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Biomedical subjects

C H Tan

Publications and source records attributed to C H Tan.

At least 127 records · Page 7Linked to original sources
PubMed1989

Blood steroid levels in the goldfish: measurement of six ovarian steroids in small volumes of serum by reverse-phase high-performance liquid chromatography and radioimmunoassay.

A reliable and rapid technique for the measurement of estradiol-17 beta, testosterone, progesterone, 17 alpha-hydroxyprogesterone, 17 alpha,20 beta-dihydroxy-4-pregnen-3-one, and cortisol by reverse-phase high-performance liquid chromatography and radioimmunoassay in single female goldfish serum samples of 50 microliters was developed. The steroids were extracted with Sep-Pak C18 cartridges after heat treatment. While estradiol-17 beta was assayed directly after extraction, the other steroids were separated on a mu Bondapak C18 stainless-steel column with acetonitrile:water (53:47, v/v) in 17 min under isocratic conditions, and then quantitated by specific radioimmunoassays. The technique was validated and was shown to be highly accurate, precise, sensitive, and specific for measuring those particular ovarian steroids in goldfish serum. The steroid levels measured in this way were not significantly different from those measured after separation on a Nova-Pak C18 column with methanol:water (56:44, v/v), where all the individual steroids could be completely resolved during a 40-min run. With this technique, the steroid levels in the serum of the goldfish during the secondary yolk stage, the tertiary yolk stage, and at 0 hr after ovulation were determined. While estradiol-17 beta was found to be significantly higher in the tertiary yolk stage, testosterone was significantly higher in the secondary yolk stage than in the other two stages. There was no significant difference in the levels of cortisol, 17 alpha-hydroxyprogesterone,17 alpha,20 beta-dihydroxy-4-pregnen-3-one, and progesterone among the three stages. We conclude that the technique is particularly useful for assaying multiple steroids in very small volumes of biological fluids.

Animals↗
PubMed1989

Efficient rescue of integrated shuttle vectors from transgenic mice: a model for studying mutations in vivo.

To study gene mutations in different organs and tissues of an experimental animal, we produced transgenic mice harboring bacteriophage lambda shuttle vectors integrated in the genome in a head-to-tail arrangement. As a target for mutagenesis, the selectable bacterial lacZ gene was cloned in the vector. The integrated vectors were rescued from total genomic DNA with high efficiency by in vitro packaging and propagation of the phages in a LacZ- strain of Escherichia coli C. The background mutation frequencies in brain and liver DNA appeared to be low, as was indicated by the absence of colorless plaques among 138,816 and 168,160 phage isolated from brain and liver DNA, respectively. Treatment of adult female transgenic mice with N-ethyl-N-nitrosourea resulted in a dose-dependent increase of the frequency of mutated vectors isolated from brain DNA, up to 7.4 x 10(-5) at 250 mg of the alkylating agent per kilogram of body weight. At this dose, in liver DNA of the same mice, mutation frequencies were approximately 3 x 10(-5). DNA sequence analysis of four mutant vectors isolated from brain DNA indicated predominantly G.C----A.T transitions. These results demonstrate the value of this transgenic mouse model in studying gene mutations in vivo. In addition to its use in fundamental research, the system could be used as a sensitive, organ-specific, short-term mutagenicity assay.

Animals↗
PubMed1988

Aminoglutethimide augments follicle-stimulating hormone-induced aromatase activity in cultured porcine granulosa cells.

The role of endogenous progestin synthesis in the modulation of FSH-induced aromatase activity was examined. Granulosa cells isolated from nonatretic medium-sized (3-5 mm) follicles of prepubertal pigs were cultured for an initial 48-h period, during which time aromatase activity was induced by FSH in the absence or presence of aminoglutethimide (AG). After induction, the cell monolayers were washed before being cultured for a further 6-h period in the presence of the substrate testosterone (0.5 microM). The aromatase activity was assessed by measuring the accumulation of estradiol during the test period. Basal aromatase activity was negligible and was unaffected by the presence of AG (0.1-100 microM) during the induction period. But when cells were cultured with FSH and AG (0.1-1000 microM) during the induction period, there was a dose-dependent, biphasic increase in the FSH-induced estradiol synthesis during the test period. Maximal enhancement was obtained with 10 microM AG (3.5-fold). Thereafter the aromatase activity declined and, at 1000 microM AG, was significantly (P less than 0.05) inhibited. At the same time, the FSH-stimulated progestin production during the induction period was inhibited in a dose-related fashion by AG. This AG-enhanced aromatase activity was dose and time dependent but was independent of the FSH concentration used. The apparent median effective dose of AG was 2.4 microM and a minimal time of 24 h or less was needed to potentiate the induction of aromatase activity by FSH. If AG was, however, added to the cell cultures during the test period, the FSH-induced aromatase activity was inhibited, showing that AG is an inhibitor of FSH-induced aromatase activity. This action of AG during the test period could be alleviated by the addition of testosterone during the induction period. The viability of the granulosa cells and the total cellular protein were not significantly (P greater than 0.05) altered by AG. These results show that the induction of aromatase activity by FSH could be enhanced by AG, which probably acts by inhibiting progestin production during the induction period, leading us to conclude that endogenous progestins might play an important role in modulating the induction of aromatase activity by FSH.

