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Biomedical subjects

S C Chan

Publications and source records attributed to S C Chan.

At least 127 records · Page 7Linked to original sources
PubMed1985

Immunopharmacology of the atopic diseases.

The atopic conditions, atopic dermatitis, asthma, and allergic rhinitis, may arise as a result of infiltrating bone marrow-derived cells into skin or respiratory mucosae. Release of inflammatory factors from these cells could account for cutaneous vascular instability and pruritus in atopic dermatitis. Erythema and itch have been induced by experimental stress interviews and by blind food challenges. In the latter, increased plasma histamine was detected and correlated with cutaneous reactions. Basophils from patients with atopic dermatitis have increased histamine release after exposure to immunologic or nonimmunologic lectin stimuli. This increased releasability may relate to inadequate cyclic AMP regulation of cell function. We have found that leukocytes of patients with atopic dermatitis have elevated phosphodiesterase activity and consequently reduced intracellular cyclic AMP. Exposure of the cells to a phosphodiesterase inhibitor caused considerable reduction in histamine release. Similarly, exposure of atopic B lymphocytes to a phosphodiesterase inhibitor greatly reduced the high spontaneous IgE synthesis in mononuclear leukocyte cultures. Elevated leukocyte phosphodiesterase activity may also serve as a marker for the atopic diathesis. We have found elevated enzyme activity in umbilical cord blood from newborns with atopic parents, suggesting that this defect may relate to a genetically determined defect. These studies have provided insight into basic abnormalities associated with atopic dermatitis and the atopic diathesis. Defects of regulatory mechanisms in immune and inflammatory cells may help explain the seemingly disparate disorders of physiologic, pharmacologic, and immunologic systems in atopy.

Asthma↗
PubMed1985

Basophil histamine release in atopic dermatitis and its relationship to disordered cyclic nucleotide metabolism.

Maximal histamine release (HR) from leucocytes, in response to Concanavalin A (Con A) was significantly higher in a group of 16 adults with moderate to severe atopic dermatitis (AD) when compared to 13 non-atopic adults. In a further 4 adults with AD, HR was similar to that in the normals, suggesting the existence of 'high releaser' and 'low releaser' subsets within the AD group. Leucocyte cyclic AMP phosphodiesterase (PDE) activity was significantly higher in the 'high releaser' group compared to the 'low releaser' and normal groups. High and low HR responses showed strong correlations with high and low PDE. Pre-treatment of leucocytes from 'high releasers' with the experimental PDE PDE inhibitor RO-20-1724 reduced the HR to normal levels. These findings suggest that increased histamine 'releasability' in AD is related to abnormalities in cyclic nucleotide regulation. No significant HR could be demonstrated in response to a range of concentrations of methacholine in 'high releaser' atopics and normals. Methacholine also did not affect HR in response to maximal Con A stimulation in 'high releaser' atopics. Basophil percentages within the leucocyte preparation and the histamine content per basophil, were not significantly different between the atopics and normals. Con A-stimulated histamine release did not correlate significantly with serum IgE levels.

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1985

Enrichment of dog leukocyte subpopulations using density gradients.

Density gradient methods have been used for the enrichment of leukocyte subpopulations from human blood. We have adapted a three step method utilizing centrifugation on Hypaque-Ficoll (HF), double density HF (ddHF) and Percoll density gradients to separate lymphocytes, monocytes and basophils from dog blood. After HF separation, both Basenji-Greyhound (BG) and mongrel dogs had similar percentages of lymphocytes and monocytes, but BG dogs had significantly greater (p greater than 0.025) numbers of granulocytes. When cells from HF gradients were spun directly on Percoll, the presence of granulocytes decreased the purification of leukocyte subpopulations. An intervening separation on a ddHF gradient removed granulocytes without a selective loss of lymphocytes or monocytes. When cells from ddHF gradients were further separated on Percoll, 2 distinct cell layers resulted. Layer 1 was monocyte rich with 72.4 +/- 6.5% monocytes, 26.6 +/- 6.2% lymphocytes, and 0.8 +/- 1.8% granulocytes. Layer 2 was lymphocyte rich with 74.3 +/- 11.1% lymphocytes, 21.4 +/- 7.8% monocytes and 3.9 +/- 4.2% granulocytes. Viability as determined by Trypan blue exclusion was above 90%. Using cAMP-Phosphodiesterase (PDE) as a marker, higher PDE activity was present in the monocyte rich layer. This was similar to that reported in humans. Basophil numbers were enhanced by the use of a 1-step separation on a ddHF gradient. The percentage of basophils was increased 10-fold over baseline levels, with a viability of 90%. These techniques can be used to purify various canine leukocyte subpopulations and provide cells for biochemical analysis.

