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L K Chong

Publications and source records attributed to L K Chong.

10 recordsLinked to original sources
PubMed1998

Salmeterol inhibition of mediator release from human lung mast cells by beta-adrenoceptor-dependent and independent mechanisms.

1. The long-acting beta2-adrenoceptor agonist, salmeterol (10(-9)-10(-5) M), inhibited the IgE-mediated release of histamine from human lung mast cells (HLMC) in a dose-dependent fashion. Additional beta-adrenoceptor agonists were studied and the rank order of potency for the inhibition of histamine release from HLMC was isoprenaline > salmeterol > salbutamol. Approximate EC50 values for the inhibition of histamine release were 10 nM for isoprenaline and 100 nM for salbutamol. An EC50 value for salmeterol could not be calculated because maximal responses to salmeterol were not observed over the concentration range employed. 2. Both salmeterol and isoprenaline inhibited the generation of sulphopeptidoleukotrienes (sLT) more potently and more efficaciously than the release of histamine from immunologically-activated HLMC. Salmeterol (EC50 < 0.1 nM) was more potent than isoprenaline (EC50 0.4 nM) at attenuating sLT generation. 3. The beta-adrenoceptor antagonist, propranolol (1 microM), and the selective beta2-adrenoceptor antagonist, ICI 118,551 (0.1 microM), both caused rightward shifts in the dose-response curve for the inhibition of histamine release by isoprenaline. The antagonism of salmeterol effects by propranolol and ICI 118,551 was more complex. At lower concentrations (< 1 microM) of salmeterol, both antagonists shifted the dose-reponse curve to salmeterol rightward. At a higher concentration (10 microM) of salmeterol, neither ICI 118,551 nor propranolol was an effective antagonist of the salmeterol-mediated inhibition of histamine release. 4. Prolonged exposure (4 h) of HLMC to isoprenaline (1 microM) caused an approximately 50% reduction in the effectiveness of a second exposure to isoprenaline (10 microM) at inhibiting the release of histamine. whereas this pretreatment did not affect the salmeterol (10 microM) inhibition of histamine release. 5. Isoprenaline (10(-9)-10(-5) M) caused a dose-dependent increase in total cell cyclicAMP levels in purified HLMC which paralleled the inhibition of histamine release. Salmeterol (10(-9)-10(-5) M) was considerably less potent than isoprenaline at increasing HLMC cyclicAMP levels. 6. In summary, these data indicate that salmeterol is an effective inhibitor of the stimulated release of mediators from HLMC. The present data also suggest that salmeterol may act to inhibit mediator release from HLMC by beta-adrenoceptor-dependent and independent mechanisms.

Adrenergic beta-2 Receptor Agonists↗
PubMed1998

Influence of receptor reserve on beta-adrenoceptor-mediated responses in human lung mast cells.