Aminoglutethimide↗
PubMed1988

Acute intermittent porphyria (AIP)--an unusual cause of acute confusional state. A case report.

Acute intermittent porphyria (AIP) presenting as acute confusional state without the classical features of recurrent abdominal pain, constipation, vomiting is uncommon. Such presentation in a young Malay man after a mild upper respiratory tract infection is reported. This is the first case of AIP with psychiatric symptomatology to be reported locally. The subsequent neuropsychiatric changes and interesting EEG changes are briefly discussed.

Acute Disease↗
PubMed1987

The inhibition of 3 beta HSD activity in porcine granulosa cells by 4-MA, a potent 5 alpha-reductase inhibitor.

Addition of 17 beta-N,N,-diethylcarbamoyl-4-methyl-4-aza-5 alpha-androstane-3-one, a potent 5 alpha-reductase inhibitor, to granulosa cell cultures inhibited the FSH-stimulated progesterone synthesis during both the initial 48 h induction period and the subsequent 6h test period in a dose-dependent fashion. Besides being a more potent inhibitor of FSH-stimulated progesterone synthesis than testosterone, 4-MA also synergized with the androgen to inhibit progesterone synthesis. These results indicate that 4-MA has a direct inhibitory action on 3 beta-HSD.

3-Hydroxysteroid Dehydrogenases↗
PubMed1987

Prostaglandin synthesis and its inhibition by cyclic AMP and forskolin in postpartum follicles of the guppy (Poecilia reticulata).

Prostaglandin synthesis in vitro and its inhibition by dibutyryl cyclic AMP and forskolin were examined in postpartum follicles of the guppy. The follicles were cultured for up to 72h under chemically-defined conditions, and the synthesis of prostaglandins was determined by radioimmunoassay. The results show unequivocally that the ovarian follicles devoid of embryos can synthesize PGE and PGF from both endogenous precursors as well as added arachidonic acid. That this could be inhibited by indomethacin is indicative of active synthesis rather than release of preformed prostaglandins. Dibutyryl cAMP and forskolin also inhibited prostaglandin synthesis in a dose-dependent manner, as did 3-isobutyl-l-methylxanthine. A similar effect was also evident in vitellogenic oocytes surrounded by follicle layers at the yolk-globule stage of development. While the physiological significance of this action of the cyclic nucleotide remains uncertain at this stage, it is hypothesized that it could explain the non-occurrence of ovulation in this viviparous fish soon after final oocyte maturation.

Animals↗
PubMed1987

Late functional, biochemical and histological changes in the rat lung after fractionated irradiation to the whole thorax.

The influence of fractionation of the radiation dose in up to 16 fractions on late occurring effects in rat lungs was studied. Measurement of the breathing frequency (BF) was used to monitor changes in lung function during the 76-wk follow-up period. At the end of this period the lungs were examined histologically and the hydroxyproline (HP) content of the lung tissue was determined as a biochemical indicator of the amount of fibrosis. Dose-response curves for BF could only be made at 76 wk post-irradiation. No dose-response curves could be constructed for HP/mg dry tissue; dose-related increases observed in HP/total left lung were entirely attributable to increases in lung dry weight. Histopathology data indicated a dose-related increase in two histological alterations, septal fibrosis and granulomatous pneumonia. The alpha/beta ratios were calculated for each of the endpoints using both the Fe-plot and Direct Analysis method. The calculated values for alpha/beta were in the range of 1.1 to 3.6 Gy, corresponding well to published data obtained in mouse lungs. The number of tissue rescuing units (TRU's) was also calculated. It was smaller for BF than for the histopathology endpoints, suggesting different target structures. However, the difference was not significant due to large confidence intervals.

