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Summary. A method for measurement of bovine plasma FSH has been established with an interassay coefficient of variation of 15·5% over the workable range of the assay. Compared to values at the end of pregnancy (42–122 ng/ml), FSH concentrations were greater (P < 0·01) between 0 and 20 days post partum in 3 cows.

Summary. LH values between −2 and +4 h (0 h =LH peak) were higher than baseline. FSH values were also raised at this time and between + 16 and + 30h. Oestradiol values between −20 and 0 h were higher than during + 4 to + 20h.

Summary. An antibody, raised in ewes against synthetic GnRH, was injected intravenously (a) before the expected synchronous surges of LH and FSH or (b) before the second FSH surge, i.e. at 0 or 20 h after the onset of behavioural oestrus. Control ewes were given normal sheep plasma at the same times.

One of the 4 ewes given antibody at 0 h showed blockade of the LH surge, whereas in the other 3 ewes there was an average delay of 16 h in the appearance of the LH peak and in 1 of the 3 there was a surge of FSH coincident with the delayed LH surge; no other surges of FSH were detected in any of the ewes between 0 and 48 h after the onset of oestrus. Endoscopic examination at 48–52 h showed follicles of 6–8 mm diameter with a papilla in all the 4 ewes but 2 ewes showed an abnormal luteal phase (32 and 34 days), indicated by plasma progesterone values.

The magnitude and the time of appearance of the second FSH surge was unaffected in all the 4 ewes treated with antibody 20 h after the onset of oestrus: 2 of the ewes had 6–8 mm follicles at the time of endoscopy, while the other 2 ewes and the controls had fresh corpora lutea haemorrhagica. Apart from 1 ewe with a long luteal phase all the other 5 treated and control ewes had a fertile oestrus 19·0 ± 0·4 days after the experiment.

Administration of antibody 2 h after a 4-h infusion of oestradiol (3 μg/h) to 3 anoestrous ewes completely blocked the LH surge in one and produced a 6 and 3 h delay of an LH surge of decreased magnitude in the other 2 ewes. The FSH surge was blocked in 2 ewes and in the other was asynchronous with the LH surge.

The results suggest that GnRH controls the preovulatory synchronous LH and FSH surges, but not the second FSH surge.

Brunner, Donaldson & Hansel (1969) showed that ACTH suppressed corpus luteum formation when administered during Days 2 to 8 of the bovine oestrous cycle. We found that adrenal suppression by betamethasone treatment from Day 10 increased the length of the bovine oestrous cycle: plasma concentrations of oestradiol and androstenedione were unaltered but testosterone levels were reduced by 13% (Kanchev, Dobson, Ward & Fitzpatrick, 1976). The present experiment was undertaken to investigate the effect on plasma steroids of adrenal stimulation achieved by a synthetic adrenocorticotrophin (ACTH).

Department of Veterinary Clinical Studies, University of Liverpool, Leahurst, Neston, Cheshire

(Received 9th October 1974)

In veterinary clinical practice, HCG is widely used as a commercially available substitute for LH to induce follicular rupture and CL formation in infertile cows. The present study was undertaken to determine the effect of HCG on ovarian function in normal cows before conducting a similar investigation in infertile animals.

The HCG was administered before the expected increase in endogenous LH concentration at oestrus and the effect on oestradiol-17β concentration in jugular venous blood was monitored. Hormonal changes occurring during the endogenous release of gonadotrophins at normal oestrus in the cow were also studied.

Blood samples were collected into heparinized evacuated bottles from the jugular veins of eight untreated control Friesian cows every 4 hr during the oestrous period. A group of five experimental cows were bled twice daily at 08.00 and 17.30 hours, starting

Summary.

Five Friesian maiden heifers and four Friesian or Ayrshire cows were assigned to each of four treatments, namely: (1) controls ; (2) melengestrol acetate (MGA) 1 mg orally daily for 14 days; (3) MGA+2500 i.u. HCG at 72 hr after the last administration of MGA; and (4) MGA+0·5 mg oestradiol benzoate on Day 1 of MGA treatment, +2500 i.u. PMSG at 36 hr+2500 i.u. HCG at 72 hr after the last administration of MGA.

