Induction of shape transformation in sea urchin coelomocytes by the calcium ionophore A23187.

We have investigated the ability of the Ca++ ionophore A23187 to induce the transformation of petaloid sea urchin coleomocytes to the filopodial form. The response of individual cells to different media was observed with time-lapse phase-contrast video microscopy. In the presence of 1 mM CaCl2, isotonic medium containing 1-5 microM A23187 produces a similar shape transformation to that caused by hypotonic shock. Higher concentrations of ionophore (10-20 microM) induce the formation of filopodia that are thinner and less rigid than those generated by hypotonic shock or low doses of ionophore. A23187 also induces shape transformation in highly flattened cells that do not respond fully to hypotonic shock. The induction of cytoplasmic alkalinization by NH4Cl, methylamine-HCl, or the Na+ ionophore monensin does not induce shape transformation, suggesting that increased intracellular pH is not the stimulus for this process. Ultrastructural changes in cytoskeletal organization were examined in negatively stained detergent-extracted cells. Low doses of ionophore produce filopodia that are indistinguishable from those of hypotonically shocked cells, with actin filament bundles that are straight and cohesive along their entire length. High concentrations of ionophore produce filopodia with filament bundles that branch repeatedly and splay apart near their tips, forming loops and irregular curves. These results suggest that an increase in intracellular free Ca++ concentration acts as the trigger that stimulates coelomocyte shape transformation, but that abnormally high concentrations of intracellular Ca++, produced by high doses of ionophore, interfere with actin filament bundling.