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Biomedical subjects

C H Tan

Publications and source records attributed to C H Tan.

At least 19 recordsLinked to original sources
PubMed1992

Adenosine modulates cell growth in human epidermoid carcinoma (A431) cells.

Adenosine mediates many physiological functions via activation of extracellular receptors. The modulation of cell growth by adenosine was found to be receptor-mediated. In A431 cells adenosine evoked a biphasic response in which a low concentration (approximately 10 microM) produced inhibition of colony formation but at higher concentrations (up to 100 microM) this inhibition was progressively reversed. Evidence for the involvement of A1 (inhibitory) and A2 (stimulatory) adenosine receptors in regulating cell growth of these tumor cells was obtained through plating efficiency studies based on the relative potency of adenosine agonists and antagonists. When both A1 and A2 receptors were blocked, colony formation or growth was not inhibited at low concentrations of adenosine but was inhibited at high adenosine concentrations.

Adenosine↗
PubMed1992

Cytotoxic effect of plant polyphenols and fat-soluble vitamins on malignant human cultured cells.

In vitro studies showed that several flavonoids, tannic acid, gallic acid and fat-soluble vitamins inhibited HeLa and Raji lymphoma cell growth. The inhibition trend exhibited by these compounds was similar for both cell lines, and their growth was inhibited dose dependently. Butein, (10 microM), the most potent anti-proliferative agent, exerted 30% growth inhibition and was more effective on HeLa cells. Retinol (100 microM) inhibited cell proliferation completely. Tannic acid was twice as potent as its monomer gallic acid. From structure-activity consideration, the C2,3-double bond of the flavonoid molecule was important for activity. Flavonoid aglycones were more effective than their corresponding glycosides in suppressing cell growth.

Cell Division↗
PubMed1992

Biological activities of Synanceja horrida (stonefish) venom.

Some biological and neurochemical properties of the venom of stonefish (Syanceja horrida) were investigated. The venom exhibited oedema-inducing, haemolytic, hyaluronidase, thrombin-like, alkaline phosphomonoesterase, 5' nucleotidase, acetylcholinesterase, phosphodiesterase, arginine esterase, and arginine amidase activities. Recalcification clotting time, prothrombin, and kaolin-cephalin clotting times were increased 1.7-2.3- and 2.4-fold respectively. The LD50 (i.v. mouse) was 300 micrograms/Kg. Its effects on uptake and stimulation of neurotransmitter synthesis and release were observed in rat brain synaptosomes. In the presence of 100 micrograms venom, uptake of [methyl-3H] choline in rat brain synaptosomes was inhibited 70%, while that of 4-amino-n-[U-14C] butyric acid was inhibited 20%. The toxin also stimulated the release of [3H]-acetylcholine from the synaptosomes.

Animals↗
PubMed1992

Steroid metabolism by ovarian follicles and extrafollicular tissue of the guppy (Poecilia reticulata) during oocyte growth and gestation.

In the viviparous guppy, fertilization and gestation are intrafollicular. Fully developed embryos are ovulated at the end of gestation just prior to parturition. The metabolism in vitro of various radiolabeled steroid precursors by isolated ovarian follicles at various stages of the reproductive cycle and extrafollicular (EF) tissue of the guppy was investigated. While estradiol-17 beta was one of the end products of metabolism in vitellogenic follicles, 17 alpha, 20 beta-P and several 5-reduced metabolites were synthesized by postvitellogenic follicles. The yield of 17 alpha, 20 beta-P, however, was much lower than some 5 beta-reduced metabolites synthesized by postvitellogenic follicles. Gestation stage follicles rapidly converted the precursors into 5-reduced and polar 7-hydroxylated steroids, and their glucuronides. Although postpartum follicles showed very poor potential for steroid metabolism, they synthesized estradiol-17 beta from testosterone. These results demonstrate distinct changes occurring in the steroidogenic potential of the follicles during the reproductive cycle. Unlike in other viviparous vertebrates, no particular steroid seems to be involved in maintaining gestation in the guppy; all the steroid precursors are converted into highly polar metabolites and their conjugates during gestation, thereby facilitating their excretion. The EF ovarian tissue also synthesized 7-hydroxylated steroids and their glucuronides, providing evidence for the first time that the teleost ovarian EF tissue plays a role in steroidogenesis. The possible physiological significance of the synthesis of the novel polar steroids by the follicles and the EF tissue is discussed.

