Carcass and meat quality of finishing Friesian steers fed the ß-adrenergic agonist L-644,969.

The effect of the ß-adrenergic agonist L-644,969 on selected parameters of carcass and meat quality was examined in Friesian steers. Four groups of 18 steers were individually offered ad libitum a pelleted diet that contained 0, 0·25, 1·0, or 4·0 ppm L-644,969 for 12 weeks prior to slaughter. L-644,969 quadratically increased carcass weight (3·7, 9·3, and 8·5%, P < 0·001) and altered the distribution of lean meat such that a greater (0·3-5%; P < 0·01) proportion was in the more valuable cuts. There were no effects of L-644,969 on carcass-chill loss and on the water-holding capacity of the longissimus thoracis et lumborum (LTL) muscle. The intramuscular-fat concentration of the LTL was decreased (27-50%; P < 0·01) and the effects on muscle ultimate pH were small and commercially unimportant. Fibre-optic-probe measurements of the LTL indicated darker (P < 0·01) meat due to ß-agonist treatment. L-644,969 increased the shear force required to cut through cooked muscle from the LTL (159%, 209%, and 217%, P < 0·001). It is concluded that L-644,969 treatment improved the quantity and distribution of lean in the carcass but impaired meat quality, primarily through a reduction in tenderness.

Growth, feed efficiency and carcass composition of finishing Friesian steers fed the beta-adrenergic agonist L-644,969.

The beta-adrenergic agonist L-644,969 was evaluated to determine its effects on growth performance and carcass composition of Friesian steers. L-644,969 is the R,R isomer of 6-amino [[(1-methyl-3-phenylpropyl) amino] methyl]-3-pyridine methanol dihydrochloride. Four groups of 18 steers, averaging 380 kg body weight, were individually given ad libitum access to a pelleted concentrate diet that contained either 0, .25, 1.0 or 4.0 ppm L-644,969 for the final 12 wk of the finishing period. Live weight gain was not affected by L-644,969, but feed consumption was linearly reduced (5.5, 6.3 and 15.7%; P less than .01) and feed conversion efficiency was linearly increased (16, 25 and 31%; P less than .01) relative to unmedicated controls, respectively. In addition, L-644,969 quadratically increased carcass weight (3.7, 9.3 and 8.5%; P less than .01) and dressing percentage (2.7, 7.9 and 7.9%; P less than .001). The proportion of trimmed fat in the carcass was quadratically reduced (14.5, 29 and 36%; P less than .001) and yield of lean meat quadratically increased (6.7, 13 and 15.6%; P less than .001). beta-adrenergic agonist treatment altered the distribution of lean meat such that a greater (P less than .001) proportion of the total lean was in the hind portion of carcasses from treated animals. Based on these findings, we suggest that L-644,969 may have utility as an agent to improve efficiency of production of lean beef.

The synergistic effects of His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 on growth hormone (GH)-releasing factor-stimulated GH release and intracellular adenosine 3',5'-monophosphate accumulation in rat primary pituitary cell culture.

His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 (GHRP-6) stimulated GH release from rat primary pituitary cells in a time- and dose-dependent manner. Stimulation was observed after a 15-min, but not a 4-h, incubation. The concentrations of GHRP-6 required for half-maximal and maximal stimulation were 7 x 10(-9) and 10(-7) M, respectively. GH release induced by GHRP-6 was not affected by the addition of either naloxone or the GRF antagonist [N-Ac-Tyr1,D-Arg2]GRF-(1-29)-NH2. The latter inhibited GRF-stimulated GH release by shifting the dose-response curve to the right. His-D-Trp-D-Lys-Trp-D-Phe-Lys-NH2, an analog of GHRP-6, inhibited GH release stimulated by GHRP-6 without affecting that induced by GRF. When present together at maximal concentrations, GHRP-6 and GRF produced a synergistic effect on GH release. GHRP-6 had no effect on intracellular cAMP levels, whereas GRF increased intracellular cAMP concentrations by 3-fold. Combined treatment of pituitary cells with GRF and GHRP-6 resulted in a potentiation of the GRF-induced increase in cAMP levels. Basal GH release was reduced by 30% after pretreatment with GHRP-6 (10(-7) M) for 1 h. Pretreatment with GHRP-6 also decreased the subsequent response to GHRP-6, but not GRF. In contrast, pretreatment with GRF for 1 h had no effect on the subsequent action of GHRP-6 or GRF on GH release. The desensitization induced by GHRP-6 was completely reversed within 1 h after removal of the peptide. Results from this study indicate that GHRP-6 and GRF stimulated GH release from somatotrophs via different receptors and through discrete mechanisms.

