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 were secreted concomitantly with pulses of FSH during all sampling periods. However, during the control period separate FSH pulses were detected resulting in a shorter (P < 0·01) interpulse interval for FSH than LH (93 versus 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 oestradiol 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 oestradiol 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 decline, 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 4·2 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.
It is concluded that: (1) progesterone and oxytocin secretion from the corpus luteum is initially increased and then dramatically decreased by a luteolytic dose of PG; (2) the reduction of progesterone concentrations below a certain threshold level in the presence of low oestradiol concentrations probably eliminates the negative feedback effect on gonadotrophin secretion, thereby allowing the frequency and amplitude of LH pulses, and to a lesser extent the frequency of FSH pulses, to increase; and (3) 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 oestradiol.
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