17-alpha-Hydroxyprogesterone↗
PubMed1992

Steroid production by ovarian follicles of the viviparous guppy (Poecilia reticulata) and its regulation by precursor substrates, dibutyryl cAMP and forskolin.

Production in vitro of estradiol-17 beta, testosterone, 17 alpha-20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-P), 17 alpha-hydroxyprogesterone, and progesterone by follicles of the guppy at various stages of oocyte growth and gestation was investigated. Basal production of estradiol-17 beta was highest in 0.8- and 1.2-mm follicles, whereas that of testosterone and 17 alpha,20 beta-P was highest in 1.6-mm (postvitellogenic) follicles. Levels of these steroids declined after fertilization and were undetectable in late gestation and postpartum follicles. 17 alpha-Hydroxyprogesterone and progesterone levels were low at all stages. Thus, none of these steroids appears to be involved in maintaining gestation. Regulation of estradiol-17 beta and 17 alpha,20 beta-P secretion by vitellogenic (1.0 mm) and postvitellogenic follicles by precursor substrates, dbcAMP (0.1 to 10 mM) and forskolin (1 to 100 microM), was also investigated. Vitellogenic follicles synthesized increased quantities of estradiol-17 beta in the presence of exogenous testosterone, whereas estradiol-17 beta production by postvitellogenic follicles was not altered by testosterone. These results suggest decreased aromatase activity in the postvitellogenic follicles. Dibutyryl cAMP and/or forskolin stimulated testosterone and estradiol-17 beta production by vitellogenic follicles but did not stimulate conversion of testosterone to estradiol-17 beta, suggesting that the adenylate cyclase system stimulates estradiol-17 beta production by stimulating testosterone production but does not mediate conversion of testosterone to estradiol-17 beta. Postvitellogenic follicles synthesized increased quantities of 17 alpha,20 beta-P in response to 17 alpha-hydroxyprogesterone in a dose-dependent manner. Although 1 microM of forskolin stimulated 17 alpha,20 beta-P production by postvitellogenic follicles in the absence of exogenous 17 alpha-hydroxyprogesterone, 100 microM of forskolin inhibited 17 alpha,20 beta-P production. Dibutyryl cAMP, however, did not affect 17 alpha,20 beta-P production. In the presence of 50 ng of 17 alpha-hydroxyprogesterone, dbcAMP (10 mM) and forskolin (1 to 100 microM) suppressed 17 alpha,20 beta-P production. It is suggested that cAMP mediates 17 alpha,20 beta-P production up to a certain threshold level, beyond which it inhibits 17 alpha,20 beta-P production.

17-alpha-Hydroxyprogesterone↗
PubMed1992

Effects of neurohypophyseal and adenohypophyseal hormones, steroids, eicosanoids, and extrafollicular tissue on ovulation in vitro of guppy (Poecilia reticulata) embryos.

