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H. J. Howard and J. H. Britt

Summary. The experimental objective was to evaluate how a spontaneously formed corpus luteum (CL) differed in its response to prostaglandin (PG) F-2α, given during the first 5 days after ovulation, from a CL induced during dioestrus with hCG. Sixteen Holstein heifers were used during each of 2 consecutive oestrous cycles. During the first cycle (sham cycle), heifers were given no PGF-2α (control) or PGF-2α (25 mg, i.m.) on Day 2, 4 or 6 (oestrus = Day 0). During the second cycle (hCG-treated cycle), heifers were given hCG (5000 i.u., i.m.) on Day 10, followed by no PGF-2α (control) or PGF-2α on Day 12, 14 or 16, corresponding to 2,4 or 6 days after the ovulatory dose of hCG. A new ovulation was induced in 13 of 16 heifers given hCG on Day 10. Luteolysis did not occur immediately in heifers given PGF-2α on Day 2 or 4 during the sham cycle, but concentration of progesterone in serum during the remainder of the cycle was lower in heifers given PGF-2α on Day 4 than in sham controls or heifers given PGF-2α on Day 2 (P < 0·05). Luteolysis occurred immediately in heifers given PGF-2α on Day 6 of the sham cycle or on Day 12,14 or 16 of the hCG-treated cycle, with concentration of progesterone in serum decreasing to < 1 ng/ml within 2 days. Intervals between periods of oestrus were shorter in heifers given PGF-2α on Day 6 of the sham cycle (8·3 ± 0·3 days) than in control heifers (20·5 ± 0·9 days) or those given PGF-2α on Day 2 or 4 (20·8 ± 0·6 and 18·4 ± 0·5 days; P < 0·05) of the sham cycle. During the hCG-treated cycle, interoestrous intervals were shorter in heifers given PGF-2α on Day 12, 14 or 16 (19·5 ± 0·5, 18·8 ± 0·5 and 20·8 ± 1·3 days) than those of hCG-treated controls (25·5 ± 1·8 days; P < 0·05). It was concluded that CL induced by treatment with hCG on Day 10 of the oestrous cycle regress after exogenous PGF-2α given within 2 days after ovulation, whereas spontaneously formed CL respond to PGF-2α only after Day 5.

Keywords: cattle; progesterone; gonadotrophin; corpus luteum; prostaglandin F-2α

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Oestrous synchronization in cattle has not received widespread acceptance because of a period of transitory infertility which occurs following progestagen withdrawal. This reduced fertility has been attributed to changes in the time required for sperm capacitation (Lauderdale & Erickson, 1970), changes in the amount and physicochemical characteristics of cervical mucus (Johnson & Ulberg, 1965; Hill, Lamond, Henricks, Dickey & Niswender, 1971), changes in body temperature (Johnson & Ulberg, 1965; Long, Nipper & Vincent, 1969), reduced fertilization rate (Wagner, Veenhuizen, Gregory & Tonkinson, 1968), and histochemical changes in the endometrium (Wordinger, Dickey & Hill, 1970). All of these physiological aberrations can be attributed to hormonal changes. This experiment was conducted to determine changes in peripheral blood plasma progesterone concentration during and subsequent to oestrous synchronization with melengestrol acetate (MGA—The Upjohn Company) and to compare

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L. A. C. Sesti and J. H. Britt