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1985

Altered phosphorylation as a possible basis for cyclic nucleotide abnormalities in atopic dermatitis.

Exposure of normal mononuclear leukocytes (MNL) to histamine causes heterologous desensitization accompanied by beta adrenergic receptor alterations and increased cyclic AMP-specific phosphodiesterase (PDE) activity. We have demonstrated similar abnormalities in MNL from patients with atopic dermatitis (AD). Considering that these dual changes might be due to altered phosphorylation, we have studied cyclic AMP-dependent protein kinase (PK-A) in untreated and histamine-treated normal MNL and in AD cells. We found that basal or endogenous phosphorylative activity was two-fold higher in preparations from patients with AD. The activity ratios of protein kinase increased significantly in normal MNL after histamine exposure. In contrast, cells from patients with AD failed to show this increase. These findings correlate with similarly increased PDE activity after histamine stimulation of normal, but not AD cells. The increased PK-A activity in both atopic and histamine-desensitized MNL correlates with membrane receptor changes and elevated PDE activity. This enchaned phosphorylation may account for the varied physiological and immunological abnormalities that have been described in AD.

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1984

Increased leukocyte sensitivity to phosphodiesterase inhibitors in atopic dermatitis: tachyphylaxis after theophylline therapy.

Previous studies have demonstrated elevated cyclic 3',5'-adenosine monophosphate specific phosphodiesterase (PDE) activity in mononuclear leukocytes (MNL) from patients with atopic dermatitis (AD) and with allergic respiratory disease. We have studied the effect of PDE inhibitors on MNL from normal subjects and from patients with AD. We measured PDE activity in MNL after in vitro exposure to Ro20-1724, theophylline, and isobutylmethylxanthine. The effect of PDE inhibitors was also measured in MNL of patients during and after oral theophylline therapy. There was a dose-response effect of Ro20-1724 on PDE activity of AD cells. This activity was significantly more inhibited than that in MNL of normal subjects or in MNL of theophylline-treated patients with AD. The inhibition pattern of PDE in MNL of treated patients with AD did not vary significantly from the pattern observed in MNL from normal patients. Two weeks after oral theophylline was stopped, the PDE activity in MNL was once again susceptible to in vitro inhibition. The relative resistance of MNL to PDE activity in normal subjects to in vitro PDE inhibitors may be due to the existence of different PDE enzyme forms or the presence of endogenous inhibitors. The clinical importance of the tachyphylaxis observed is uncertain, but our results suggest that intermittent dosing with PDE inhibitors may provide a therapeutic advantage in treatment of AD and other atopic diseases.

Administration, Oral↗
PubMed1984

Elevated umbilical cord blood leukocyte cyclic adenosine monophosphate-phosphodiesterase activity in children with atopic parents.

We found cyclic adenosine monophosphate-phosphodiesterase (cAMP-PDE) activity to be significantly elevated in the umbilical cord blood mononuclear leukocytes of forty-two children with one atopic parent (3.2 +/- 0.3) and in eighteen children with two atopic parents (4.2 +/- 0.7) as compared to twenty-two children with two nonatopic parents (1.9 +/- 0.2) (p less than 0.005). Among those newborns with one atopic parent, mean PDE activities were significantly elevated in cord blood cells regardless of whether only the father was atopic, thus eliminating the possibility of transplacental enzyme passage. Follow-up evaluations of twenty-four children, age 6 months or older, suggest that infants with elevated PDE activity at birth have an increased likelihood of developing dermatitis and/or "atopic features." Further long-term studies will determine the predictive value of newborn mononuclear leukocyte PDE activity.

