EFFECTS OF MELATONIN ON MICROTUBULE ASSEMBLY DEPEND ON HORMONE CONCENTRATION - ROLE OF MELATONIN AS A CALMODULIN ANTAGONIST

Melatonin may play a key role in cytoskeletal rearrangements through i ts calmodulin antagonism. In the present work, we tested this hypothes is by studying melatonin effects on both microtubule polymerization in vitro and cytoskeletons in situ. Microtubule assembly is a dynamic pr ocess inhibited by Ca2+/calmodulin. Calmodulin antagonists prevent the inhibition by binding to Ca2+-activated calmodulin, thus causing micr otubule enlargement. In the presence of calmodulin (5 mu M) and CaCl2 (1 mM), polymerization at equilibrium was inhibited by 40%. Complete r eversal of the Ca2+/calmodulin effect on microtubules was observed wit h 10(-9) M melatonin or with 10(-5) M trifluoperazine or 1 mu g/ml of compound 48/80. In the absence of Ca2+/calmodulin, melatonin at 10(-5) M inhibited tubulin polymerization like 10(-4) M trifluoperazine does . Melatonin effects on microtubule assembly at both nanomolar and micr omolar ranges were corroborated in cytoskeletons in situ. Therefore, i t is suggested that at a low concentration (10(-9) M), cytoskeletal me latonin effects are mediated by its antagonism to Ca2+/calmodulin. At a higher concentration (10(-5) M), non-specific binding of melatonin t o tubulin occurs, thus overcoming the melatonin antgonism to Ca2+/calm odulin. The results support the hypothesis that under physiological co nditions, melatonin synchronizes different body rhythms through cytosk eletal rearrangements mediated by its calmodulin antagonism.