1. The effects of the beta-adrenoceptor agonists isoprenaline and salbutamol on IgE-mediated histamine release from human lung mast cells (HLMC) were evaluated. Both agonists (10(-10)-10(-5) M) inhibited histamine release in a dose-dependent manner and isoprenaline (pD2, 8.3+/-0.1, mean+/-s.e.mean) was more potent than salbutamol (7.3+/-0.1). Moreover, the mean data indicated that salbutamol was a partial agonist when compared with isoprenaline. However, there was a large degree of interexperimental variability because, in 11 of 32 experiments, salbutamol was a full agonist and, in 21 of 32 experiments, a partial agonist relative to isoprenaline. These data suggest that different HLMC preparations possess variable receptor reserves. 2. The effect of the irreversible beta-adrenoceptor antagonist, bromoacetylalprenolol menthane (BAAM), on the inhibition of IgE-mediated histamine release by both isoprenaline and prostaglandin E2 (PGE2) was assessed. Whereas BAAM (100 nM) antagonized the isoprenaline inhibition of histamine release from activated HLMC, BAAM had no effect on the PGE2 inhibition. Pretreatment of HLMC with the beta2-selective competitive antagonist, ICI 118551 (100 nM), protected against the loss in responsiveness to isoprenaline following treatment with BAAM. 3. Concentrations of 1, 10 and 100 nM of BAAM caused dose-dependent rightward shifts in the dose-response curve for the isoprenaline inhibition of histamine release. Furthermore, there was a dose-dependent reduction in the maximal inhibitory response obtained with isoprenaline following treatments with increasing concentrations of BAAM. Although the rightward shifts in the isoprenaline dose-response curves, with a given concentration of BAAM, were similar in all experiments, there was some variability in the depression of the maximal response in individual experiments. Thus, in 6 of 16 experiments, BAAM (1 nM) did not depress the maximal response to isoprenaline, whereas in 10 of 16 experiments there was a depression (7 to 49% reduction) in the maximal response. These data suggest that different HLMC preparations possess variable receptor reserves. 4. Isoprenaline was more potent as an inhibitor in those HLMC preparations in which there was a larger receptor reserve (i.e. preparations in which the maximal inhibitory response to isoprenaline was unaffected by pretreatment with 1 nM BAAM). 5. The influence of receptor reserve on the inhibition by salbutamol of histamine release from HLMC was evaluated. There was a good correlation (r=0.77) between receptor reserve and the maximal response (relative to isoprenaline) obtained with salbutamol. Thus, HLMC preparations with larger receptor reserves were more responsive to salbutamol. 6. Receptor reserve influenced the desensitization of beta-adrenoceptor-mediated responses in HLMC. Cells were incubated (24 h) with isoprenaline (1 microM), washed and then the ability of a second isoprenaline (10(-10)-10(-5) M) exposure to inhibit histamine release was assessed. The pretreatment caused a reduction in the isoprenaline inhibition of histamine release although the extent of desensitization was highly variable, ranging from essentially negligible levels in some preparations to substantial reductions (93% desensitization) in the ability of isoprenaline to inhibit histamine release. There was a reasonable correlation (r=0.59) between receptor reserve and desensitization. Preparations that possessed a larger receptor reserve were more resistant to desensitization. 7. Collectively, these data suggest that a receptor reserve exists for the beta-adrenoceptor-mediated inhibition of histamine release from HLMC but that the size of this reserve varies between HLMC preparations. Moreover, the size of this receptor reserve may influence the sensitivity of HLMC to beta-adrenoceptor agonists and the susceptibility of individual HLMC preparations to desensitization.

Adrenergic beta-Agonists↗
PubMed1997

Protection by dexamethasone of the functional desensitization to beta 2-adrenoceptor-mediated responses in human lung mast cells.

1. The beta-adrenoceptor agonist, isoprenaline, inhibited the IgE-mediated release of histamine from human lung mast cells (HLMC) in a dose-dependent manner. Maximal inhibitory effects were obtained with 0.1 microM isoprenaline. However, the inhibition of histamine release from HLMC by isoprenaline (0.1 microM) was highly variable ranging from 33 to 97% inhibition (mean, 59 +/- 3%, n = 27). 2. Long-term (24 h) incubation of HLMC with isoprenaline led to a subsequent reduction in the ability of a second exposure of isoprenaline to inhibit IgE-mediated histamine release from HLMC. The impairment in the ability of isoprenaline (0.1 microM) to inhibit histamine release following desensitizing conditions (1 microM isoprenaline for 24 h) was highly variable amongst HLMC preparations ranging from essentially negligible levels of desensitization in some preparations to complete abrogation of the inhibitory response in others (mean, 65 +/- 6% desensitization, n = 27). 3. The ability of HLMC to recover from desensitization was investigated. Following desensitizing conditions (1 microM isoprenaline for 24 h), HLMC were washed and incubated for 24 h in buffer and the effectiveness of isoprenaline (0.1 microM) to inhibit IgE-mediated histamine release from HLMC was assessed. The extent of recovery was highly variable with some HLMC preparations failing to recover and others displaying a complete restoration of responsiveness to isoprenaline (mean, 40 +/- 6% recovery, n = 23). 4. The effects of the glucocorticoid, dexamethasone, were also investigated. Long-term (24-72 h) treatments with dexamethasone (0.1 microM) had no effect on IgE-mediated histamine release from HLMC. Additionally, long-term (24-72 h) treatments with dexamethasone (0.1 microM) had no effect on the effectiveness of isoprenaline to inhibit histamine release. However, long-term (24-72 h) treatments with dexamethasone (0.1 microM) protected against the functional desensitization induced by incubation (24 h) of HLMC with isoprenaline (1 microM). The protective effect was time-dependent and pretreatment of HLMC with dexamethasone for either 24, 48 or 72 h prevented desensitization by either 15 +/- 7, 19 +/- 5 or 51 +/- 10%, respectively (n = 5-7). 5. HLMC preparations which were relatively refractory to isoprenaline even after withdrawal (24 h) from desensitizing conditions responded more effectively to isoprenaline (0.1 microM) if dexamethasone (0.1 microM) was also included during the recovery period (19 +/- 9% recovery after 24 h in buffer; 50 +/- 8% recovery after 24 h with dexamethasone, n = 5). 6. These data indicate that the responses of different HLMC preparations to isoprenaline, the susceptibility of HLMC to desensitization and the ability of HLMC to recover from desensitizing conditions varies markedly. Dexamethasone, which itself has no direct effects on IgE-mediated histamine release from HLMC, protected HLMC from the functional desensitization to beta-adrenoceptor agonists. Because beta 2-adrenoceptor agonists and glucocorticoids are important in the therapeutic management of asthma and as the HLMC is probably important in certain types of asthma, these findings may have wider clinical implications.