In the viviparous guppy, oocyte maturation is followed by intrafollicular fertilization and gestation. The fully developed embryos are ovulated at term just prior to parturition. Various agents were tested in vitro for their effects on ovulation of embryos in isolated follicles of the guppy. Arachidonic acid (10 and 100 microM), PGE2, PGF2 alpha, and 6-keto-PGF1 alpha (0.1 microgram/ml) induced ovulation, while PGE1, 15-keto-PGF2 alpha, leukotriene B4, 5-, 12-, and 15-HETEs (0.01 to 0.1 microgram/ml), cortisol, 11-deoxycortisol (25 and 250 ng/ml), estradiol-17 beta, testosterone, 17 alpha,20 beta-P, progesterone (5 and 50 ng/ml), isotocin, vasotocin (0.02 to 2 microgram/ml), and guppy pituitary extract (one and two glands per fish) did not. Extrafollicular (EF) ovarian tissue cocultured with isolated follicles induced ovulation, and the medium levels of PGE and PGF in such incubations were higher than those in the control. Indomethacin, the cyclooxygenase inhibitor, did not inhibit ovulation induced by arachidonic acid and EF tissue, although it inhibited PGE and PGF production. NDGA, the lipoxygenase inhibitor, did not inhibit ovulation induced by arachidonic acid or EF tissue. A combination of eicosanoids synthesized by follicles and EF tissue may be involved in the induction of ovulation. Dibutyryl cAMP inhibited ovulation induced by PGE2, PGF2 alpha, 6-keto-PGF1 alpha, and EF tissue suggesting that a low level of cAMP may be associated with ovulation in the guppy.

Animals↗
PubMed1992

Prostaglandins and teleost neurohypophyseal hormones induce premature parturition in the guppy, Poecilia reticulata.

Administration of PGE2, PGF2 alpha, isotocin, and vasotocin to pregnant guppies during late gestation induced premature parturition. Indomethacin (10 micrograms/g body wt), the cyclooxygenase inhibitor, did not inhibit parturition at term, suggesting that some lipoxygenase metabolites may be involved in the induction of parturition. Pituitary extract, as well as LH-RH, did not induce premature parturition. It may be that in the guppy, in which ovulation occurs at the end of gestation just prior to parturition, gonadotropin is not involved in the induction of ovulation and parturition.

Animals↗
PubMed1992

Steroid metabolism in teleost gonads: purification and identification of metabolites by high-performance liquid chromatography.

A simple, efficient, and comprehensive technique for the purification, identification, and quantitation of the common steroid metabolites synthesized by the gonads of teleosts involving five systems of high-performance liquid chromatography (HPLC) was developed. Steroid standards were identified in HPLC by UV absorption at 254 nm or 280 nm, by differential refractive index, or by using radioactive standards. Metabolites that do not absorb UV light and are not resolved in the isocratic HPLC systems were identified in thin-layer chromatography following purification by HPLC. By using this technique, most of the steroid metabolites, including some polar metabolites, synthesized by the gonadal tissues of the teleosts can be purified within three steps of chromatography. The HPLC systems reported here are also useful in identifying the chromium trioxide oxidized products of metabolites, such as triols and tetrols, which considerably narrows down the number of probable metabolites.

Animals↗
PubMed1992

Electrochemically modulated optrode for glucose.

One of the problems with fibre-optic sensors is the difficulty of finding reversible indicator reagents. This is a particular problem for fibre-optic glucose sensors. The development of an electrochemically modulated fibre-optic probe or optrode has been proposed as a convenient solution. Here the indicator reagent is regenerated electrochemically. In this work a design is proposed that offers considerable advantages in practical applications. In particular, it can be used in the same way as conventional optrodes. The optimization of working parameters and the application of the optrode to flow analysis under steady-state and flow-injection conditions is described.

Biosensing Techniques↗
PubMed1992

Legionella species isolated from cooling towers in Kuala Lumpur.

Three building complexes in Kuala Lumpur were surveyed for the presence of legionellae in cooling towers. The organisms were grown from 12 out of 46 samples of water collected from 30 towers. L. pneumophila serogroups 1 and 7 were the commonest serogroups isolated. None belonged to the Pontiac subgroup of L. pneumophila serogroup 1.

Legionella↗
PubMed1991

Progestins and cortisol delay while estradiol-17 beta induces early parturition in the guppy, Poecilia reticulata.