High performance liquid chromatography of steroids in bovine adrenal glands: changes during the postpartum interval.

High performance liquid chromatography was utilized to characterize changes in the content of various steroids in adrenal glands with time postpartum. Adrenal glands were obtained from 32 beef cows on either days, 7, 14, 28, 42 or 56 after parturition (n = 6-8/group). Of the major glucocorticoids, only cortisone significantly changed with time postpartum. Adrenal content of cortisone was 56% lower (P less than 0.05) on day 14 than on days 7, 28, 42 and 56. Average adrenal content (microgram/g) of cortisol, cortisone, corticosterone and 11-dehydrocorticosterone was 3.3, 2.4, 1.8 and 1.8, respectively. All other adrenal steroids measured were less than 0.8 micrograms/g. Correlation coefficients among the adrenal steroids measured indicated that terminal products (e.g. 11-dehydrocorticosterone and cortisone) of glucocorticoidogenesis were positively correlated (r = 0.56-0.64) with each other (P less than 0.05), whereas cortisone was negatively correlated (P less than 0.05) with both 17 alpha-hydroxyprogesterone (r = -0.49) and progesterone (r = -0.58). The latter suggests that a product (cortisone) inhibition mechanism may exist in bovine adrenal glucocorticoidogenesis. Collectively, these data suggest that cortisone should be considered a potential factor in the regulation of physiologic functions in postpartum cattle.

Modulation of growth hormone-releasing factor-induced release of growth hormone from bovine pituitary cells.

Growth hormone (GH)-releasing factor (GRF) at concentrations of 10(-12) through 10(-7) M for 6 hr linearly increased GH release (b1 = 10.4 +/- .3) from bovine anterior pituitary cells in culture. Maximum release of GH (262% above controls) occurred at 10(-7) M GRF. In contrast, GH release-inhibiting factor (SRIF) at 10(-12) through 10(-5) M had no effect on basal concentrations of GH. In a second experiment, as the proportion of SRIF relative to GRF increased, SRIF suppression of GRF-induced GH release from anterior pituitary cells increased. In a third experiment, anterior pituitary cells cultured in media containing fetal calf serum (FCS) were treated with cortisol (0 or 10 ng/ml media) for 24 hr before exposure to 10(-13) through 10(-7) M GRF. GRF linearly increased GH secretion (b1 = 7.4 +/- .3) and cortisol augmented this response (b1 = 10.5 +/- .6). However, when cells were cultured in media containing dextran-charcoal treated FCS, cortisol did not alter GRF-induced GH release. Our results demonstrate that GH response of bovine anterior pituitary cells to GRF was modulated negatively by SRIF. However, augmentation of GRF-induced GH release by cortisol was evident only when cells were cultured in media supplemented with untreated FCS.

Relationship between weaning and secretion of luteinizing hormone, cortisol and transcortin in beef cows.