The objectives of this study were (1) to determine whether release of GnRH in vitro was related to concentrations of LH and FSH in serum and pituitaries and to oestradiol in serum of sows at mid- or late lactation, and (2) to determine whether weaning at mid-lactation changes concentrations of these hormones from values expected at mid-lactation to values expected at late lactation. Multiparous crossbred sows were killed on day 14 (n = 5) or day 28 (n = 5) of lactation or on day 14 post partum after litters had been weaned on day 10 (n = 5). Blood samples were taken every 6 h for 4 days before sows were killed, and the preoptic suprachiasmatic area, medial basal hypothalamus, stalk median eminence, anterior pituitary and ovaries were collected at slaughter. Sows killed on day 14 after having their litters weaned on day 10 had more (P < 0.01) preovulatory follicles (> 6 mm in diameter) than lactating sows killed on day 14 or 28 (7.0 ± 1.2 versus 0.2 ± 0.1 and 1.5 ± 0.8, respectively). Concentrations of LH, FSH and oestradiol in serum during 90 h before slaughter were greater (P < 0.05) in weaned sows and lactating sows killed on day 28 than in lactating sows killed on day 14 (LH: 0.72 ± 0.3 and 0.68 ± 0.3 versus 0.45 ± 0.2 ng ml−1; FSH: 39.3 ± 2.7 and 57.3 ± 4.0 versus 28.8 ± 1.6 ng ml−1; oestradiol: 10.9 ± 1.6 and 5.6 ± 0.7 versus 2.7 ± 0.2 pg ml−1, respectively). Weights of anterior pituitaries did not differ among groups, but concentrations of LH in anterior pituitaries increased (P < 0.05) and concentrations of FSH in pituitaries tended to increase although the increase was not significant (P < 0.10) from day 14 to day 28 of lactation, whereas pituitary concentrations of gonadotrophins in weaned sows were intermediate (LH: 0.45 ± 0.2, 0.94 ± 0.2 and 0.65 ± 0.1 mg g−1; FSH: 8.7 ± 1.2, 13.8 ± 2.6 and 10.9 ± 1.2 mg g−1 for day 14 lactating, day 28 lactating and weaned groups, respectively). Content of GnRH in the preoptic area, and medial basal hypothalamus did not differ among groups. Content of GnRH in stalk median eminence was lower (P < 0.05) in weaned sows, but was similar among sows killed on day 14 or day 28; however, the proportion of residual GnRH released by the stalk median eminence in response to a challenge with K+ in vitro was greater (P < 0.05) for sows at day 14 than for weaned sows and sows at day 28 (21.5 ± 4.5 versus 13.7 ± 2.8 and 7.0 ± 1.6%, respectively), indicating an increase in rate of release of GnRH in vivo and a consequent stimulation of synthesis and release of LH and FSH. These results are consistent with the hypothesis that suckling/lactation alters endogenous rate of GnRH secretion, thereby influencing secretion of gonadotrophins and follicular development.

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H. J. Howard, D. E. Morbeck and J. H. Britt

Summary. The experimental objective was to evaluate how continuous infusion of oxytocin during the anticipated period of luteolysis in cattle would influence secretion of progesterone, oestradiol and 13,14-dihydro-15-keto-prostaglandin F-2α (PGFM). In Exp. I, 6 non-lactating Holstein cows were infused with saline or oxytocin (20 IU/h, i.v.) from Day 13 to Day 20 of an oestrous cycle in a cross-over experimental design (Day 0 = oestrus). During saline cycles, concentrations of progesterone decreased from 11·0 ± 2·0 ng/ml on Day 14 to 2·0 ± 1·3 ng/ml on Day 23; however, during oxytocin cycles, luteolysis was delayed and progesterone secretion remained near 11 ng/ml until after Day 22 (P < 0·05). Interoestrous interval was 1·6 days longer in oxytocin than in saline cycles (P = 0·07). Baseline PGFM and amplitude and frequency of PGFM peaks in blood samples collected hourly on Day 18 did not differ between saline and oxytocin cycles. In Exp. II, 7 non-lactating Holstein cows were infused with saline or oxytocin from Day 13 to Day 25 after oestrus in a cross-over experimental design. Secretion of progesterone decreased from 6·8 ± 0·7 ng/ml on Day 16 to <2 ng/ml on Day 22 of saline cycles; however, during oxytocin cycles, luteolysis did not occur until after Day 25 (P < 0·05). Interoestrous interval was 5·9 days longer for oxytocin than for saline cycles (P < 0·05). In blood samples taken every 2 h from Day 17 to Day 23, PGFM peak amplitude was higher (P < 0·05) in saline (142·1 ± 25·1 pg/ml) than in oxytocin cycles (109·8 ± 15·2 pg/ml). Nevertheless, pulsatile secretion of PGFM was detected during 6 of 7 oxytocin cycles. In both experiments, the anticipated rise in serum oestradiol concentrations before oestrus, around Days 18-20, was observed during saline cycles, but during oxytocin cycles, concentrations of oestradiol remained at basal levels until after oxytocin infusion was discontinued. We concluded that continuous infusion of oxytocin caused extended oestrous cycles, prolonged the secretion of progesterone, and reduced the amplitude of PGFM pulses. Moreover, when oxytocin was infused, pulsatile secretion of PGFM was not abolished, but oestrogen secretion did not increase until oxytocin infusion stopped.