Animals↗
PubMed1987

Induction of aromatase activity in porcine granulosa cells by FSH and cyclic AMP.

This paper examines the involvement of mRNA and protein synthesis in the induction of aromatase activity by FSH and also the site of action of androgens on FSH induction of aromatase activity in porcine granulosa cells. Treatment of cells with FSH or dbcAMP for an initial 48 h (induction period) resulted in an induction of aromatase activity, as measured by the production of estradiol during a subsequent 6h test period. Addition of testosterone to cells cultured with dbcAMP during the induction period enhanced the aromatase activity during both the induction and test periods. However, while DHT has no effect during the induction period, it significantly inhibited the aromatase activity during the test period. Culturing the granulosa cells with actinomycin D or cycloheximide during the induction period resulted in a dose-dependent inhibition of FSH induction of aromatase activity, suggestive of the synthesis of new protein(s). These results indicate clearly that FSH induction of aromatase activity involves the synthesis of mRNA and proteins.

1-Methyl-3-isobutylxanthine↗
PubMed1987

Sodium-independent synaptosomal uptake of leucine.

The uptake of leucine by rat brain synaptosomes was investigated in a Na+-free medium. It was found that leucine was transported into synaptosomes by 2 uptake processes which were both extremely sensitive to inhibition by 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid. These 2 components were distinguishable from each other by kinetic analysis and by their relative sensitivity to inhibition by selected amino acids.

Amino Acids↗
PubMed1986

Inhibition of follicle-stimulating hormone induction of aromatase activity in porcine granulosa cells by thyroxine and triiodothyronine.

We examined the effects of T3 and T4 on basal and FSH-induced aromatase activity in granulosa cells isolated from medium-sized follicles (4-6 mm) of prepubertal pigs. Treatment of cells with T3 or T4 alone during an initial 48-h induction period did not result in any significant change in aromatase activity, as measured by 17 beta-estradiol accumulation during the subsequent 6-h test period, when testosterone (0.5 microM) was added as substrate. However, when cells were cultured with FSH and T3 or T4 during the induction period, a definite dose-dependent inhibition of FSH-induced aromatase activity was demonstrated. This inhibition was not altered by the presence of a binding protein (BSA). The inhibition of FSH-induced aromatase activity by T4 and T3 is a true biological effect, as inactive iodocompounds (MIT and DIT) and iodide had no significant effect on the gonadotropin-induced aromatase activity. Furthermore, the viability of cells was unaffected by the thyroid hormones, and total cellular protein did not change significantly. These results indicate that thyroid hormones might play an important role in modulating FSH-induced aromatase activity, and that the elevated plasma estrogen levels in some cases of hyperthyroidism are not due to increased ovarian secretion.

Animals↗
PubMed1986

FSH-induced aromatase activity in porcine granulosa cells: non-competitive inhibition by non-aromatizable androgens.

The aim of this study was to examine the inhibitory effect of the non-aromatizable androgens on FSH-stimulated aromatase activity in porcine granulosa cells. The cells were isolated from medium-sized follicles (4-6 mm) of prepubertal pigs, and cultured under chemically defined conditions in the presence of FSH (1 microgram/ml, NIADDK-oFSH-S13) with and without the androgens for an initial 48-h induction period. Subsequently, the spent medium was replaced with fresh medium containing only testosterone as substrate and the cells were reincubated for a further 6 h. The conversion of this steroid to oestradiol-17 beta during this latter 'test' period was taken as a measure of the aromatase activity. The addition of 5 alpha-dihydrotestosterone (DHT) into cultures of FSH-stimulated cells during the induction period resulted in a definite dose-dependent inhibition (30-70%) of the aromatase activity expressed in the test period. This inhibitory action, of the mixed non-competitive type, is characterized by a decrease in the apparent Vmax and an increase in the Km value, suggestive of an androgen inhibition of FSH-stimulated aromatase synthesis. This inhibition was also shown by the other 5 alpha- and 5 beta-reduced androgens: 5 beta-androstanedione was the most effective, while DHT was the least. Other steroids such as pregnenolone and progesterone were inhibitory, but testosterone and diethylstilboestrol were stimulatory. These results suggest an important mechanism for the intrafollicular control of oestrogen synthesis, involving a possible reciprocal relationship between aromatase and 5 alpha-reductase activities.

Animals↗
PubMed1985

The role of climatic conditions and histamine release in exercise-induced bronchoconstriction.