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1983

Increased leukocyte histamine release with elevated cyclic AMP-phosphodiesterase activity in atopic dermatitis.

Maximal leukocyte histamine release in response to concanavalin A was significantly higher in a group of 16 adult patients with moderate to severe atopic dermatitis when compared to 13 adult nonatopic, normal subjects. In a further four atopic dermatitis patients, histamine release was similar to that in the normal group suggesting the existence of "high-releaser" and "low-releaser" subsets within the atopic dermatitis group. Leukocyte cAMP phosphodiesterase activity was significantly higher in the high-releaser group than in the low-releaser and normal groups. High and low histamine release responses showed strong correlations with high and low phosphodiesterase activities. Pretreatment with the experimental cAMP phosphodiesterase inhibitor Ro-20-1724 in high releasers reduced the histamine release to normal levels. These findings suggest that increased histamine "releasability" in atopic dermatitis is related to abnormalities in cyclic nucleotide regulation. Basophil percentages within the leukocyte preparation and the histamine content per basophil were not significantly different between the atopics and normals. Histamine release did not correlate significantly with serum IgE levels.

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1982

Elevated leukocyte cyclic AMP-phosphodiesterase in atopic disease: a possible mechanism for cyclic AMP-agonist hyporesponsiveness.

Previous studies have documented that leukocytes from atopic individuals show reduced cyclic AMP (cAMP) responsiveness to isoproterenol, histamine, and prostaglandin E1. We questioned whether this blunted response was due to rapid enzymic breakdown of cAMP. We measured cAMP-phosphodiesterase activity in mononuclear leukocytes of patients with atopic dermatitis and allergic rhinitis and noted consistent, significantly elevated levels. Kinetic studies showed differences in two of three leukocyte enzyme forms between normals and patients. Isolated lymphocytes from atopic dermatitis patients lacked a low Km phosphodiesterase form and showed an increase in the high Km, high-activity form seen in normal monocytes. The increased phosphodiesterase activity provides an explanation for the blunted cAMP responsiveness in atopic leukocytes and may reflect a basic biochemical characteristic relevant to abnormal immunocellular regulation in atopic disease.

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1982

Functional desensitization due to stimulation of cyclic AMP-phosphodiesterase in human mononuclear leukocytes.

We have previously demonstrated heterologous desensitization of human mononuclear leukocytes (MNL) by incubation of low (e.g., 10(-6)M) concentrations of histamine, isoproterenol and prostaglandin E1. Subsequent exposure of the cells to stimulating (e.g., 10(-3) to 10(-5)M) concentrations of any one of the three agonists shows reduced cAMP responses. Possible mechanisms for the subnormal responsiveness include rapid degradation of cAMP. In this report we demonstrated time-dependent elevation of cAMP-phosphodiesterase (PDE) activity following agonist desensitization. The increased enzyme activity was accompanied by a mirror-image decrease in cAMP responsiveness. The effect was rapid and prolonged, with recovery time proportional to desensitization time. Cycloheximide failed to inhibit the increase of cAMP-PDE activity caused by short-term histamine exposure, but partially diminished the elevation as the result of chronic histamine desensitization. We observed three different kinetic forms of cAMP-PDE in MNL, designated as I, II and III, each with distinctive Km and Vmax. Histamine desensitization increased the activity of form II and drastically reduced that of form I. Also we observed a possible conversion of lymphocyte cAMP-PDE from form I to the form II more characteristic of monocytes. Agonist-induced increases in cAMP-PDE activity and changes in enzyme kinetic characteristics represent a potentially important mechanism of functional desensitization. This may have significant effects on biological regulation of cyclic nucleotides.