Jugular vein blood was sampled daily over the oestrus period in the control animals and daily after the last dose of MGA until the day after the first oestrus. The concentrations of progesterone and total oestrogens in peripheral plasma samples were assayed by radioimmunoassay without chromatographic separation.

In the control animals, the total oestrogen concentration increased from 7 pg/ml 4 days before oestrus to 14 pg/ml on the day of oestrus, then decreased to 9 pg/ml 1 day after oestrus. Treatment with MGA alone did not alter this pattern at the first synchronized oestrus. Administration of HCG 72 hr after MGA withdrawal apparently depressed total oestrogen concentration. This was counteracted by prior treatment with PMSG. A comparison of the data from animals receiving MGA only and from controls indicates no adverse effect on release of oestrogens from ovarian follicles after cessation of MGA treatment.

The progesterone concentration in control animals decreased from 9 ng/ml 4 days before oestrus to 0·35 ng/ml on the day of oestrus. The MGA-treated animals had low progesterone values for several days before oestrus. Three of the sixteen animals treated with gonadotrophins had progesterone concentrations higher than basal values.

Department of Veterinary Clinical Studies, University of Liverpool, Leahurst, Neston, Cheshire, and Imperial Chemical Industries Limited (Pharmaceuticals Division), Mereside, Alderley Park, Macclesfield, Cheshire

(Received 14th June 1974)

Tervit, Rowson & Brand (1973) have described the induction of fertile oestrus in cattle by intramuscular injection of a low dose (1 mg) of I.C.I. 79,939, an analogue of prostaglandin F_2α, (Binder & co-authors, 1974; Dukes, Russell & Walpole, 1974) but neither this compound nor prostaglandin (PG) F_2α, given in a single dose at any one time, will induce oestrus in all cattle in a herd since animals treated before Day 5 of the oestrous cycle will not respond (Lauderdale, 1972; Liehr, Marion & Olsen, 1972; Rowson, Tervit & Brand, 1972a, b; Hill, Dickey & Henricks, 1973; Cooper & Furr, 1974). Cooper (1974) and Cooper & Furr (1974) have described a regimen which overcomes this difficulty and provides a potentially valuable

Summary. Betamethasone (a synthetic glucocorticoid, 15 mg) was administered i.m. twice daily for 10 days to 4 regularly cycling dairy cows, beginning on Day 10 of the oestrous cycle. Luteal function, monitored by plasma progesterone, was extended by 7, 9, 19 and 20 days, respectively. Luteal function in the next cycle was normal. Endogenous cortisol values were suppressed for 14, 13, 34 and 27 days, respectively. Pituitary responsiveness to 20 μg GnRH was assessed by LH measurement on Days − 1, + 3 and + 7 relative to the start of betamethasone treatment. There was a progressive decrease in peak LH concentrations after each GnRH challenge compared to control cows. Hourly measurements of PGF-2a metabolite during the expected period of luteolysis failed to reveal normal increases.

It is suggested that betamethasone caused prolonged luteal function, either by directly inhibiting PGF-2α release, or by suppressing pituitary stimulation of follicular growth and hence lowering oestradiol concentrations, since it is known that PGF-2α and oestradiol act synergystically to cause luteolysis.

Summary. Four cows released an LH surge after 1·0 mg oestradiol benzoate administered i.m. during the post-partum anoestrous period with continuing low plasma progesterone. A similar response occurred in the early follicular phase when plasma progesterone concentration at the time of injection was < 0·5 ng/ml. Cows treated with a progesterone-releasing intravaginal device (PRID) for 8 days were injected with cloprostenol on the 5th day to remove any endogenous source of progesterone. Oestradiol was injected on the 7th day when the plasma progesterone concentration from the PRID was between 0·7 and 1·5 ng/ml. No LH surge occurred. Similarly, oestradiol benzoate injected in the luteal phase of 3 cows 0·9–2·1 ng progesterone/ml plasma) did not provoke an LH surge. An oestradiol challenge given to 3 cows 6 days after ovariectomy induced a normal LH surge in each cow. However, when oestradiol treatment was repeated on the 7th day of PRID treatment, none released LH.

It is concluded that ovaries are not necessary for progesterone to inhibit the release of LH, and cows with plasma progesterone concentrations > 0·5 ng/ml, whether endogenous or exogenous, did not release LH in response to oestradiol.

Keywords: oestradiol; progesterone; LH; cows