The effects of suckling on secretion of luteinizing hormone, cortisol and transcortin were investigated in anovulatory postpartum cows. On d 35 postpartum, calves were separated from 12 cows to prevent suckling and eight calves continued to suckle their dams ad libitum. Between 35 and 41 d postpartum, samples of jugular blood were collected every 15 min for two periods of 6 h/d. In non-suckled cows, frequency of pulses and basal luteinizing hormone increased but amplitude of pulses did not change. Concentrations of total cortisol in serum of cows were not altered during 3 d after weaning calves and did not differ among intervals before, during and after a suckling event. Affinity of transcortin for cortisol was not affected by postpartum interval or treatment. Capacity of transcortin to bind cortisol tended to increase after weaning. We found no evidence to support the hypothesis that suckling reduces binding capacity of transcortin or increases unbound cortisol. Differences in preovulatory secretion of luteinizing hormone between suckled and non-suckled cows could not be accounted for by differences in secretion of cortisol. In beef cows that are fed to satisfy requirements for energy and have average body condition, we conclude that negative modulation of luteinizing hormone by suckling is not mediated by cortisol.

Relationships between histological signs of atresia, steroids in follicular fluid, and gonadotropin binding in individual bovine antral follicles during postpartum anovulation.

Two experiments were conducted to determine the relationship between histological signs of atresia, gonadotropin binding, and steroids in fluid of medium-sized bovine follicles during postpartum anestrus. In Experiment I, ovaries of 21 cows were removed on Days 7, 14, 28, 42, or 56 after parturition. In Experiment II, ovaries of 29 cows were removed between Days 20 and 30 postpartum after 48 or 96 h of either saline (0.9% NaCl, 5 ml) or luteinizing hormone-releasing hormone (LHRH; 500 ng/5 ml saline) injections given every 2 h via jugular cannulas. Two to 10 follicles, 4.0-7.9 mm in diameter, were removed per pair of ovaries. Follicles were classified as normal, intermediate, atretic, or luteinized-atretic, depending on their micromorphology. In both Experiments I and II, follicles classified as normal had 50-80% lower (p less than 0.05) concentrations of progesterone and 2- to 7-fold greater (p less than 0.05) concentrations of estradiol than atretic follicles. However, concentrations of androstenedione and gonadotropin-binding sites were similar in normal and atretic follicles. Atretic follicles had degenerative granulosa with several pyknotic nuclei, thick theca, and little distinction between theca and granulosa. Intermediate follicles showed slight signs of degeneration and had 2- to 3-fold greater (p less than 0.05) concentrations of progesterone than normal follicles. Concentrations of estradiol did not differ (p greater than 0.10) between normal and intermediate follicles. Equal proportions of normal and atretic medium-sized follicles were located on the ovaries bearing the corpus albicans from pregnancy (CAP).(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of surface diameters and dissected diameters of bovine ovarian follicles.

Comparisons were made between diameters of 54 bovine follicles greater than 5.9 mm from 32 pairs of ovaries measured on the ovarian surface and diameters of the same follicles subsequently dissected from the ovaries. Seventy-eight percent of follicles measured on the ovarian surface were within 1.9 mm of the size measured after dissection. The remaining 22% of follicles measured on the surface had diameters recorded that were 2.0 to 3.9 mm different than their diameter after dissection. Surface diameter tended to underestimate dissected diameter for small follicles (less than 8.0 mm) and to overestimate dissected diameter for large (greater than or equal to 12.0 mm) follicles. The correlation coefficient between surface and dissected follicular diameters was .83. We conclude that measuring the diameter of the largest follicles on the ovarian surface and after dissection yield approximately equivalent results.

Effects of intermittent injections of LHRH on secretory patterns of LH and FSH and ovarian follicular growth during postpartum anovulation in suckled beef cows.

