Intracellular calcium and cAMP regulate directional pigment movements in teleost erythrophores

Teleost pigment cells (erythrophores and melanophores) are useful models for studying the regulation of rapid, microtubule-dependent organelle transport. Previous studies suggest that melanophores regulate the direction of pigment movements via changes in intracellular cAMP (Rozdzial and Haimo, 1986a; Sammak et al., 1992), whereas erythrophores may use calcium- (Ca²⁺- ) based regulation (Luby-Phelps and Porter, 1982; McNiven and Ward, 1988). Despite these observations, there have been no direct measurements in intact erythrophores or any cell type correlating changes of intracellular free Ca²⁺ ([Ca²⁺](i)) with organelle movements. Here we demonstrate that extracellular Ca²⁺ is necessary and that a Ca²⁺ influx via microinjection is sufficient to induce pigment aggregation in erythrophores, but not melanophores of squirrel fish. Using the Ca²⁺-sensitive indicator, Fura-2, we demonstrate that [Ca²⁺](i) rises dramatically concomitant with aggregation of pigment granules in erythrophores, but not melanophores. In addition, we find that an erythrophore stimulated to aggregate pigment will immediately transmit a rise in [Ca²⁺](i) to neighboring cells, suggesting that these cells are electrically coupled. Surprisingly, we find that a fall in [Ca²⁺](i) is not sufficient to induce pigment dispersion in erythrophores, contrary to the findings obtained with the ionophore and lysed-cell models (Luby-Phelps and Porter, 1982; McNiven and Ward, 1988). We find that a rise in intracellular cAMP ([cAMP](i)) induces pigment dispersion, and that this dispersive stimulus can be overridden by an aggregation stimulus, suggesting that both high [cAMP](i) and low [Ca²⁺](i) are necessary to produce pigment dispersion in erythrophores.