Adrenergic beta-2 Receptor Agonists↗
PubMed1995

5,10-Dideazatetrahydrofolic acid reduces toxicity and deoxyadenosine triphosphate pool, expansion in cultured L1210 cells treated with inhibitors of thymidylate synthase.

5,10-Dideazatetrahydrofolic acid (DDATHF) reduces de nova purine biosynthesis by inhibiting glycinamide ribonucleotide transformylase. ICI D1964 and CB3717 are folate-based inhibitors of thymidylate synthase (TS). Fluorodeoxyuridine (FdUrd) following metabolism to FdUMP also inhibits TS. In cultured L1210 cells DDATHF reduced the toxicity of ICI D1694, CB3717 and FdUrd in a concentration-dependent manner. This protection correlated with a prevention of the increase in intracellular dATP pools seen in cells exposed to the TS inhibitors alone. The possibility that DDATHF protection might be due to competition for cell entry is not likely since CB3717 and FdURD but not ICI D1694 enter the cell by independent transport processes. Exogenous hypoxanthine (HX) had no effect on the toxicity of TS inhibitors. However, HX increased the protective effect of DDATHF from ICI D1694 toxicity, had no effect on the CB3717-DDATHF interaction, and reduced the protective effect of DDATHF on FdUrd toxicity. HX prevented the fall in dATP levels caused by DDATHF addition to cells treated with TS inhibitors. HX had different effects on dTTP levels in cells treated with DDATHF and quinazoline TS inhibitors compared to FdUrd. Together these results support the hypothesis that imbalance in dTTP and dATP pools is an important determinant of cytotoxicity in antifolate-treated cells. In addition, these findings suggests that intracellular reduced folates interconvert to catalyse reactions in metabolically perturbed cells.

Animals↗
PubMed1995

Functional desensitization of beta agonist responses in human lung mast cells.

The beta adrenergic agonist isoprenaline inhibited the IgE-triggered release of the preformed mediator histamine from human lung mast cells (HLMC) in a dose-dependent fashion. After prolonged (> or = 4 h) preexposure of HLMC to isoprenaline, there was a subsequent diminution in the effectiveness of a second exposure of isoprenaline to inhibit the release of histamine from activated HLMC. This induced hyporesponsiveness to isoprenaline was both concentration and time dependent. Although maximal levels of desensitization were obtained after an initial prolonged (14-h) preincubation with a high (10(-5) M) concentration of isoprenaline, exposure of HLMC for a shorter (4-h) time period with a lower (3 x 10(-7) M) concentration of isoprenaline was also effective at inducing a functional desensitization to isoprenaline. The inhibitory activity of the beta 2 agonist fenoterol was attenuated after a prolonged (14-h) pretreatment step with isoprenaline (10(-5)M), whereas the inhibitory properties of other adenylate cyclase activators, prostaglandin E2 and forskolin, were not affected appreciably. Prolonged (12-h) exposure of HLMC to the beta agonists fenoterol, salbutamol, and terbutaline also induced hyporesponsive states of beta agonists, qualitatively similar to that obtained with isoprenaline. The beta receptor antagonist propranolol, if coincubated with isoprenaline during the prolonged pretreatment step, protected against the subsequent refractoriness of the HLMC to isoprenaline. The glucocorticoid dexamethasone failed to prevent the isoprenaline-induced functional desensitization. In total, these results indicate that prolonged exposure of HLMC to beta agonists induces a state of selective hyporesponsiveness to agonists that act at beta adrenoreceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic beta-Agonists↗
PubMed1979

Genetic microsurgery by laser: establishment of a clonal population of rat kangaroo cells (PTK2) with a directed deficiency in a chromosomal nucleolar organizer.

An ultraviolet laser beam was focused to a submicron spot on one of the nucleolar organizer regions of mitotic chromosomes of rat kangaroo cells in tissue culture. The daughter cells were isolated and cloned into a viable population that maintained the directed nucleolar deficiency. It is concluded that the laser can be used to delete preselected genetic regions and the genetic deletion is maintained as a heritable deficiency in subsequent daughter cells.

Animals↗