Search Results

Summary.

Cystic ovaries were experimentally produced by i.m. injection of 5 mg oestradiol valerate on Day 16 of the oestrous cycle or by s.c. injection of 100 ml antiserum to bovine LH before the onset of oestrus. The length of the cycle after both treatments was altered. The cysts formed after oestradiol injection were relatively small (2·5-3 cm diameter) and thin-walled, and were accompanied by relatively low plasma oestrone and high oestradiol levels; those after LH antiserum treatment were large (5·0-6·0 cm) and firm and associated with relatively low oestradiol and high oestrone levels. The results of the hormone assays after treatment support the theory that cystic ovaries could result from premature LH release or an insufficiency of LH at the time of ovulation.

Summary. Holstein heifers (18 pregnant, 12 inseminated but non-pregnant, and 17 cyclic) were studied during the first 18 days after oestrus. Jugular vein plasma progesterone concentrations were significantly (P < 0·05) higher in pregnant than in non-pregnant and cyclic animals between Days 10 and 18. Jugular oestradiol concentrations were also higher in pregnant than in cyclic animals between Days 6 and 16 (P < 0·05). No differences were found among the three groups in jugular LH levels during Days 1–18. Ovarian and uterine vein progesterone and oestradiol concentrations suggested that the elevated peripheral plasma concentrations of these steroids in pregnant animals were due mainly to increased ovarian production, but the embryo itself may have contributed to the oestradiol levels found at Day 17. Luteal tissues from pregnant animals contained more progesterone when incubated alone or with LH than tissues from cyclic animals (P < 0·06). However, LH-stimulated synthesis (incubated with LH minus incubated alone) was nearly identical for the three groups of tissues. Therefore, loss of responsiveness to LH does not appear to be the initial event in luteolysis. Concentrations of PGF in ovarian arterial plasma in cyclic and pregnant animals were similar at Day 18, although uterine vein concentrations were higher (P < 0·05) in the cyclic animals. Free arachidonic acid concentrations in corpora lutea at Day 18 were higher (P < 0·05) in cyclic than in pregnant animals. The data suggest that the embryo may produce one or more luteotrophic substances that stimulate increased progesterone secretion by the corpus luteum, beginning as early as Day 10 of pregnancy.

Summary. Five new concepts concerning the control of corpus luteum function in the cow have been developed in recent years.

1. Prostacyclin (PGI-2) plays a luteotrophic role. Conversely, products of the lipoxygenase pathway of arachidonic acid metabolism, particularly 5 hydroxyeicosatetetraenoic acid (5-HETE), play luteolytic roles.

2. Luteal cells arise from two sources. The small luteal cells are all of theca cell origin; the large cells found early in the cycle (Days 2–6) are mainly of granulosa cell origin. However, a population of large cells found after Day 10 of the cycle are of theca cell origin.

3. Oxytocin of luteal cell origin plays a role in development of the corpus luteum and possibly in its regression.

4. The recently described Ca²⁺-polyphosphoinositol–protein kinase C second messenger system, as well as the LH–cAMP system, is involved in control of progesterone synthesis in the bovine corpus luteum.

5. Progesterone synthesis in the small theca-derived luteal cells is primarily controlled by the cAMP system. However, elevated intracellular calcium diminishes cAMP-mediated progesterone synthesis in these cells.

These findings modify our current concepts of the mechanisms of control of progesterone secretion by the corpus luteum and suggest several new lines of research.

Summary. Holstein heifers (4/group) were injected intramuscularly with 0, 5, 10 or 25 mg 13,14-dihydro-PGF-2α on Day 10 of the oestrous cycle. Complete luteolysis and precocious oestrus occurred in 3 of 4 heifers receiving 25 mg and 1 of 4 receiving 10 mg 13,14-dihydro-PGF-2α injected i.m. These features were not affected in heifers injected with 0 or 5 mg 13,14-dihydro-PGF-2α, although plasma progesterone concentrations were depressed in all treated heifers within 75 min. LH concentrations were elevated between 5 and 8 h after 13,14-dihydro-PGF-2α in all treated heifers. The addition of 13,14-dihydro-PGF-2α to dispersed bovine luteal cells did not affect progesterone accumulation during a 2-h period. These results suggest that 13,14-dihydro-PGF-2α may play a role in PGF-2α-induced luteolysis.

Summary. Two culture systems for maintenance of Day 13–18 conceptus tissue were developed. Harvested culture media were assayed for luteotrophic activity by determining their ability to stimulate progesterone synthesis by dispersed bovine luteal cells. Significant luteotrophic activity was found in 80% of the 31 tissue culture media studied. A series of experiments carried out to determine the nature of the luteotrophic activity indicate that it is a small (M _r < 10 000), heat-labile, lipid-soluble substance that is adsorbed by dextran-coated charcoal. The nature and activity of this factor, together with its synthesis by the early bovine conceptus, suggest that it may have a significant role in stimulating progesterone synthesis by the corpus luteum during early pregnancy.

