Increase of cytosolic calcium results in formation of F-actin aggregates in endothelial cells.

In XTH-2 cells, a line derived from tadpole heart endothelia, changes of the F-actin pattern have been followed after injection of calcium and after permeabilization of the cells to calcium using the ionophore A 23187 and an external pCa of 6.0. On the dorsal surface numerous microvilli appear, the stress fibres show an interrupted pattern and F-actin accumulates at the cell periphery. These reactions are interpreted to result from contractions. Finally most of the F-actin becomes depolymerized (in about 30 min). In addition actin containing accumulations are developed in the perinuclear area. In G1 cells stress fibres are less sensitive to enhanced calcium, they persist for more than 30 min in pCa 6.0. Taking into account a relatively high cytosolic calcium concentration in many malignantly transformed cells, their more rounded appearance and lack of an ordered array of actin fibrils could be closely related phenomena.