Calcium regulation of pigment transport in vitro

Calcium has been implicated in the regulation of many cellular motility events. In this study we have examined the role of different Ca²⁺ concentrations on the in vitro transport of pigment within cultured chromatophores. Cells treated with Brij detergent for 1-2 min were stripped of their plasma membranes, leaving their cytoskeleton and associated pigment granules exposed to the external milieu. We found that retrograde pigment transport (aggregation) is induced upon addition of 1 mM MgATP^2- with 10^-7 M free Ca²⁺, while an orthograde transport (redispersal) of pigment results from lowering the concentration of free Ca²⁺ to 10^-8 M while maintaining 1 mM MgATP^2-. These Ca²⁺-regulated movements are ATP dependent but are apparently independent of cAMP and insensitive to calmodulin inhibitors. The observations reported here provide novel evidence that the concentration of free Ca²⁺ acts to regulate the direction of intracellular organelle transport.