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J. CARLSON and D. L. BLACK

Summary.

Exogenous oxytocin in intact heifers shortened the oestrous cycle and decreased luteal progesterone content and in vitro synthesis both with and without the addition of lh to the incubation medium. Unilateral ovariectomy decreased the proportion of shortened oestrous cycles following oxytocin treatment. Both the removal of one ovary and oxytocin injection are thought to upset the normal hypophysial—gonadal balance of hormones leading to increased variability in cycle length.

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S. J. Kehl and J. C. Carlson

Summary. The ability of systemic infusion of arachidonic acid and various prostaglandin (PG) compounds to induce luteolysis was examined in the Day 9 pseudopregnant rabbit. Administration of PGF-2α (25 μg/h for 6 h) elicited a decline in plasma progesterone from mean ± s.e.m. pretreatment levels of 8·83 ± 0·54 to 0·54 ± 0·05 ng/ml on the following day (P < 0·01). A shorter infusion (25 μg/h for 3 h) or a lower hourly dose rate (12·5 μg/h for 6 h) of PGF-2α was ineffectual, suggesting that a dose/duration regimen exists for PGF-2α-mediated luteolysis. PGE-2 (25 μg/h for 6 h) or sodium arachidonate (667 μg/h for 6 h) did not significantly affect luteal function. Of the PGF-2α metabolites examined, 13,14-dihydro-PGF-2α was the most effective: it was approximately 4-fold more potent as a luteolysin than PGF-2α since functional regression occurred with infusion of 6·25 μg/h for 6 h. These data illustrate that systemic infusion of PGF-2α is capable of inducing luteolysis in the rabbit. The metabolite 13,14-dihydro-PGF-2α may also be involved luteolytically in this species.

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J. C. Carlson and J. W. D. Gole

Summary. During pseudopregnancy in the rabbit, plasma progesterone levels in the posterior vena cava fell rapidly from maximal concentrations of 9·8 ± 0·7 (s.e.m.) ng/ml on Day 13 of pseudopregnancy to 2·0 ± 0·6 ng/ml on Day 15. There was no significant increase in PGF levels at this time, although administration of indomethacin to a similar group of does delayed functional regression for over 7 days. Subcutaneous injection of 1 mg PGF-2α or 35 mg arachidonic acid on Day 9 of pseudopregnancy resulted in loss of luteal function as plasma progesterone levels fell significantly from about 6 to < 1 ng/ml within 24 h. These findings indicate that the uterine factor responsible for luteolysis in the pseudopregnant rabbit may not be released in the form of PGF-2α.

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J. C. Carlson and D. G. Perrin

Summary. Intracarotid infusion of LH-RH to female rabbits stimulated a significant increase in plasma LH concentration in the jugular vein. This response varied with the reproductive state of the animal, with a greater release occurring in oestrous (spontaneous or oestrogen-induced) and non-receptive does than in pseudopregnant or ovariectomized animals. If ovariectomized rabbits were pretreated with oestrogen, the pituitary response to LH-RH was restored. These findings suggest that there is little change in pituitary sensitivity to LH-RH infusion between oestrous and non-receptive rabbits, although pseudopregnancy (high physiological levels of progesterone) or ovariectomy inhibit its ability to respond to a releasing-hormone stimulus.

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J. C. CARLSON, B. BARCIKOWSKI and J. A. McCRACKEN

Recent studies suggest that prostaglandins of the E and F series may cause release of the adenohypophysial hormones, ACTH, growth hormone (GH), prolactin and LH. Evidence for the stimulation of ACTH release from the rat pituitary by prostaglandin has been presented by de Wied, Witter, Versteeg & Mulder (1969); Peng, Six & Munson (1970) and Hedge (1972). Hertelendy, Todd, Ehrhart & Blute (1972) observed that intravenous doses (20 μg/kg) of prostaglandin E1 (PGE1) in castrated rams rapidly induced a significant increase in the plasma concentration of GH. Vermouth & Deis (1972) found that two intraperitoneal injections of PGF on Day 18 of pregnancy in rats resulted in a biphasic elevation of plasma prolactin at 12 and 20 hr. Indirect evidence exists which indicates that PGF may stimulate LH release in the rat (Labhsetwar, 1970; Orczyk & Behrman, 1972; Varavudhi & Chobdieng, 1972).

Systemic infusion of PGF

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J. C. Carlson, Anne P. Wong and D. G. Perrin

Summary. The release of LH in response to prostaglandin (PG) treatment of female rabbits in various reproductive states was compared with the surge following mating. Intracarotid infusion of PGE-2 or PGF-2α (0·3–900 μg/h) into non-receptive and pseudopregnant does resulted in small, 2–4-fold elevations in jugular vein LH concentration. Similar doses of PGF-2α in oestrogen-pretreated, oestrous does stimulated a 13-fold increase in plasma LH levels. Mating resulted in a much larger release of LH, as plasma levels increased approximately 60-fold from 1·1 ± 0·2 (S.E.M.) ng/ml to 67·8 ± 10·5 ng/ml. These results indicate that PG can stimulate the hypothalamic—hypophysial axis to release LH in non-receptive, pseudopregnant and oestrogen-pretreated, oestrous rabbits.