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Abstract
PURPOSE: Studies were conducted to investigate the influence of melatonin on adenylate cyclase activity in cultured human and rat retinal pigment epithelial (RPE) cells. METHODS: Adenylate cyclase activity was assessed by measurement of cAMP levels in cultured RPE cells using a specific cAMP-binding protein isolated from bovine adrenal cortex to detect cellular cAMP by competition with a standard amount of tritiated cAMP. The effects of melatonin on basal cAMP levels and those induced by the direct activator of adenylate cyclase, forskolin, were studied. RESULTS: Exposure of human RPE cells to 100 microM of melatonin had no effect on basal cAMP levels, but it caused a 41% reduction in the forskolin (5 microM) stimulation of cAMP. This melatonin-induced reduction in forskolin-stimulated adenylate cyclase activity was dose dependent, with half-maximal (EC50) reduction at 4.2 x 10(-10) M. 2-iodomelatonin, 6-chloromelatonin, and 6-hydroxymelatonin mimicked the melatonin effect with EC50 values of 3.5 x 10(-10) M, 4.3 x 10(-9)M, and 1.9 x 10(-7)M, respectively. Preexposure of cells to pertussis toxin (100 ng/ml) for 18 hours completely attenuated the ability of melatonin to influence the forskolin stimulation of cAMP levels. Propranolol did not influence the action of melatonin but did antagonize the ability of serotonin to reduce the forskolin-elevated cAMP levels. Thus, melatonin receptors are distinct from serotonin receptors. Melatonin receptors negatively linked to cAMP metabolism are also associated with cultured hooded rat RPE cells. Melatonin and iodomelatonin caused dose-dependent reductions in forskolin-stimulated cAMP production with half-maximal values of 2.4 x 10(-9)M and 3.28 x 10(-9)M, respectively. CONCLUSIONS: The results show that human and rat RPE cells possess melatonin receptors negatively coupled to adenylate cyclase and sensitive to pertussis toxin.