Keywords: corpus luteum; prostaglandin; progesterone; oxytocin; cow

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V. H. Lee, J. H. Britt and B. S. Dunbar

Laminin, a major component of the basal lamina, is known to be important in the differentiation of epithelial cells. The outer granulosa cell layer of ovarian follicles is attached to a basal lamina surrounding the follicle and it has been demonstrated that proteins of the basal lamina can alter the steroidogenic capacity and cytoskeletal composition of mature granulosa cells. The present studies were carried out to evaluate the developmental expression of laminin proteins in the basal lamina of ovarian follicles during early stages of granulosa cell differentiation. Ovaries from sexually immature pigs (0–20 weeks of age) and rabbits (0–12 weeks of age) were used for this study because formation of primordial follicles and initiation of follicular growth occur partially or completely postnatally. Specific antibodies were made against laminin β1/γ1 chains using protein purified by high resolution two-dimensional polyacrylamide gel electrophoresis. These antibodies were used in immunohistochemical localization studies to demonstrate that laminin is associated with the basal lamina surrounding egg clusters and their connections to the ovarian surface epithelium and ovarian rete during early ovarian development. In addition, laminin was observed to associate with a continuous matrix that surrounds forming primordial follicles as they are isolated from the egg clusters. Laminin is localized in the basal lamina of primordial, primary, secondary, and tertiary follicles of both pig and rabbit ovaries. Laminin proteins are not only expressed throughout early follicular development in pigs and rabbits, but are also found surrounding the germ cell compartments prior to initiation of meiosis and formation of primordial follicles. These results demonstrate that laminin proteins are deposited in the basal lamina well before granulosa cells undergo morphological differentiation. This pattern of expression suggests that laminin proteins alone do not control changes in granulosa cell morphology during early development but may be required to maintain cell lineage commitment.

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J. D. Armstrong, J. H. Britt and N. M. Cox

Summary. Primiparous sows were fed to appetite during lactations that occurred during winter or summer, and 11·4 ± 0·4 pigs per litter were weaned at 23·5 ± 0·1 days of age. Sows were slaughtered at 0 or 72 h after weaning or blood samples were collected until 24 h after onset of oestrus. Sows that lactated during summer consumed less food and lost more (P < 0·05) weight, heartgirth and backfat than those that lactated during winter. Weaning-to-oestrus interval was greater (P < 0·05) in summer (224 ± 25 h) than in winter (93 ± 13 h). Content of GnRH in the hypothalamus and concentrations of LH in the anterior pituitary and serum were lower (P < 0·05) after weaning in summer than winter. The numbers of visible ovarian follicles < 5 mm in diameter at weaning were lower (P < 0·05) in summer than in winter. In contrast to LH, FSH concentration in serum was higher (P < 0·10) in summer than winter, but FSH values in the anterior pituitary were lower (P < 0·05) in summer than in winter. Post-weaning patterns of secretion of oestradiol and follicular development differed between winter and summer. For example, in some sows weaned during the summer, transient surges of oestradiol occurred repeatedly during 0 to 280 h after weaning without provoking surges of LH. These results indicate that the period of post-weaning anoestrus in summer is prolonged because of altered activity of the hypothalamic– pituitary axis, possibly because of changes in sensitivity to the feedback of oestradiol. Lower feed intake during lactations that occur during summer may predispose the endocrine system to the aberrations.