Fertilization and gestation are intrafollicular in the guppy (Poecilia reticulata), and ovulation occurs at the end of gestation prior to parturition. In this study, the effects in vivo of the ovarian steroids, progesterone, 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-P), cortisol and estradiol-17 beta, the antiprogestin RU 486, and aromatase inhibitor, 4-hydroxyandrost-4-ene-3,17-dione (4-HAD), on gestation and parturition were studied in the guppy. Progesterone (0.05 and 0.10 micrograms/ml of water), 17 alpha,20 beta-P (0.01 micrograms/ml and greater), cortisol (0.10 micrograms/ml) and 4-HAD (0.10 micrograms/ml) all prolonged gestation presumably by inhibiting ovulation. 17 alpha,20 beta-P was most effective in inhibiting ovulation and parturition for up to 36 days postpartum. This inhibition was reversed when fish were transferred to steroid-free water. Besides extending gestation, 17 alpha,20 beta-P and 4-HAD also inhibited development of vitellogenic oocytes. Estradiol-17 beta (0.05 and 0.10 micrograms/ml) and RU 486 (10 micrograms/g body weight) both induced premature parturition presumably by accelerating onset of ovulation. These results, together with our previous observations on the steroid profile in the guppy, strongly suggest roles for estradiol-17 beta and cortisol in regulating ovulation and parturition.

Animals↗
PubMed1991

Tardive dyskinesia in elderly psychiatric patients in Singapore.

The prevalence of tardive dyskinesia (TD) in 514 psychiatric patients who were 60 years and above in Singapore was found to be 27.6%. This is low compared with other studies done on elderly psychiatric patients in the West. Among the different ethnic groups, the Eurasians have the highest prevalence of 53.8% compared to the Chinese, Malays or Indians. Inpatients have a higher prevalence (31.5%) than outpatients (10.5%). Higher prevalence was also found in women, who were 2.2 times more common than men. 50% of the mentally retarded patients were found to have TD.

Affective Disorders, Psychotic↗
PubMed1990

Inhibition of sodium-dependent L-leucine uptake in rat brain synaptosomes.

Synaptosomes isolated from adult rat cerebral cortices were used for studying the uptake of L-leucine by the Na(+)-dependent route. Three non-metabolizable amino acid analogues, which had been used previously to discriminate the Na(+)-dependent A-type uptake system of animal cells, were employed in this study. It was found that Na(+)-dependent uptake of leucine was insensitive to inhibition by 2-aminoisobutyric acid (AIB) and N-methylaminoisobutyric acid (MeAIB) whereas N-methylalanine (NMA) was markedly inhibitory. Inhibition by NMA was stereospecific--only the L-isomer had a pronounced effect. Na(+)-dependent uptake of leucine as well as its inhibition by L-NMA were rather insensitive to changes in pH from 6 to 9. Kinetic analysis of inhibition by L-NMA of Na(+)-dependent uptake revealed a non-competitive type of inhibition with a Ki value of approximately 0.5 mM.

Alanine↗
PubMed1990

Steroid hormone profile during gestation and parturition of the guppy (Poecilia reticulata).

The guppy is an ovoviviparous teleost with fertilization and gestation occurring intrafollicularly, and ovulation and hatching at around parturition. The levels of estradiol-17 beta (E), testosterone (T), cortisol (F), 17 alpha-hydroxyprogesterone (17-OHP), 17 alpha-20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P), and progesterone (P) in the guppy at different stages of gestation were measured in 60 microliters of serum with a highly specific, sensitive, precise, and accurate procedure involving high-performance liquid chromatography and radioimmunoassay. Levels of E and T increased during vitellogenic growth of oocytes and declined sharply after fertilization, increasing again gradually during late gestation to reach prefertilization levels at periparturition. F levels decreased significantly during fertilization followed by an increase during gestation and a sharp decline again at periparturition. The level of 17-OHP was significantly lower at periparturition than at other stages, while that of 17,20 beta-P was significantly higher in periparturition than in late gestation. There was no significant change in the levels of P at all stages of gestation, suggesting that, unlike mammals, P may not be involved in maintaining gestation in this fish. Changes in the levels of E and T seem to be related to oocyte growth rather than gestation. The role of F in gestation is uncertain since the effect of stress could not be discounted or defined in this study. Thus, on the present results, the role of any particular steroid in maintaining gestation of the guppy is not obvious. The elevated levels of E at term may facilitate parturition in addition to its role in vitellogenesis during oocyte growth.

