The control of granule movement in fish melanophores.

The effects of electrical field stimulation, alpha-2-adrenoceptor activation and manipulation of the intracellular machinery on aggregation and dispersion of melanosomes within melanophores of Labrus ossifagus were studied. Neither melatonin nor MSH exerted any direct effects on the melanophores, and furthermore MSH alone did not affect the responses elicited by electrical stimulation or by noradrenaline. Omitting extracellular Ca2+ blocked the nerve-mediated aggregation but was without effect on noradrenaline-induced aggregation. Colchicine and demecolcine inhibited the melanosome aggregation induced by noradrenaline. The characteristics of aggregation and dispersion were found to diverge in several ways. In melanophores made permeable by dispersion were found to diverge in several ways. In melanophores made permeable by using saponin, cyclic AMP caused dispersion. The dynein-ATP-ase inhibitor vanadate prevented the melanosome aggregation within saponin-treated melanophores, while vandate was without effect on the cyclic AMP-induced dispersion. The melanosome transport in saponin-treated melanophores could be activated even in the absence of Ca2+, indicating that the melanosome movements were completely Ca2+-independent.