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Y Song, CS Pang, EA Ayre, GM Brown and SF Pang

Song Y, Pang CS, Ayre EA, Brown GM, Pang SF. Melatonin receptors in the chicken kidney are upregulated by pinealectomy and linked to adenylate cyclase. Eur J Endocrinol 1996;135:128–33. ISSN 0804–4643

The effect of pinealectomy on the characteristics of melatonin receptors in the chicken kidney was studied. One-day-old chicks were operated and kept under a 12 h/12 h light/dark photoperiod. Six weeks after operation, the animals were sacrificed at mid-light and mid-dark. Serum melatonin was determined by radioimmunoassay and kidney melatonin receptors were studied by radioreceptor assay using the melatonin agonist 2-[125I]iodomelatonin as the radioligand. Pinealectomy significantly reduced the mid-dark serum melatonin level and abolished the diurnal rhythm of 2-[125I]iodomelatonin binding in the kidney. The density of 2-[125I]Iodomelatonin binding sites in the kidney at mid-dark was increased significantly to a value comparable to the mid-light density after pineal ablation. Our results suggest that melatonin receptors in the chicken kidney are directly regulated by melatonin in the circulation. The coupling of kidney melatonin receptors to adenylate cyclase was investigated. The basal and forskolin-stimulated cAMP production in chicken kidney explants was studied following melatonin or melatonin plus pertussis toxin treatment. Levels of cAMP in chicken kidney explants were extracted and determined by radioimmunoassay. Melatonin had no effect on basal cAMP levels. However, melatonin significantly inhibited the forskolin-stimulated cAMP accumulation at a concentration of 10 pmol/l. Inhibitory effects of melatonin on the forskolin-stimulated cAMP increase in the chicken kidney were totally blocked by preincubating the kidney tissue with 1.0 μg/ml pertussis toxin. Our results suggest that kidney melatonin receptors may modulate the adenylate cyclase leading to biological responses in the renal system.

SF Pang, Department of Physiology, Faculty of Medicine, University of Hong Kong, Hong Kong

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AMS Poon, ZM Liu, F Tang and SF Pang

Poon AMS, Liu ZM, Tang F, Pang SF. Cortisol decreases 2[125I]iodomelatonin binding sites in the duck thymus. Eur J Endocrinol 1994;130:320–4. ISSN 0804–4643

The immunosuppressive effect of chronic glucocorticoid treatment on 2[125I]iodomelatonin binding in the duck thymus was studied. Two-week-old ducks were injected intraperitoneally with either 1 mg of cortisol per day (experimental group) or an equivalent volume of vehicle (control group) in the middle of the light period for 7 days. 2[125I]Iodomelatonin binding assays were performed on thymic membranes. Cortisol injection reduced the body weight gain, size of the bursa of Fabricius and absolute weights of the primary lymphoid organs but had no effect on the spleen weights. The relative weights of the spleen were increased while those of the primary lymphoid organs were unchanged. The density of the thymus 2[125I]iodomelatonin binding sites was decreased while the affinity was not affected. The modulation of the thymic 2[125I]iodomelatonin binding sites by changes in the immune status of the duck suggests that these binding sites represent physiologically relevant melatonin receptors and that melatonin exerts its action on the lymphoid tissues directly. Our findings support the hypothesis that the thymus is the target site for the immunomodulatory interactions between the pineal melatonin and the adrenal steroids. A possible inhibitory influence of adrenal steroids on the immuno-enhancing effect of melatonin is also suggested.

AMS Poon, University of Hong Kong, 5 Sassoon Rd, Hong Kong