Changes in numbers of ovarian follicles and coincident secretion of pituitary gonadotropins were characterized in suckled, anovulatory beef cows injected iv with 500 ng of luteinizing hormone-releasing hormone (LHRH) every 2 h for 48 or 96 h, starting 21.4 +/- .4 d after parturition. Two hours after the last injection, all cows were ovariectomized. Compared with saline-injected controls, LHRH had no effect on baseline or overall concentrations of luteinizing hormone (LH) in serum (P greater than .10), but increased (P less than .05) frequency and decreased (P less than .05) amplitude of LH pulses. Luteinizing hormone-releasing hormone increased (P less than .05) baseline concentration of follicle stimulating hormone (FSH) in serum and frequency of FSH pulses, but decreased (P less than .05) pulse amplitude. Overall concentrations of FSH increased 20% (P less than .10). Exogenous LHRH did not affect diameter of the two largest follicles or numbers of follicles 1.0 to 3.9 mm, 4.0 to 7.9 mm or greater than or equal to 8.0 mm in diameter. These data suggest that increasing the frequency of episodic LH and FSH pulses in postpartum cattle by intermittent administration of LHRH did not increase mean circulating levels of LH, or alter size and numbers of ovarian follicles within the 96-h period of injections. Thus, induction of ovulation in anovulatory cows treated with low-dose injections of LHRH cannot be explained on the basis of an increase in mean concentrations of LH or numbers of antral follicles within 96 h after initiation of injections.

Effects of intermittent injections of LHRH on specific binding of 125I-labeled gonadotropins to granulosa and theca, and concentrations of steroids in serum and ovarian follicles during postpartum anovulation in suckled beef cows.

To examine ovarian follicular response to low-dose injections of luteinizing hormone-releasing hormone (LHRH), 32 anovulatory, suckled beef cows were allotted to one of four treatment groups and injected with either saline or 500 ng LHRH every 2 h for 48 or 96 h, starting 21.4 +/- .4 d after parturition. Two hours after the last injection of LHRH, cows were ovariectomized and 10 to 15 ovarian follicles per pair of ovaries were removed and categorized by diameter as small (1.0 to 3.9 mm), medium (4.0 to 7.9 mm) or large (greater than or equal to 8.0 mm). Injections of LHRH did not affect (P greater than .10) steroid levels in small follicles or numbers of gonadotropin receptors in small and medium follicles. Concentrations of progesterone in fluid of medium follicles increased 1.5-fold (P less than .05) after 96 h of LHRH, whereas concentrations of estradiol and androstenedione were unchanged. In fluid of large follicles, concentrations of progesterone were fourfold greater (P less than .05) in LHRH-treated than in control cows at 48 h, but by 96 h progesterone was twofold greater (P less than .05) in control than LHRH-treated cows. In large follicles, concentrations of estradiol were unchanged (P greater than .10) after 48 h of LHRH injections but after 96 h estradiol was twofold greater (P less than .05) in LHRH-treated than control cows. Increased concentrations of estradiol in large follicles coincided with increased numbers of binding sites for human chorionic gonadotropin (hCG) but not follicle stimulating hormone (FSH) in granulosa and theca.(ABSTRACT TRUNCATED AT 250 WORDS)

Anovulation in postpartum suckled beef cows. II. Associations among binding of 125I-labeled gonadotropins to granulosa and thecal cells, and concentrations of steroids in serum and various sized ovarian follicles.

To determine if specific binding of 125I-labeled gonadotropins to granulosa and thecal cells, or concentrations of steroids in ovarian follicles change during the postpartum anovulatory period, 21 suckled beef cows were slaughtered on d 7, 14, 28, 42 or 56 after parturition (n = 4 to 6 per d). After slaughter, 10 to 15 follicles were dissected from each pair of ovaries and categorized by diameter: small (1.0 to 3.9 mm), medium (4.0 to 7.9 mm) or large (greater than or equal to 8 mm). Progesterone (221 to 612 ng/ml), androstenedione (48 to 94 ng/ml) and estradiol (2.7 to 23.9 ng/ml) did not change (P greater than .10) in fluid of small or medium follicles from d 7 to 42 to 56 after parturition. Similarly, specific binding of human chorionic gonadotropin (125I-hCG) or follicle stimulating hormone (125I-oFSH) to homogenates of small, medium or large follicles did not change (P greater than .05). In contrast, progesterone in fluid of large follicles increased (P less than .05) 3.4-fold between d 7 and 14, but decreased (P less than .05) 55% between d 14 and 28. Concentrations of androstenedione in fluid of large follicles did not change (P greater than .10) from d 7 to 42 to 56. Concentrations of estradiol in fluid of large follicles remained constant between d 7 and 14, but increased (P less than .05) 4.2-fold between d 14 and 28. We conclude that during the postpartum anovulatory period, there is no change in steroidogenic capabilities of small or medium follicles, both of which predominantly produce progesterone.(ABSTRACT TRUNCATED AT 250 WORDS)

Anovulation in postpartum suckled beef cows. I. Associations among size and numbers of ovarian follicles, uterine involution, and hormones in serum and follicular fluid.