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J. D. Armstrong, R. R. Kraeling and J. H. Britt

Summary. Sows (N = 16) were infused intravenously for 8 h with saline or naloxone (200 mg/h) or their litters were transiently weaned for 8 h. Before infusion, 200 mg naloxone were administered to elevate quickly concentrations of naloxone. Blood samples were collected from sows at 15 min intervals for 24 h, beginning 8 h before and continuing until 8 h after imposition of treatments during the middle 8-h segment. Frequency of episodic release of LH and concentrations of prolactin were similar before, during and after infusion of saline. Average concentration of LH was greater during the last than during the middle 8-h segment when sows were given saline. Frequency of episodic release of LH increased and concentrations of prolactin decreased during infusion of naloxone or transient weaning; however, average concentration of LH increased during transient weaning, but not during infusion of naloxone. After transient weaning or infusion of naloxone, frequency of release of LH decreased, returning to pretreatment values in sows infused with naloxone but remaining above pretreatment values in sows subjected to transient weaning. At the resumption of suckling by litters in sows subjected to transient weaning, prolactin increased to levels not different from those observed during the 8-h pretreatment segment. Prolactin did not increase until 4–5 h after cessation of naloxone infusion. We conclude that continuous infusion of naloxone altered secretory patterns of LH and prolactin. Collectively these results provide evidence that the immediate effects of weaning on LH and prolactin in sows are mediated in part through a mechanism involving endogenous opioid peptides.

Keywords: LH; endogenous opioid peptides; naloxone; lactation; prolactin

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A. J. Ziecik, K. L. Esbenshade and J. H. Britt

Summary. Male (N = 8) and female (N = 8) pigs were assigned to receive saline or a potent GnRH antagonist ([Ac-d 2Nal1,d 4-Cl-Phe2,d-Trp3,d-Arg6, d-Ala10]GnRH*HOAc; 1 mg/kg body weight) at 14 days of age. The GnRH antagonist caused LH to decline (P < 0·01) from 1·7 ng/ml at 0 h to <0·5 ng/ml during 4–32 h in males and females. Concentrations of FSH in gilts declined slowly from 75 ± 8 to 56 ± 5 ng/ml (P < 0·05) at 32 h. In males FSH was low (5·7 ± 0·5 ng/ml) at 0 h and did not change significantly.

To observe the effect of long-term treatment with GnRH antagonist, 10 male and 10 female pigs, 3 days of age, were treated with saline or 1 mg GnRH antagonist per kg body weight every 36 h for 21 days. Concentrations of LH were reduced (P < 0·01) to 0·2–0·4 ng/ml throughout the experimental period in male and female piglets treated with GnRH antagonist. Plasma FSH increased in control females, but remained suppressed (P < 0·001) in females treated with GnRH antagonist. Treatment with the GnRH antagonist suppressed FSH levels in males on Days 8 and 16 (P < 0·05), not on Day 24. Treatment of females with the GnRH antagonist did not influence (P > 0·10) oestradiol-17β concentrations. Administration of GnRH antagonist to males suppressed testosterone and oestradiol-17β values (P < 0·01) and reduced testicular weight (P < 0·01). Concentration of LH/hCG receptors in testes of boars treated with GnRH antagonist was lower (P < 0·10) than in controls, but concentration of FSH receptors was not affected. Basal and potassium-stimulated release of GnRH from the stalk median eminence and medial basal hypothalamus in vitro did not differ between treatment groups. The amount of residual GnRH in hypothalamic tissue was not different in control gilts and in gilts receiving the GnRH antagonist, but it was lower (P < 0·05) in boars treated with GnRH antagonist than in control boars.

