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Biomedical subjects

F C Leung

Publications and source records attributed to F C Leung.

53 records · Page 3Linked to original sources
PubMed1984

Effects of dietary thyroid hormones on growth and serum T3, T4, and growth hormone in sex-linked dwarf chickens.

Dwarf pullets fed with either T3 or T4 at 0.1, 1.0, and 10.0 ppm for 2 weeks showed no improvement in their body weight gain as compared with birds that were fed a control diet. Birds fed T3 or T4 at 10.0 ppm showed poorer growth, body weight gain, and feed efficiency than control birds. Pullets fed 1.0 ppm of T3 showed significantly better feed efficiency than control birds. Serum T3 concentrations were significantly higher when birds were fed 1.0 ppm of T3 or 10.0 ppm of T3 or T4. Plasma T4 concentrations were also higher in T4 fed birds (1.0 ppm and 10.0 ppm), but were significantly lower in T3 fed birds (1.0 ppm and 10.0 ppm) than in control birds. In birds fed T3 or T4 at 1.0-ppm and 10.0-ppm levels serum growth hormone concentrations were significantly lower as compared with control birds. In conclusion, exogenous T3 and T4 did not correct the sex-linked dwarfism of dwarf chickens. Such dwarfism is characterized by low circulating levels of T3 and T4.

Animals↗
PubMed1983

In vivo and in vitro stimulation of growth hormone release in chickens by synthetic human pancreatic growth hormone releasing factor (hpGRFs).

The effect of the synthetic human pancreatic GH releasing factors (hpGRFs, hpGRF44 and hpGRF40) on GH release was studied both in vivo and in vitro. Four-week-old cockerels were injected iv with hpGRFs at 0.1 microgram, 1.0 microgram, and 10.0 micrograms/bird. The 0.1 microgram dose of hpGRFs had no effect on plasma GH levels, but the 1.0 microgram and 10.0 micrograms doses of hpGRFs caused a dose-dependent elevation of plasma GH. Plasma GH levels returned to basal 60 min after injection. HpGRFs, thyrotropin releasing hormone (TRH) and somatostatin (SRIF) were examined in a chicken in vitro pituitary cell culture system. In vitro hpGRFs and TRH caused a significant dose-dependent release of chicken GH, and the ability to stimulate GH was additive when hpGRF44 and TRH were added together in the culture medium. SRIF showed no consistent effect on the release of chicken GH, but it inhibited the stimulatory effect of hpGRF44 on chicken GH release. We conclude that hpGRF is a potent releaser of chicken GH in vivo and in vitro.

Animals↗
PubMed1983

Counteraction of underfeeding-induced inhibition of mammary tumor growth in rats by prolactin and estrogen administration.

The purpose of this study was to determine whether administration of estrogen or/and haloperidol (HAL), a drug that increases prolactin (PRL) secretion, could counteract the inhibitory effects of underfeeding on growth of established carcinogen-induced mammary cancers in rats. Sprague-Dawley female rats were fed half of the complete diet consumed daily by ad libitum fed controls, beginning 1 week before daily injection of estradiol benzoate (EB) or/and HAL for 3 weeks. Body weight, mammary tumor number, and mammary tumor diameter were measured at weekly intervals, and at the end of the 3 weeks blood was collected for assay of serum PRL. Full-fed (FF) control rats showed an increase by the end of 3 weeks in average body weight, mammary tumor number, and mammary tumor size whereas half-fed (HF) rats showed a significant decrease in average body weight, tumor number, and tumor size, and serum PRL concentrations when compared with these parameters in FF rats. Administration of EB to the HF rats partially prevented loss of mammary tumor size and significantly increased serum PRL levels over those of HF or FF control rats. Injection of HAL or the combination of HAL and EB completely prevented the decrease in mammary tumor number and size in the HF rats. These results suggest that regression in number and size of established mammary tumors in HF rats during a 3-week period is due primarily to a decrease in secretion of estrogen and PRL, the two hormones essential for mammary tumor growth in rats.

Animals↗
PubMed1981

Evidence for hypothalamic noradrenergic involvement in naloxone-induced stimulation of luteinizing hormone release.

A single injection of the opiate antagonist, naloxone (NAL), resulted in a fourfold increase in serum luteinizing hormone (LH) concentration 20 min after injection. To determine whether noradrenergic neurons were involved, male Sprague-Dawley rats were treated with alpha-methyl-p-tyrosine (alpha-MPT), phenoxybenzamine hydrochloride (PBH), or diethyldithiocarbamate (DDC), all anti-noradrenergic drugs. Reduction of hypothalamic norepinephrine synthesis by alpha-MPT or DDC, or blockade of the alpha-receptors by PBH, resulted in complete suppression of NAL-induced LH release. These results suggest that the NAL-induced increase in LH release is mediated in part via a hypothalamic noradrenergic mechanism.

Animals↗
PubMed1980

Effects of advancing age on the hypothalamic-pituitary-ovarian axis of the female white-footed mouse (Peromyscus leucopus).