Changes in sizes and numbers of ovarian antral follicles, uterine size and weight, serum hormones, and frequency and duration of suckling were examined during the postpartum anovulatory period in primiparous, suckled beef cows. Twenty-one anovulatory, suckled cows (n = 4 to 6/d) were slaughtered on d 7, 14, 28 and 42 to 56 after parturition. In addition, a total of 11 postpartum cows that had begun cyclic activity were slaughtered on d 28, 42 or 56. Blood was collected at 10-min intervals for 6 h 1 d before slaughter for measurement of prolactin, cortisol and progesterone in serum. Numbers of medium (4.0 to 7.9 mm) follicles increased fourfold (P less than .05) between d 7 and 42 to 56 in anovulatory cows, whereas numbers of small (1.0 to 3.9 mm) and large (greater than or equal to 8.0 mm) follicles did not change (P greater than .10). Uterine involution was complete by d 28. In anovulatory cows, a higher (P less than .05) proportion of largest (but not second-largest) follicles was opposite the ovary containing the corpus albicans from pregnancy (CAP). In addition, 90% of these largest follicles opposite the CAP had concentrations of estradiol greater than progesterone. In cyclic cows, however, first ovulations occurred with equal frequency on either ovary. Concentrations of prolactin or cortisol in serum or duration of suckling were not associated with changes in uterine or ovarian measurements. In conclusion, growth and function of the largest (but not second-largest) follicle were reduced when located on the ovary containing the CAP.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between pituitary GnRH-binding sites and pituitary release of gonadotrophins in post-partum beef cows.

Thirty primiparous suckling beef cows were slaughtered on Day 7, 14, 28, 42 or 56 after parturition. Some had resumed oestrous cyclicity by the time they were slaughtered on Days 42 and 56. Amongst acyclic cows between Days 7 and 42, pituitary LH concentrations and basal and GnRH-induced release of LH from pituitary explants doubled. Pituitary FSH concentration and basal release in FSH increased only by 15-20%, while GnRH-induced release of FSH in vitro was unchanged. During postpartum anoestrus, overall mean concentrations of serum FSH did not change, whereas overall mean concentrations and pulse amplitudes of serum LH increased. Numbers and affinity constants of GnRH-binding sites in pituitary glands remained constant during the post-partum period studied. We conclude that, under these experimental conditions, numbers and affinity constants of GnRH-binding sites in the pituitary gland of post-partum beef cows do not limit the ability of the anterior pituitary gland to release gonadotrophins.

Changes in inhibin-like bioactivity in ovulatory and atretic follicles and utero-ovarian venous blood after prostaglandin-induced luteolysis in heifers.