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1981

Heterologous desensitization of leukocytes: a possible mechanism of beta adrenergic blockade in atopic dermatitis.

Studies of mononuclear leukocytes from patients with atopic dermatitis showed depressed cyclic AMP (cAMP) responses after exposure to isoproterenol, histamine, and prostaglandin E1. Because plasma and tissue histamine levels are elevated in atopic dermatitis, we questioned whether histamine or other mediators might be responsible for cAMP abnormalities. We found that exposure of normal cells to low (10(-6)M) concentrations of histamine, isoproterenol, or prostaglandin E desensitized the cells to subsequent stimulatory concentrations of any of the agonists. This heterologous desensitization occurred within 15 min and persisted for days, with gradual recovery of cAMP responses roughly paralleling those of cells from patients with atopic dermatitis. These findings provide a possible explanation for Szentivanyi's beta adrenergic blockade theory and the depressed leukocyte cAMP response to multiple agonists in atopy.

Adrenergic beta-Antagonists↗
PubMed1980

Monitoring serum valproic acid by gas chromatography with electron-capture detection.

I have developed a procedure to monitor, by gas chromatography with electron-capture detection, the concentration of valproic acid in serum. The valproic acid is derivatized into its phenacyl ester. The assay is very sensitive, measuring as little as 100 pg of valproic acid easily and accurately. Solvent extraction and concentration of the drug are not necessary; instead, one only has to precipitate the serum proteins. n-Octanoic acid is used as the internal standard. The peak-height ratio of phenacyl valproate to phenacyl octanoate is linearly related to concentration up to 200 mg of valproic acid per liter.

Acetonitriles↗
PubMed1980

Reconstructive middle-ear surgery for chronic otitis media.

Chronic otitis media can be divided into the non-cholesteatomatous ears and the cholesteatomatous ears. The non-cholesteatomatous ears are most suitable for reconstructive middle ear surgery and the majority of the patients can expect a dry trouble-free ear with good hearing after the operation. Surgery for the cholesteatomatous ear is not only difficult but must be exact as residual disease can lead to fatal intra-cranial complications. Unlike the non-cholesteatomatous ears, not all cholesteatomatous ears are suitable for tympanic reconstruction. In both types of ears, eradication of disease takes priority and precedes tympanic reconstruction. Basically, tympanic reconstruction consists of repair of the tympanic membrane defect or myringoplasty and repair of the ossicular defect or ossiculoplasty. A scheme or surgical management for chronic otitis media is outlined. Between May 1977 and October 1979, 165 cases were seen by the author in the ENT Department, Singapore General Hospital and operated in accordance with this scheme of management and the results are analysed.

Cholesteatoma↗
PubMed1979

Cyclic AMP derivatives stimulate the chondrogenic differentiation of the mesoderm subjacent to the apical ectodermal ridge of the chick limb bud.

Recent studies indicate that one of the major functions of the apical ectodermal ridge (AER) of the embryonic chick limb bud is to maintain mesenchymal cells directly subjacent to it (i.e., cells extending 0.4-0.5 mm from the AER) in a labile, undifferentiated condition. Furthermore, when mesenchymal cells are freed from the AER's influence, either artifically or as a result of normal polarized proximal-to-distal limb outgrowth, they are freed to commence cytodifferentiation. In a preliminary attempt to investigate at a molecular level the mechanism by which the AER exerts its "negative" effect on the cytodifferentiation of subridge mesenchymal cells, we have examined the effect of a variety of agents that elevate cyclic AMP levels on the chondrogenic differentiation of the unspecialized subridge mesoderm of the limb bud in an organ culture system. Dibutyryl- and 8-hydroxy-cyclic AMP elicit a dose-dependent increase in the rate and amount of cartilage matrix formation and a corresponding dose-dependent increase in sulfated glycosaminoglycan accumulation by subridge mesoderm explants. The stimulatory effect of suboptimal concentrations of cyclic AMP derivatives is potentiated by the addition of theophylline. The stimulatory effect is limited to cyclic AMP derivatives, since dibutyryl-cyclic GMP and 5'-AMP have no effect. Thus agents that elevate intracellular cyclic AMP levels stimulate the chondrogenic differentiation of the unspecialized subridge mesoderm of the embryonic chick limb bud.