Animals↗
PubMed1990

Effects of lithium on platelet ionic intracellular calcium concentration in patients with bipolar (manic-depressive) disorder and healthy controls.

Lithium is an effective drug in the treatment of both manic and depressive episodes of bipolar disorder. Lithium has been shown to block the metabolism of the intracellular second messenger inositol-1,4,5-trisphosphate which is involved in the rise in ionic intracellular calcium [( Ca++]i) which triggers neurotransmitter release and other cellular changes in secretory cells. We have measured the effect of lithium on [Ca++]i dynamics in platelets from bipolar patients stabilized with lithium treatment, and from healthy controls. Both resting [Ca++]i and the thrombin stimulated increase in [Ca++]i were higher in bipolar patients than in controls. Lithium added in vitro tended to increase the thrombin-stimulated rise in [Ca++]i. The use of the fluorescent Ca++ probe fura-2 in human platelets provides a useful method to investigate the mechanism of lithium's action in bipolar disorder and to study Ca++ related systems which may be abnormal in bipolar disorders.

Adult↗
PubMed1990

Early and late effects of fractionated irradiation and the kinetics of repair in rat lung.

The thorax of WAG/Rij rats was irradiated with fractionated doses of X rays. Irradiation schedules were designed either to allow virtually complete repair of sublethal damage between subsequent fractions by fractionating at 6-h intervals, or to result in incomplete repair by allowing only 1-h intervals between subsequent fractions. Combination of the data from both experimental series permitted the calculation of alpha/beta ratios and values for the repair halftime T1/2. The animals were monitored by assessment of the breathing frequency and by recording deaths. At the end of the experiments, 18 months after treatment, the hydroxyproline content of the lung tissue was determined as a biochemical indicator of radiation-induced fibrosis, and an histopathological analysis was performed. Early endpoints, indicative of radiation-induced pneumonitis, resulted in an alpha/beta ratio of 3.5 Gy and a T1/2 value of 0.95 h. Late endpoints were presumed to be indicative of radiation-induced fibrosis. Based on the combined analysis of data from three different late endpoints, the mean alpha/beta ratio was 2.3 Gy, and the T1/2 value was 1.13 h. The difference in alpha/beta ratio and T1/2 value between early and late endpoints was not significant, since the 95% confidence limits were overlapping. For each individual early or late endpoint as well as for the two early or the three late endpoints combined, there was a trend for lower alpha/beta ratios and higher T1/2 values associated with low doses per fraction. However, widely overlapping confidence limits indicated that again the differences were not significant.

Animals↗
PubMed1990

Carbon paste-tetrathiafulvalene amperometric enzyme electrode forthe determination of glucose in flowing systems.

The development of a carbon paste-tetrathiafulvalene amperometric enzyme electrode for the determination of glucose in flowing streams is described. The enzyme electrode is operated in a flow-through detector based on the wall-jet configuration under flow injection (FI) and steady-state (SS) conditions. Under FI conditions, high precision (0.6%) and sample throughput (120 samples h-1) are possible. Moreover no pre-conditioning of the electrode is required. The flow system is suitable for the determination of glucose in whole blood without sample dilution. With proper orientation of the jet with respect to the enzyme electrode, high accuracy can be obtained under SS conditions.

Blood Glucose↗