Peromyscus leucopus, with an average lifespan of 48 months, showed unchanged levels of serum luteinizing hormone (LH), estradiol, progesterone, and pituitary LH and prolactin, between the ages of 12 and 48 months. Hypothalamic LH-releasing hormone (LHRH), norepinephrine and dopamine also remained unchanged with advancing age. Ovarian and uterine weight decreased with age, although the changes in uterine weight were not statistically significant. These data indicate that the hypothalamic-pituitary-ovarian axis remains intact with increasing age, accounting for the maintenance of fertility in these animals. The lack of significant changes in these parameters is in very marked contrast to those in the aging laboratory mouse and rat, which show derangements in their reproductive systems midway through their lifespans.

Aging↗
PubMed1980

Inhibition of mammary tumor growth by dexamethasone in rats in the presence of high serum prolactin levels.

Female Sprague-Dawley rats with established 7, 12-dimethylbenz(a)anthracene-induced mammary tumors were given daily s.c. injections of 50 microgram dexamethasone per rat, 0.5 mg haloperidol per kg, or both for 3 weeks. Control rats received the injection vehicles only. Mammary tumor growth was measured at weekly intervals for 21 days, and blood was collected on Days 10 and 21 of treatment for assay of prolactin. Dexamethasone produced significant regression of mammary tumors and reduced serum prolactin levels, whereas haloperidol significantly increased mammary tumor growth and greatly elevated serum prolactin levels. When dexamethasone and haloperidol were injected together, there was significant regression of mammary tumors despite markedly elevated serum prolactin levels. No significant differences in specific prolactin binding to membrane preparations of mammary tumors from these animals were observed in any treatment group. These results indicate that dexamethasone, a synthetic glucocorticoid, can directly inhibit mammary tumor growth in the presence of elevated serum prolactin levels produced by haloperidol, and this inhibition is not due to a reduction of prolactin binding sites in the tumor tissue.

Animals↗
PubMed1978

Relationship between bioassay and radioimmunoassay estimates of prolactin in rat serum.

RIA and bioassay (BA) estimates of the concentration of PRL ([PRL]) in rat serum were compared using homologous assay systems. A rat mammary gland organ culture BA was developed which allowed objective and quantitative measurement of serum [PRL]. the serum samples were tested at two doses against three doses of the RP-1 rat PRL standard and the responses were quantified by a histometric procedure. The index of precision of the BA ranged from 0.1--0.4 and the secretory response was not affected by physiological concentrations of rat GH, T3, or by gonadal steroids. Serum samples taken from female rats during the estrous cycle, from suckled lactators, from rats bearing a PRL-secreting pituitary tumor, and from females that were treated with perphenazine were assayed. A highly significant correlation (r = 0.99) was obtained between the two assay methods with 21 samples. However, the regression coefficient was 4.20 +/- 0.55.Thus, although the two assays are highly correlated for measuring serum [PRL], the RIA measures only about 25% of the hormone that is detected by the mammary gland organ culture BA.

Animals↗
PubMedDate not supplied

New variations in intron 4 of growth hormone gene in Chinese native chickens.

Polymorphism in intron 4 of chicken growth hormone (cGH) gene was studied in 20 Chinese native chicken populations and broiler or layer populations. A total of eight restriction digestion profiles were identified in intron 4 and confirmed by sequencing. Among 20 populations, there were distinctively different allele numbers and frequencies of intron 4 restriction fragment length polymorphisms (RFLPs) between Chinese native chickens and broilers or layers. Two new alleles, allele D and allele E, were identified in Taihe Silkies. Allele D was also identified in other Chinese native breeds and a 50 bp fragment deletion was identified in allele E.

Animals↗
PubMedDate not supplied

Molecular characterization of seven Chinese isolates of infectious bursal disease virus: classical, very virulent, and variant strains.

Seven infectious bursal disease virus (IBDV) strains isolated from China have been characterized in this study, including a classical strain CJ801, an attenuated strain GZ911, a variant strain GZ902, and four very virulent strains G9201, G9302, F9502, and HK46. With the use of reverse transcription-polymerase chain reaction, the full-length VP2 genes were amplified and the hypervariable regions were sequenced. Protein sequences of the hypervariable region (a.a. 143-382) of the field isolates confirmed their identities. CJ801 has the highest identity to the classical strains STC and 52/70. GZ902 has the highest identity to the American variant strains A, E, and GLS, and they share unique amino acid residue at positions 249K and 254S, which are not present in standard serotype 1 strains. Attenuated strain GZ911, like other cell culture-adapted strains, has substitutions at positions 279(D to N) and 284 (A to T) as well as in the serine-rich heptapeptide region. Hence, these substitutions may take an important role in the reduced virulence of these strains. The four very virulent strains have the highest identity to the European very virulent strain UK661 and Japanese strain OKYM. These strains share unique amino acid residues at positions 222A, 256I, and 294I, which are not present in other less virulent strains. The very virulent strains isolated in Guangdong (G9201, G9303) and Fujian (F9502) Provinces have one to five amino acid substitutions at the two hydrophilic domains of VP2 comparing with UK661 and OKYM, indicating that new very virulent strains are evolving. Phylogenetic analysis suggests that Chinese very virulent IBDVs and European very virulent strains are derived from similar origin.

Amino Acid Sequence↗