The objectives of this study were to develop a bioassay for measuring inhibin bioactivity in untreated samples of bovine follicular fluid (BFF) and then examine changes in inhibin bioactivity in ovulatory and atretic follicles and utero-ovarian venous blood during the periovulatory period in heifers. A rat pituitary cell culture system was used to bioassay inhibin-like activity. Addition of 0.005 to 1 microliter untreated (whole), unfiltered charcoal-stripped, or filtered whole BFF to pituitary cultures caused a linear suppression of LHRH-induced FSH release but had no effect on LH secretion. Steroids in BFF did not suppress FSH secretion, since removal of steroids from BFF with charcoal did not remove the FSH-suppressive activity in BFF. Addition of ether extracts of BFF caused a slight but nonparallel suppression of FSH secretion; however, heating these extracts removed most of this suppressive activity. Removal of BFF from pituitary cultures completely restored the capacity of pituitary cultures to respond to LHRH. It was concluded that the inhibin bioassay was specific for detecting inhibin-like activity in fluids from individual follicles without interference of steroids. Within 12 h after a prostaglandin (PG) injection during the luteal phase of heifers, LH levels in serum increased 2- to 4-fold and remained at this level until the occurrence of the preovulatory gonadotropin surge. In contrast, FSH did not change before the gonadotropin surge. Inhibin bioactivity was measured in all follicles (greater than or equal to 6 mm) 0, 12, 24, 36, 48, 60, and 72 h after and in utero-ovarian venous serum 0, 24, and 36 h after PG-induced luteolysis. From 0-36 h after PG administration, inhibin-like activity increased linearly in presumed ovulatory follicles and utero-ovarian venous serum. Then, from 48-72 h after PG treatment, before the preovulatory LH surge, inhibin activity decreased in ovulatory follicles. After the surge but before ovulation, inhibin-like activity increased in ovulatory follicles. Inhibin-like activity in atretic follicles did not change after PG treatment and was lower in atretic than ovulatory follicles. Since a single hypothalamic releasing factor, LHRH, may control the secretion of LH and FSH, increased secretion of inhibin from preovulatory follicles before the preovulatory LH and FSH surges could account for the absence of a presurge rise in FSH in blood, as was observed for LH during this time in heifers. Diminished follicular production of inhibin during the gonadotropin surge could explain the preovulatory release of FSH along with LH during this time.

Relationship between pituitary responsiveness to Gn-RH and number of Gn-RH-binding sites in pituitary glands of beef cows.

Changes in the ability of Gn-RH to induce gonadotrophin release with time after synchronization of oestrus was determined in 4 groups of 6 cows each. Cows were given Gn-RH at 40-min intervals for 6 h beginning at -24, 0, 18 or 36 h (time 0 = removal of progestagen implant). Changes in concentration (ng/ml) of serum LH after Gn-RH averaged 2.9, 6.2, 6.4 and 33.4, whereas serum FSH averaged 25.7, 35.8, 35.8 and 97.3. Thus the responsiveness of the pituitary to Gn-RH had increased by 36 h after implant removal. Other groups of cows subjected to the same synchronization scheme were slaughtered at 0 h, 24 h or at various times after onset of oestrous behaviour. Gn-RH binding to crude pituitary membrane preparations was assessed. There was no apparent change in the affinity constant of Gn-RH-binding sites with time after synchronization. The number of Gn-RH-binding sites remained unchanged until the period of oestrus when a significant decline with time was detected. We conclude that the increase in pituitary responsiveness to Gn-RH that occurs before the preovulatory gonadotrophin surge was not directly associated with changes in number or affinity of pituitary Gn-RH-binding sites in crude pituitary membrane preparations.

Cortisol inhibits and adrenocorticotropin has no effect on luteinizing hormone-releasing hormone-induced release of luteinizing hormone from bovine pituitary cells in vitro.

Suckling causes a delay in onset of estrus and ovulation in cattle postpartum. In addition, the suckling stimulus causes the release of corticoids presumably via ACTH. Since any hormone released by suckling is a potential inhibitor of gonadotropin secretion and/or ovulation, we investigated the effects of ACTH and cortisol on LHRH-induced LH release from bovine pituitary cells in vitro. Anterior pituitary glands were obtained from cows killed during the luteal phase of an estrus cycle (days 5-15). Pituitary cells, disaggregated enzymatically, were grown in Dulbecco's medium containing 10% dextran charcoal-stripped fetal calf serum. On day 5, cultures were washed and reincubated in serum free medium containing the hormone being tested. After 6 h of incubation, LHRH was added in 10 microliters medium and incubation continued for an additional 6 h. ACTH at 4.3 X 10(-9), 4.3 X 10(-8), and 4.3 X 10(-7) M had no effect on basal or LHRH-induced LH release. Cortisol at 12.1 ng/ml decreased (P less than 0.001) the slope of LHRH response curve (b1 = 2.9 vs. 5.5 for controls). To determine if this effect was specific for cortisol, we compared cortisol, dexamethasone, and progesterone. LHRH-induced LH release (percent of control) was decreased (P less than 0.001) by 12.1 ng/ml cortisol (98%), 1, 5, and 10 ng/ml dexamethasone (60%, 71%, and 88%), but not by 3.1 ng/ml progesterone. The inhibitory effect of cortisol was reversible. Thus, LHRH-induced LH release (percent of controls) at 0, 24, 48, and 72 h after a 24-h exposure to 12.1 ng/ml cortisol was 19%, 89%, 100%, and 115%, respectively. We have demonstrated that cortisol at concentrations found normally in blood of cows postpartum will inhibit LHRH-induced LH release from bovine pituitary cells. This observation is consistent with the hypothesis that cortisol released by suckling may inhibit gonadotropin secretion postpartum and as such may prolong the anovulatory interval postpartum.