Animals↗
PubMed1979

In vitro studies on the morphogenesis and differentiation of the mesoderm subjacent to the apical ectodermal ridge of the embryonic chick limb-bud.

It has been suggested that one of the major functions of the apical ectodermal ridge (AER) of the embryonic chick limb-bud is to maintain mesenchymal cells directly subjacent to it (i.e. cells extending 0.4-- 0.5 mm from the AER) in a labile, undifferentiated condition. We have attempted to directly test this hypothesis by subjecting the undiffertiated subridge mesoderm of stage-25 embryonic chick wing-buds to organ culture in the presence and absence of the AER and the ectoderm that normally surrounds the mesoderm dorsally and ventrally. During the period of culture, control explants comprised of the subridge mesoderm capped by the AER and surrounded by the dorsal/ventral ectoderm undergo progressive morphogenesis characterized by polarized proximal to distal outgrowth and changes in the contour of the developing explant, and ultimately form a structure grossly resembling a normal distal wing-bud tip. In contrast, explants from which the AER and dorsal/ventral ectoderm have been removed (minus ectoderm explants) or from which just the AER has been removed (minus AER explants) form compact, rounded masses exhibiting no signs of morphogenesis. During the polarized proximal to distal outgrowth control explants undergo during the first 3 days of culture, as cells of the explant become located greater than 0.4--0.5 mm from the AER, they concomitantly undergo a sequence of changes indicative of their differentiation into cartilage. However, those cells which remain 0.4--0.5 mm from the AER during this period retain the characteristics of non-specialized mesenchymal cells. In marked contrast to control explants, virtually all of the cells of minus ectoderm explants initiate chondrogenic differentiation during the first day of culture. Cells comprising the central core of minus AER explants also initiate chondrogenic differentiation during the first day of culture, but in contrast to minus ecotderm explants, non-chondrogenic tissue types form along the periphery of the explants subjacent to the dorsal/ventral ectoderm. These results indicate that the AER maintains cells directly subjacent to it in a labile, undifferentiated condition, and that when mesenchymal cells are freed from the AER's influence either artificially or as a result of normal polarized outgrowth, they are freed to commence cytodifferentiation. The results further suggest that the dorsal/ventral ectoderm may have an influence on the differentiation of the mesenchymal cells directly subjacent to it, once the cells have been removed from the influence of the AER.

Animals↗
PubMed1978

Interactions between chondroitin sulfate and concanavalin A.

Chondroitin sulfate, the major extracellular matrix glycosaminoglycan, formed an insoluble complex with concanavalin A at pH 5.4 or below. Concanavalin A (500 microgram/ml) reacted only within a relatively narrow concentration range of chondroitin sulfate (optimally between 5 and 50 microgram/ml) at pH 5.4 in 0.05 M buffer. Similar precipitin-like interactions were seen between concanavalin A and hyaluronic acid or heparin. No precipitating complexes formed between concanavalin A and the glycosaminoglycans at these concentrations in physiological salt solutions (approx. 0.15 M) unless the pH was below 4.5. Precipitating self-aggregates of concanavalin A appeared to be promoted by chondroitin sulfate at pH 7.3, but no significant precipitation occurred between the reactants at this pH even at very high concentrations, nor did soluble complexes form as determined by affinity chromatography on Sephadex G-200 or fractionation on Bio-Gel P-200. Thus, binding between the lectin and glycosaminoglycans appeared to depend upon reversible non-specific electrostatic interactions observed only at low Ph and low ionic strength. Stable interactions were not seen in experiments using physiologically balanced salts at near neutral pH.

Chondroitin↗