Keywords: pig; GnRH antagonist; LH; FSH

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D. E. Morbeck, W. L. Flowers and J. H. Britt

A cell culture system was developed to study the function of porcine granulosa cells from primary and secondary follicles. Primary follicles were isolated from 1- to 3-day-old pigs. Secondary follicles were isolated from 50- to 60-day-old pigs. Follicles were isolated after a digestion for 15 min with 0.25% trypsin followed by 15 min with 1000 U DNAase. Follicles were plated at 100 primary follicles or 30 secondary follicles per well in 48-well plates and cultured in media containing 10% fetal bovine serum (FBS). During initial plating, follicles attached to the plate and cells spread from the point of attachment. This resulted in monolayer cultures of granulosa cells from primary or secondary follicles. On day 4 of culture, media were replaced with 0.5 ml media containing one of the following treatments: control (media only); 10% FBS; 100 ng FSH; 2 mmol 8-bromo-cAMP l−1 or 50 ng epidermal growth factor (EGF). Media and cells were harvested on day 6, after 2 days of treatment. FBS and EGF increased DNA in granulosa cell cultures from primary or secondary follicles (P < 0.01). Treatment with 8-bromo-cAMP increased DNA in granulosa cell cultures from primary but not from secondary follicles (P < 0.05). Conversely, treatment with FSH increased DNA in granulosa cell cultures from secondary but not from primary follicles (P < 0.05). Progesterone release was greater in granulosa cell cultures from primary and secondary follicles treated with 8-bromo-cAMP than in controls (P < 0.001); however, progesterone concentrations were not influenced by FBS, EGF or FSH in granulosa cells from primary or secondary follicles. In summary, EGF and 8-bromo-cAMP stimulated proliferation of granulosa cells from primary and secondary follicles, and 8-bromo-cAMP also stimulated progesterone secretion. In contrast, FSH had no effect on progesterone secretion or proliferation of granulosa cells from primary follicles, but stimulated proliferation of granulosa cells from secondary follicles.

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B. F. King, J. H. Britt, K. L. Esbenshade, W. L. Flowers and J. J. Ireland

The aim of this study was to determine whether immunoneutralization of inhibin altered compensatory ovarian hypertrophy. Crossbred postpubertal gilts actively immunized with a synthetic bovine inhibin peptide fragment (bINH) conjugated to human alpha globulins (HAG, n = 4 gilts) or HAG alone (control; n = 5) were unilaterally ovariectomized at mid-cycle. After unilateral ovariectomy, the remaining ovary was removed between day 8 and day 12 of the subsequent oestrous cycle. The number of corpora lutea per ovary was determined at each ovariectomy. Blood samples were collected at frequent intervals beginning 1 h before and continuing until the first oestrus after unilateral ovariectomy, and serum concentrations of FSH, LH, progesterone and oestradiol were determined. Inhibin antibody titres were estimated from the percentage of125I-labelled bINH bound to serum diluted 1:4000. At unilateral ovariectomy, the number of corpora lutea per ovary was similar for bINH:HAG-immunized and control gilts (8.6 ±0.7 versus 7.6 ± 0.6). During the next oestrous cycle after unilateral ovariectomy, the number of corpora lutea on each remaining ovary had doubled (P < 0.05) in controls compared with the number of corpora lutea per ovary in the previous cycle. In contrast, the number of corpora lutea remained unchanged in bINH:HAG-immunized gilts. Titre of anti-inhibin antibodies in bINH:HAG-immunized gilts was 9 ± 1% at unilateral ovariectomy compared with 0% for controls. Alterations in serum concentrations of hormones after unilateral ovariectomy did not differ between treatment groups. Compensatory ovarian hypertrophy was blocked after unilateral ovariectomy in immunized gilts independent of alterations in serum hormones, duration of oestrous cycle, or normal ovulation rate per ovary. Thus, it is concluded that inhibin or inhibin α subunits are positive local stimulators of compensatory ovarian hypertrophy in postpubertal gilts.