D-valine medium maintains prolactin production in primary culture.

Prolactin secretion by bovine pituitary cells in L-valine-containing medium decreases approximately 96% from day 3 to day 11 of culture. We hypothesized that this decrease was caused by overgrowth of these cultures by fibroblasts. Our present objective was to maintain the synthesis and secretion of PRL by bovine pituitary cells in culture. We attempted this by growing pituitary cells in D-valine-containing medium to achieve selective suppression of fibroblast growth. Substitution of D-valine for L-valine in Earle's or Swim's medium resulted in undiminished PRL synthesis and release over a 30-day culture period. In contrast, comparable measures for cells maintained in medium with L-valine decreased more than 90% from day 5 to day 20 of culture and remained low thereafter. Cells cultured in medium containing D-valine retained their ability to release PRL in response to thyrotropin-releasing hormone throughout the 30-day culture period, although there was a decrease in magnitude of response with time. Similarly, estradiol increased PRL release by pituitary cells maintained in D-valine, but this stimulatory effect was no longer demonstrable by day 20 of culture. The amount of growth hormone (GH) and luteinzing hormone (LH) released into the medium decreased with time and this decrease was independent of the valine isomer contained in the medium. We conclude that substituting D-valine for L-valine in culture medium allows PRL synthesis and release to persist undiminished for at least 30 days in culture.

Effect of dosage and frequency of injection of luteinizing hormone releasing hormone on release of luteinizing hormone and follicle stimulating hormone in estradiol-treated steers.

The objective was to determine how estradiol (0 vs 1 mg) and changes in the dosage of luteinizing hormone releasing hormone (LHRH; 1,000 ng/steer vs 1 ng/kg body weight) and frequency of LHRH injection (25 vs 50 min) affect LH and follicle stimulating hormone (FSH) release in steers. In steers pretreated with estradiol peak concentrations of LH in serum after LHRH averaged 14.4 ng/ml, which was greater (P less than .001) than peak concentrations in steers given oil (7.4 ng/ml). Increasing the dosage of LHRH from 1 ng/Kg body weight (approximately or equal to 300 ng/steer) to 1,000 ng/steer increased (P less than .001) peak LH values from 7.5 to 14.4 ng/ml. Furthermore, increasing the frequency of LHRH injections from once every 50 min to once every 25 min increased (P less than .001) LH release, but only in steers given estradiol. Estradiol reduced basal concentrations of FSH by 65% and then increased LHRH-induced FSH release by 276% (P approximately .07) relative to values for steers given oil. Only when 1,000 ng LHRH was given every 25 min to steers pretreated with estradiol were LH and FSH release profiles similar to the preovulatory gonadotropin surges of cows in magnitude, duration and general shape. The results demonstrate that increases in the dosage or frequency of LHRH pulses increase LHRH-induced release of LH, but not of FSH. Furthermore, these results are consistent with the hypothesis that in cows, estradiol increases responsiveness of the gonadotrophs to LHRH and then increases the magnitude and frequency of pulses of LHRH secretion beyond basal levels, thereby causing the preovulatory gonadotropin surges.