We examined the role of climatic conditions on exercise induced asthma in patients in the tropics. Ten asthmatic patients exercised on a cycle ergometer achieving similar maximal heart rates, oxygen consumption, work load, and minute ventilation on three occasions, breathing warm humid air, cool drier air and cold dry air. Blood for plasma histamine was collected before and after exercise breathing cold air. The exercise-induced bronchoconstriction (EIB) was least in warm air, maximal in cold air and intermediate in cool air. Post exercise venous histamine was raised in only 2 out of 7 patients. We conclude that temperature and humidity have a direct role in EIB and that mediator release is probably the result rather than the cause of EIB.

Adolescent↗
PubMed1983

Is Hibiscus rosa sinensis Linn. a potential source of antifertility agents for males?

The effects of oral administration of aqueous and alcoholic extracts of flowers of H. r. sinensis (250 mg/kg BW/day, 30 days) on the reproductive organs of male rats have been examined. The results indicate that the organ weights were unaffected by the extracts: weights of the testis, epididymis, ventral prostate, and seminal vesicle of the treated animals were not significantly different from those of the controls. The testis and epididymis of the rats also showed normal histological features, irrespective of treatment. No apparent toxicity of the extracts was discernible.

Animals↗
PubMed1982

2-bromopalmitate inhibition of stimulatory effect on LH in isolated luteal cells.

The effect of 2-bromopalmitate, an inhibitor of fatty acids oxidation, on progesterone synthesis in isolated luteal cells was studied. Incubation of the cells with increasing concentrations (0 - 2 mM) of the inhibitor resulted in an initial enhancement of progesterone synthesis, both in the presence and absence of LH (1 microgram/ml). In the LH-treated cells, this stimulation in steroid synthesis becomes markedly impaired at 2 mM 2-bromopalmitate. In the control cells, however, progesterone synthesis was sustained at the elevated level. At high concentrations of 2-bromopalmitate, the stimulatory effect of LH (relative to the controls) on steroidogenesis was progressively diminished, until it was completely abolished at an inhibitor concentration of 2 mM. The oxidation of labelled palmitic acid by luteal cells was also effectively inhibited by 2-bromopalmitate (1 mM). The results indicate that the steroidogenic effect of LH is mediated, in part, by fatty acid oxidation, and were explained in terms of the interaction between carbohydrate and fatty acid oxidation in supporting ovarian steroidogenesis.

Animals↗
PubMed1982

Activation and transport of fatty acids in ovarian mitochondria: effect of Lh.

Enzymes of fatty acid activation and transport were studied in luteinized rat ovaries. Luteal mitochondria were found to contain high levels of palmitoyl-CoA synthetase and carnitine palmitoyl-transferase activities. In addition, studies on the effect of palmitate concentration on palmitoyl-CoA synthetase activity revealed the possible existence of two forms of the enzyme: Km values of 0.34 mM and 21.33 mM, with Vmax of 3.64 and 66.67 nmoles/min/mg mitochondrial protein respectively, were obtained for the two activities. Similar kinetic data for carnitine palmitoyl-transferase activity in intact mitochondria are a Km of 21 microM and a Vmax of 18.2 nmoles/min/mg mitochondrial protein. Only one activity of this enzyme could be detected in luteal mitochondria. It appears that the activities of both enzymes were not affected by prior administration of LH in vivo. The possibility that this negative finding was due to the experimental procedures employed, rather than a reflection of the situation in vivo, could not be discounted, although its more likely that these two enzymes are probably not locus of LH stimulation. The results indicate that fatty acid oxidation is an important metabolic capability of luteal mitochondria, and support the view regarding the lipid nature of the respiratory fuel of ovarian tissue.

Acyltransferases↗
PubMed1981

Melatonin concentrations in human serum, ventricular and lumbar cerebrospinal fluids as an index of the secretory pathway of the pineal gland.

Melatonin was extracted from human sera, ventricular and lumbar cerebrospinal fluids taken within an hour from 25 patients undergoing neurosurgical operations such as ventricular shunts and craniotomy. Using a radioimmunoassay procedure, it was possible to measure the melatonin concentrations in 3 groups of patients: (a) those with obstructive hydrocephalus, (b) those with communicating hydrocephalus, and (c) those with normal cerebrospinal fluid flow. It was demonstrated that melatonin levels were higher in sera compared with those in lumbar and ventricular cerebrospinal fluids thus supporting the view that melatonin is primarily secreted into the bloodstream.

Cerebral Ventricles↗