Summary. In preliminary studies 5 of 6 heifers given oxytocin subcutaneously on Days 2–6 of the oestrous cycle had depressed progesterone levels by Day 5, and each had a precocious oestrus and a normal preovulatory LH surge. Oxytocin treatments given once daily on Days 5 and 6, or twice daily on Days 5 and 6 or 4, 5 and 6 resulted in shortened cycles and lower jugular progesterone concentrations in 1 of 5, 2 of 4 and 4 of 5 heifers, respectively.

On the basis of these data, uterine venous and ovarian arterial catheters were inserted into 17 animals on Day 3 of the oestrous cycle and oxytocin treatments were given twice daily on Days 4, 5 and 6. Oxytocin treatment significantly depressed jugular progesterone levels by Day 8 and increased uterine venous prostaglandin (PG) F concentrations between 30 and 240 min after each injection. No changes of PGF concentrations occurred in uterine venous blood in control heifers, or in ovarian arterial blood from oxytocin-treated or control animals. It therefore appears that some factor other than PGF or a mechanism other than local veno–arterial transfer of PGF is involved in mediating the inhibitory effects of oxytocin on bovine corpus luteum function.

Summary.

The mean interval to the first observed post partum oestrus was 14 days in milked cows. In contrast, none of eleven nursed animals showed behavioural oestrus by Day 30 post partum, although two ovulated before that time. Only one of seven anaemic animals (haemoglobin levels of 9 to 10 g/100 ml) showed behavioural oestrus by Day 22 post partum; two others ovulated, but showed no signs of oestrus. Thyroprotein-fed cows also had prolonged periods of post partum anoestrus.

Pregnancy corpora lutea were quite degenerate by Day 7 post partum. At Day 14 post partum, they were approximately 9·0 mm in diameter and remained at this size, at least to Day 30. There was an increase in follicular size following parturition. At Day 7, the mean diameter of the largest follicle was 9·6 mm; at Day 14, it was 11·3 mm; at Day 30, the largest follicle averaged 13·0 mm.

Neither the presence of suckling calves, nor anaemia delayed uterine involution. Caruncular tissue sloughed and was expelled from the uterus between Day 7 and Day 14 post partum. Regeneration of the surface epithelium over the caruncles occurred by growth from the surrounding tissue, and was complete in a majority of animals slaughtered at Day 30 post partum.

Summary.

There was a non-significant rise in pituitary lh concentration from Day 7 post partum (2·07 μg/mg fresh anterior pituitary) to Day 30 post partum (2·62 μg/mg). Neither nursing of calves nor anaemia caused a significant change in pituitary lh content. Both control and thyroprotein-fed non-post partum cows had significantly higher levels of pituitary lh, 7·4 μg and 6·9 μg respectively, than did the post partum cows.

There was no detectable tsh in pituitaries of five of six thyroproteinfed cows. tsh concentration ranged from 2·0 to 9·5 mu nih-tsh-b3/mg fresh pituitary in the remaining animals, but there were no significant differences between any of the groups. Thyroprotein-fed animals also had significantly lower thyroid cell heights than all other groups.

Adrenal data indicate a positive correlation exists between progesterone and cortisol in the same gland (r = 0·67, P<0·01). The only group difference of significance was that the nursed group had a significantly lower adrenal progesterone than did the milked group (0·63 versus 1·76 μg/g). Cortisol content was also lower in the nursed animals, but not to a significant extent.

Summary. Bovine blastocysts were collected at Days 13, 15 and 16 and placed in TCM-199 supplemented with 5% fetal calf serum; some blastocysts were immediately frozen while the others were cultured for 48 h and then frozen. Samples (tissue + medium, 5–12/group) were thawed, homogenized and analysed by radioimmunoassays. Measurable amounts of progesterone were found in all blastocysts but values were higher (P < 0·01) after culture. Testosterone was not found in the cultured or uncultured blastocysts at Day 13, but was detectable on Days 15 and 16 and in greater amounts (P < 0·05) in the cultured blastocysts. PGF and PGE-2 were increased (P < 0·05) in the cultured blastocysts on all 3 days. Oestradiol was measurable in some but not all blastocysts. It is suggested that PG synthetase and enzymes capable of synthesizing progesterone, testosterone and, possibly, oestradiol are present in these early bovine blastocysts.

Summary. Sixteen virgin Holstein heifers were used to study the role of oxytocin in luteal function. Oxytocin (100 i.u.) was administered on Days 2–5 (Exp. 1) or Days 2–6 (Exp. 2), and the corpora lutea were collected and weighed on Day 6 or Day 8, respectively. Plasma progesterone values and tissue concentrations of progesterone and oxytocin were determined by radioimmunoassay. In addition, the tissue level of the oxytocin-specific transcript was measured by dot-blot hybridization. Expression of the oxytocin-specific transcript was not affected by oxytocin treatment in either experiment. In contrast, plasma progesterone values and tissue progesterone and oxytocin concentrations were all decreased after 5 days of oxytocin treatment (Exp. 2), but not after 4 days (Exp. 1). Since oxytocin concentrations were reduced while the level of its transcript remained unaffected, oxytocin injections might be influencing either peptide processing or release.

Keywords: cattle; oxytocin; progesterone; corpora lutea